Crucifer Seed Crop Pests, Parasites, and the Potential for IPM in Northern Thailand

Transcription

1 61 Crucifer Seed Crop Pests, Parasites, and the Potential for IPM in Northern Thailand Brent Rowell, Arkom Jeerakan, and Satit Wimol Vegetable Crops Technology Division, Maejo Institute of Agricultural Technology, Sansai, Chiang Mai 50290, Thailand Abstract Seed production of brassica vegetables has expanded rapidly in northern and northeastern Thailand during , and simplified IPM techniques are urgently needed. A quick and simple sampling procedure for Chinese kale (Brassica oleracea var. alboglabra Bailey) seed crops was developed based on the close association found between the number of diamondback moth, Plutella xylostella (L.), larvae per plant and the percentage of plants infested. Insects were counted twice weekly in the Chinese kale seed crop in Major pests were diamondback moth and armyworm (Spodoptera litura F.); minor pests included fleabeetles (Phyllotreta sinuata Steph.), aphids, cabbage looper (Trichoplusia ni Hübner), Helicoverpa armigera Hübner, and cabbage webworm (Hellula undalis F.). Diamondback moth populations peaked in January but declined rapidly thereafter; this decline coincided with increasing crop maturity and 45-50% Cotesia plutellae Kurdj. parasitism. Spodoptera litura populations peaked in early December but also declined rapidly; low numbers of S. litura coincided with increased parasitism by Snellenius ( = Microplitis)? manilae Ashmead. Rearing of fieldcollected diamondback moth larvae and pupae from Chiang Mai and northeast Thailand revealed a surprising diversity of parasites including Cotesia plutellae, Diadromus collaris Grav., Macromalon orientale Herr., lsotima sp., Brachymeria excarinata Gahan, and B. Iasus Walker. Simplified IPM for Diamondback Moth in Crucifer Seed Crops Commercial production of crucifer seed began only recently in Thailand but is rapidly expanding. Several hundred small farmers in the northeast grew Brassica seed crops on more than 50 ha during the dry season; more than 75 % of this area was planted in Chinese kale (Brassica oleracea var. alboglabra Bailey). The quantity of Chinese kale seed imported is still higher than for any other vegetable crop, ranging from 120 to 150 t annually with a value of US$240, ,000. Local production has supplied less than 25% of this amount leaving ample room for further expansion. Crucifer seed production is problematic in a tropical climate where temperatures can reach 40 C during the cool season and where insects like the diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Yponomeutidae), can attain as many as 25 generations per year. Unfortunately, DBM populations reach their highest levels during the winter growing season. The long season is both an advantage and disadvantage in terms of pest management. The crop must be protected over a long period with heavy expenditures for pesticides in outbreak years. On the other hand, there is ample time for buildup of beneficial insect populations if growers are careful in choosing insecticides not detrimental to parasites and predators. Fortunately there is no concern over the cosmetic appearance of the product. Although bilateral and multilateral aid agencies have promoted and supported IPM projects in developing countries for some years now, it is difficult to cite examples of sustained and 55 1

2 552 Rowell, Jeerakan and Wimol effective pest management programs in Asia. In one of the rare cases where researchers returned to evaluate adoption of an intensely promoted IPM scheme, it was found that farmers no longer monitored insects in their fields in spite of all the training they had received--counting insects was just too difficult and time-consuming (Adalla et al. 1989). Most farmers and researchers in Western countries would agree and hire professional scouts or graduate students to do their counting. Many have observed that IPM technologies used in developed countries cannot be transferred with any hope of success unless they are first simplified and adapted to local constraints (Goodell 1984; O Neil et al. 1989; Smith 1983). Given the dearth of IPM technicians in the field (especially for horticultural crops), it looks as if most small farmers in Asia will need to do their own sampling and make their own decisions for the forseeable future. They are not likely to use any complex sampling plans or economic thresholds offered them by researchers without drastic simplification. Chinese kale seed fields were planted at Maejo with the following objectives in mind: (1) to test an IPM scheme using simple action thresholds based on feeding damage in conjunction with selective insecticides (prior to testing in farmers fields); (2) to verify and document what we considered to be major and minor insect pests at different growth stages of a Brassica seed crop; and (3) to try out a standard terminology for describing growth stages for Chinese kale seed production. General Materials and Methods Insect pests were sampled from a Chinese kale seed production field planted at the Maejo Institute of Agricultural Technology, 10 km north of Chiang Mai, Thailand. Maejo lies in the Chian Mai valley 700 km north of Bangkok at an elevation of 300 m. We sampled from a 440 m field that had been planted as part of a study of insect pollinators of Chinese kale and which included open and caged plots in a randomized complete block arrangement. There were no untreated or conventionally treated check plots for statistical comparisons with plots treated according to thresholds; however, a 230 m² plot planted adjacent to the IPM field was sprayed weekly and used for yield and economic comparisons with the experimental field. An open-pollinated Chinese kale selection from the East West Seed Company was transplanted on 30 October 1989 at 5 weeks after seeding. Seedlings were transplanted into raised beds 5 m long and 1.5 m wide with 50 cm between double rows and 40 cm between plants; other cultural practices currently recommended for commercial Brassica seed production in Thailand were followed. Sampling We first attempted sampling lepidopterous larvae by counting new feeding holes/plant as this appeared to be one of the easiest and quickest methods available (Chalfant et al. 1979; Workman et al. 1980; Chelliah and Srinivasan 1986). This approach was soon abandoned, however, since it was difficult to separate new and old damage and because clusters of Spodoptera litura F. (Lepidoptera: Noctuidae) larvae made so many feeding holes as to render the counts meaningless. Other workers have experienced similar problems with this technique (Cartwright et al. 1987). Sampling based on the percent of plants infested seemed promising in terms of time and simplicity (Morisak et al. 1984), but must be based on a previously established relationship between the number of larvae per plant and the percentage of plants infested (Kirby and Slosser 1981). We consequently decided to count all larvae during the dry season and to record the number of plants infested with one or more DBM larvae in order to determine this relationship.

3 Crucifer Pest IPM in Thailand 553 These data were used to formulate simpler DBM thresholds based on the percentage of plants infested. All insects were counted on five plants in a row from open plots in each of the four blocks of the experimental field. Every other plant in a row was sampled. Averages from all blocks (n = 20 plants) were used to compare with thresholds and in statistical analyses. Plots were sampled twice weekly beginning on 9 November at 10 days after transplanting and continuing until 8 February (2 weeks before harvest) for a total of 24 samples. Aphid numbers were not recorded, but a plant was counted as infested if more than 20 individuals were observed. The obvious presence of parasites or predators was also noted. Growth stages Both crop susceptibility and insect populations vary considerably during the long growing season for seed crops. The crop was divided into the following four growth stages: (1) seedling- -from seeding until transplanting at 6-8 true leaves; (2) preflowering--from transplanting until bolting; (3) flowering/pod setting--from the time first flowers appear until most plants have finished flowering; most seed pods (siliques) will have set during this period, and (4) seed/pod maturity--from the end of flowering until harvest. Action thresholds After abandoning the feeding damage approach, we used action thresholds based on number of larvae per plant. These thresholds were best guesses based on experience with seed production here and on tests of thresholds in Thailand and elsewhere. Apparently none have been used or published for Brassica seed production in Asia, and those we used for the early crop stages were adapted from trials with fresh market cabbage (Rushtapakornchai and Vattanatangum 1982). The thresholds we tried were as follows: Flea Cabbage Growth stage beetles DBM looper Aphids Seedling 1 /plant % plants infested Preflowering 1 /plant 3/plant 0.3/plant 40% plants infested Flowering / podsetting - 2/plant - 40% plants infested Insecticide treatments Broad-spectrum products are widely used in Thailand with the frequent destruction of pollinators and natural enemies. Insecticides used in this study were chosen for both their effectiveness and selectivity; products toxic to bees could not be used during the flowering period. Bacillus thuringiensis Berliner (Florbac FC, 8500 IU/mg) was used exclusively for control of DBM and cabbage looper (Trichoplusia ni Hübner, Lepidoptera: Noctuidae). Pirimicarb (Pirimor) was used for aphid suppression and mevinphos (Phosdrin 24EC) for control of Hellula undalis Fabricius (Lepidoptera: Pyralidae). Although mevinphos is a broad-spectrum product with high mammalian toxicity, its systemic action and short residual make it an attractive choice against species which feed within plant tissues. Bacillus thuringiensis remains effective against DBM in the north at moderate rates of kg/ha in contrast to the apparent tolerance to this material observed in central Thailand. We chose trichlorfon (Dipterex) for use against S. litura, although treatment was not necessary during the study. When the characteristic feeding damage of Spodoptera was encountered during sampling, we inspected the entire field and removed by hand any new larval clusters and egg masses. The granular systemics carbofuran (Furadan 3G) or isazophos (Miral 2.5G) were recommended for control of fleabeetles (Phyllotreta sinuata Stephens (Coleoptera: Chrysomelidae)) in the seedbed before transplanting, but treatments were also not necessary for this insect.

4 554 Rowell, Jeerakan and Wimol Growth stages Results and Discussion The growth stages described earlier were important in determining when different thresholds would be applied and in deciding what plant parts were to be sampled. In actual sampling practice, however, only the preflowering and flowering/pod setting stages were really necessary. Plants in the seedbed (seedling stage) will probably be treated as needed and not according to any thresholds; it should not be necessary to sample or treat during the seed/pod maturity stage in most years. Pest management vs. weekly sprays The field treated according to thresholds was sprayed only four times compared to 10 in the adjacent observation plot which was treated weekly up until 2 weeks before harvest. Insecticide costs for the IPM field were US$98/ha (392 Baht/rai) or about one-third those of the field treated weekly (US$271/ha or 1084 Baht/rai). The six additional weekly sprays were all of B. thuringiensis plus a sticker; sprays applied later in the season were more costly and timeconsuming because of the much larger plant surfaces covered. For example, 1.3 l/ha of B. thuringiensis product was applied at 9-24 days after transplanting compared with 2.5 I/ha applied at full flowering (66-73 days after transplanting). Seed yields were measured from 3m X 5m, four-row plots from each block of the IPM field and from two plots of the same size in the adjacent field treated weekly. Yields from the fourth block were omitted because of the poor drainage and plant growth in that section of the field. The mean seed yield from three blocks of the IPM field was 870 kg/ha (139 kg/rai) compared with 946 kg/ha (151 kg/rai) in the plots treated weekly. Not counting labor costs and using a seed buy-back price of US$3.00/kg, returns after insecticide costs were US$2511 /ha (10,044 Baht/rai) for the IPM plots compared to US$2567/ha (10,268 Baht/rai) for the weeklytreated plots. The field treated according to action thresholds showed only slightly more insect damage than the field treated weekly; higher yields from the latter were due in part to better drainage conditions and more vigorous plant growth compared to the IPM plots. Action thresholds and sampling It appears that the threshold for DBM of 2 larvae/plant during the flowering/pod-setting stage might be too high and that larvae/plant would be safer, especially during the critical early flowering period. A higher threshold of 3.5, 4 or even higher is possible toward the end of flowering. Treatment for DBM and other pests should not be necessary after flowering in most years; populations of Cotesia plutellae Kurdjumov (Hymenoptera: Braconidae) should be high during this period if broad-spectrum insecticides were avoided earlier in the season. The threshold of 0.3 larvae/plant for cabbage looper is probably acceptable during the early preflowering stage until bolting (about 35 days after transplanting) when a higher threshold of /plant might be more appropriate. Loopers appear to cause little damage after bolting; their numbers may have been held in check by parasites and predators. The percentage of plants infested with one or more DBM larvae was closely associated (r2 = 0.97) with the average number of larvae/plant (Fig. 1). New thresholds based on the percentage of plants infested can be calculated using the quadratic curve fitted to these data within the range of x = larvae/plant. Using the resulting equation y = x-0.097x² (y = % of plants infested and x = larvae/plant), count thresholds I, 1.5, 2, and 3 larvae/plant are equivalent to 50, 65, 77, and 85% of plants with one or more DBM larvae, respectively. Although a sequential sampling plan based on these data would be desirable from a statistical standpoint, this technique is probably too complicated for use by the majority of growers. Given the fact that most fields here are smaller than 0.2 ha, it should be possible to use percentage

5 Crucifer Pest IPM in Thailand 555 thresholds in a quick and simple sampling procedure based on casual inspection of a standardized sample of 10 plants for the presence/absence of DBM larvae. The counting obstacle is eliminated and the grower knows immediately when a threshold has been reached (without calculations). Simple boards with movable pegs or slides on a scale painted in contrasting colors above and below thresholds could also help growers keep track of sampling. We hope that this simplified sampling method for DBM can be tested in the fields of a few commercial seed growers; we believe there is a good opportunity for private seed companies, using their own trained extension staff and input supply, to promote pest management and the use of selective insecticides among their contract growers in north and northeast Thailand Predicted line Actual data No. larvae/plant Fig. I. The relationship between the proportion of infested Chinese kale plants and the mean number of DBM larvae per plant. Data points are means of four replications. DBM Major and Minor Crucifer Seed Crop Pests Major Pests DBM is considered the most serious pest by both Brassica seed and fresh market growers in north and northeast Thailand. Although easier to control in the north than in the central region, DBM is still the major pest. Infestations vary considerably from year to year and even from field to field. In some years the insect is difficult to find and treatments unnecessary while in other years weekly or even more frequent sprays are not able to control the pest. Populations are generally lowest following the June-October rainy season but build up rapidly in December and January. The most critical period for seed producers is the time from about the first week of December until the end of January. Local seed growers have learned that early planting is essential both to ensure flowering during the coolest months and to avoid major DBM damage to seedlings and young plants in the field. Assuming a grower has transplanted on time (mid October to early November), DBM will begin to become a problem during the early to midflowering period. At this time flowers and newly formed seed pods must be protected.

6 556 Rowell, Jeerakan and Wimol Our data from the seed field at Maejo illustrate the scenario just described (Fig. 2). The average number of larvae per plant was low from seeding in late September until bolting at about 45 days after transplanting. The population peaked at almost 3.5 larvae/plant in January during the flowering period but declined without treatment after the cessation of flowering. The rapid decline in the DBM population in February was probably associated with the increasing maturity of the seed crop and with C. plutellae parasitism; 45-50% parasitism was recorded from laboratory-reared DBM larvae collected from other crucifer fields at Maejo in February (parasitism by C. plutellae and other species reared from DBM at Maejo is discussed later). Of the four insecticide treatments applied during the 5-month growing season (arrows in Fig. 2), only two of these were applied specifically for DBM. The first treatment (B. thuringiensis) on 23 November was made for control of cabbage looper, although most of these larvae were observed outside the plots sampled. The second treatment a week later (mevinphos) was for H. undalis larvae, several of which were feeding on flower primordia in the sampled plots. The following two treatments (B. thuringiensis + pirimicarb) on 21 December and 4 January were applied for DBM larvae observed on flowers during the critical early and mid-flowering period. The population had reached levels of 1.5 and 1.6 larvaelplant flower cluster for these dates, respectively, but had not yet exceeded the threshold which had been set at 2 larvae/flower cluster. Flower clusters infested with aphids had exceeded the threshold of 40% on both dates. We made a decision not to treat for DBM in spite of high larval counts after 4 January because flowering had almost ceased and many cocoons of C. plutellae were observed in the field. DBM larvae were rarely observed after early February when seed pods were maturing. We had also counted DBM larvae in plots that had been caged with nylon netting for a pollination experiment; DBM populations within the caged plots were very high (> 100 larvae/plant) when the cages were removed on 25 January. This was at a time when very few DBM were to be found in the open-field plots. The plots had been caged on 18 December at 50% flowering and had received the same insecticide treatments as the open plots. 4 cabbage looper Days after transplanting Fig. 2. DBM and cabbage looper population fluctuations in a Chinese kale seed production field at Maejo, Chiang Mai, Data points are means of four replications. Arrows indicate insecticide treatments for DBM and/or other species (see text).

7 Crucifer Pest IPM in Thailand 557 Armyworm Spodoptera litura is considered a major pest only during the preflowering stage. The adults appear to be abundant at the end of the rainy season when the characteristic feeding damage is often observed on foliage of cruciferous crops. Although the larvae can be destructive to young plants, they appear to cause little damage to flowers or developing seed pods; we seldom observed Spodoptera larvae in flower clusters or on the upper plant parts. The average number of larvae observed on the plants was highest during the preflowering stage from November to mid December (Fig. 3). Variability in the data probably reflect the sporadic occurrence of the insect s egg masses and subsequent larval clusters within the field; this irregularity might preclude inclusion of this species in any quick and simple sampling procedure. The apparent rapid decline and disappearance of the insect after the beginning of flowering reflects more the fact that only the upper plant parts were sampled at this time than absence of the species. In addition, the decline is probably associated with a high degree of parasitism by Snellenius ( = Microplitis)? manilae Ashmead (Hymenoptera: Braconidae) in December. Cocoons of this parasite were first found attached to young instar S. litura larvae on cauliflower at Maejo in mid November and then in the experimental plots in early December. Fifteen larvae were parasitized out of 18 examined from a single cauliflower plant on 28 November 1989; six out of 10 larvae examined from 4 to 18 December in our experimental seed field were parasitized. A single individual of a small unidentified ichneumonid (hyperparasite) was also reared from a group of Snellenius cocoons collected from cauliflower in November 1989 at Maejo. No insecticides were applied specifically for armyworms although some hand-picking of eggs and new clusters was attempted at intervals during the preflowering period Days after transplanting Fig. 3. Spodoptera litura population fluctuations in a Chinese kale seed production field at Maejo, Chiang Mai, Data points are means of four replications. Arrows indicate insecticide treatments applied for other insect pests (see text).

8 558 Rowell, Jeerakan and Wimol Cabbage looper Minor Pests Trichoplusia ni, like the armyworm, can be potentially very damaging to young plants from the seedling stage until bolting, but appears to cause little damage to the seed crop thereafter. The first B. thuringiensis treatment on 23 November was applied specifically for looper suppression although the count average of 0.1 larva/plant on this date was lower than the threshold which had been set at 0.3 larva/plant (Fig. 2). The crop was treated because a large number of loopers were observed outside the sampled plots. Although the threshold was exceeded at bolting in early December, these larvae were observed on lower leaves only and did not appear to be causing any serious damage to the crop. Loopers were seldom observed on the upper parts of the plants and none were counted on flowers or developing seed pods after 11 January. It is likely that natural enemies played an important role in reducing the seriousness of this pest, although no larvae were reared or examined for parasites during this study. More loopers were observed in the caged plots of the pollination experiment than in open-field plots indicating the possible exclusion of parasites and predators. Cabbage webworm Hellula undalis may cause occasional damage to seed fields, especially to young plants in the preflowering stage. They occurred in small numbers sporadically throughout the season. We felt it necessary to treat with insecticide once after a total of four were observed feeding on flower primordia of young plants in the sampled plots at 25 days after transplanting. Other H. undalis larvae were occasionally found feeding inside wilting plant stems and in late-blooming flower clusters. Pieris brassicae Larvae of Pieris brassicae L. (Lepidoptera: Pieridae) were not recorded within our seed fields during the sampling period but were frequently observed on Brassicas at Maejo in February and March. At this time many larvae in the field were parasitized by Apanteles sp. (glomeratus group). We reared several groups of Apanteles from parasitized Pieris larvae; Eurytoma sp. hyperparasites emerged from two Apanteles cocoon clusters. The hyperparasites emerged from 17 out of 32 Apanteles cocoons in one cluster and from all 15 cocoons of the second cluster. Pontia daplidice Two late-instar larvae of P. daplidice Rober (Lepidoptera: Pieridae) were found feeding on cabbage plants in a field near our seed plots on 3 January This is apparently the first record of this species from a low elevation in Thailand. Helicoverpa armigera Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) larvae were first observed feeding on flowers just outside the sampled plots on 8 January. Although a peak of 18 larvae per 20 plants sampled was recorded on 1 1 January, these insects did not appear to be causing significant damage. Larvae were always found feeding on the terminal flower clusters; they appeared in the field at a time when few flowers remained and most seed pods had already formed. This pest occurred at about the same time on Chinese kale in northeast Thailand where farmers complained about the difficulty of controlling it with insecticides. Although late instar larvae are large and of obvious concern to farmers, it is not known if damage to the late-blooming terminal flower clusters results in significant yield losses. The insect's tendency to feed in the

9 Crucifer Pest IPM in Thailand 559 tops of plants would appear to expose it to birds and other predators; one farmer in the northeast reported that birds often came to his seed field to feed on the large worms. He also expressed a concern that his insecticide treatments were interfering with the birds feeding activities. Aphids Aphids (species undetermined) were present during the entire season from seedling stage until harvest. The threshold of 40% was exceeded four times during the sampling period. The highest level of 60% infestation (of flower clusters) occurred during early flowering on 21 December and pirimicarb was applied (together with the B. thuringiensis treatment for DBM on that date). Pirimicarb was again applied with a B. thuringiensis treatment for DBM 2 weeks later when 55% of the flower clusters were aphid-infested. Both applications were effective and aphid counts were low for 2-3 weeks afterwards. Although aphid populations were high again on plants in February during the seed maturity stage, they appeared to be causing little damage and were not treated. Larvae and adults of Menochilus sexmaculatus F. (Coleoptera: Coccinellidae) plus adults and mummies of Aphidius sp. were frequently observed in the field during this final stage of the crop. Fleabeetles Fleabeetles (Phyllotreta sinuata) are potentially damaging to plants in seedbeds and to newly transplanted seedlings early in the season; later plantings are more susceptible. We recorded 0.2 adult/plant on 14 November at 2 weeks after transplanting and occasionally noted the presence of fleabeetles in the field thereafter. Protection of seedlings is recommended with granular systemics for later plantings or when fleabeetles are abundant. Leafminers and other Diptera Liriomyza brassicae Riley (Diptera: Agromyzidae) leafminers were present in small numbers but appeared to cause no damage to the crop. Natural enemies may control this pest when longresidual, broad-spectrum insecticides are avoided. Leafminers have occasionally damaged crucifer plantings in the North but this is thought to have occurred as the result of the destruction of natural enemies with insecticides used to control other pests. Unidentified small dipteran larvae were occasionally found causing wilting of leaves (preflowering stage) and later wilting of terminal flower clusters. The larvae burrow and feed inside leaf petioles and young flower stalks but appear to cause little damage because of their low numbers. Harlequin bug Eurydema pulchrum Westwood (Hemiptera: Pentatomidae) was first observed on the last count on 8 February during the seed maturity stage. Although adults and nymphs were frequently observed on plants after this date until harvest, it is not known if they cause any damage to the maturing seed crop. Parasites of DBM in Northern Thailand Materials and Methods Field collections of DBM larvae and pupae were made in March and April 1989 in preparation for rearing and release of imported Diadegma semiclausum Hellén (Hymenoptera: Ichneumonidae) at Maejo in Chiang Mai. The parasites did not reproduce in the high temperatures in our

10 560 Rowell, Jeerakan and Wimol laboratory, however, and none were released. Additional DBM samples were taken from crucifer fields during the dry season to determine what parasites were already present around Chiang Mai; four samples from each of two locations were made at irregular intervals throughout the growing season from 23 December 1989 to 22 March Samples were taken from crucifer crops at the Maejo campus (MIAT) and at or nearby the East-West Seed Company field station at Ban Chedi Mae Krua (BCMK) 10 km north of the Maejo campus. The last sample at Maejo on 22 March 1990 was taken from a destructive harvest of 200 potted cabbage plants which had received no insecticide treatments. Other crucifers at MIAT had been treated almost exclusively with B. thuringiensis while those at BCMK received more frequent applications of broad-spectrum materials in addition to B. thuringiensis. Rearing results are summarized in Table 1. All percentages reported below are based on the parasitized fraction of the total number of parasites and adults recovered after rearing rather than the total number of DBM larvae or pupae collected, i.e. larvae/pupae which were lost to disease or other causes were not included in the denominator. For reasons we cannot explain, % of the larvae from each batch reared could not be accounted for by emerged DBM adults, parasites, or the remains of dead larvae. In addition to samples from Chiang Mai, two collections were made from farmers fields in northeast Thailand on January No parasites emerged from 104 larvae and 49 pupae collected from a heavily DBM-infested mustard seed field 9 km south of Nakorn Phanom town. A single individual of Bruchymeria lusus Walker (Hymenoptera: Chalcididae) emerged from among 107 DBM pupae collected from a heavily infested Chinese cabbage seed field 33 km east of Loei town; no parasites were found among 154 DBM larvae reared from this field. Both fields had been treated with broad-spectrum insecticides and DBM appeared to be beyond control in the field sampled at Loei. Table I. Parasites reared from DBM larvae and pupae from Maejo (MIAT) and Ban Chedi Mae Krua (BCMK) in Chiang Mai, Thailand DBM collected Parasites emerged Date Location larvae pupae C. D. M. B. Adult Total plutellae collaris orientale excarinata DBM recovered 16 Mar.89 MlAT Apr Apr Dec Jan Feb Feb Mar Mar Mar Mar. 90 BCMK BCMK BCMK BCMK MlAT MlAT MlAT BCMK BCMK KlAT I I I I I I49 I I56 I56 I I I I I I07 Total number of DBM parasites + adults recovered after rearing. This figure was used in calculating percentages of parasitism cited in the text.

11 Crucifer Pest IPM in Thailand 56 1 Results and Discussion Egg parasites Although we did not sample for DBM egg parasites in the course of this study, the occurrence of Trichogrammatoidea bactrae Nagaraja (Hymenoptera: Trichogrammatidae) from lowland central Thailand and Trichogramma confusum from the Khao Khor highlands of Petchaboon Province (Keinmeesuke and Vattanatangum 1986; P. Keinmeesuke, pers. comm.) suggests that one or both of these species may also occur in the north. Larval parasites C. plutellae: This species appears to be the dominant larval parasite from December to March in our area (Table 1). Parasitism ranged from 12% in mid December to as high as 88% in mid April 1989 at BCMK; average parasitism for all samples was 41 %. We observed relatively few parasites in the field during the critical period in December when DBM can be very damaging; economic damage to fresh market crucifers often occurs before sufficient numbers of the parasite are present. Cotesia plutellae populations appeared to rise together with those of the host. DBM populations declined after mid January (Fig. 2) which coincided with the period when C. plutellae was abundant. Two hyperparasites emerged from C. plutellae cocoons during the course of this study. One individual, identified as Brachymeria excarinata?apantelesi Risbec. (Hymenoptera: Chalcididae) emerged from a group of 39 C. plutellae pupae from parasitized DBM larvae collected on 15 February 1990 at Maejo; a second, Brachymeria excarinata plutellae (Joseph et al. 1972) emerged from among a group of 58 pupae from parasitized DBM larvae collected on 7 March 1990 at Maejo. Macromalon orientale Kerrich (Hymenoptera: Ichneumonidae): This species was reared from both larvae and pupae of DBM and constitutes a first record of its occurrence in Thailand (and possibly Southeast Asia). It has been listed as an important larval parasite in the Assam region of India (Chacko 1968). Macromalon onentale, along with Diadromus collaris Gravenhorst (Hymenoptera: Ichneumonidae) and C. plutellae were sent to Thailand from India in 1965 but laboratory rearing was not successful and no releases were made (Rao et al. 1971). Small numbers of this parasite were reared from larvae and pupae collected late in the growing season from mid February until the last samplings on 19 April 1989 and 22 March 1990 (Table 1). Parasitism ranged from 1 % in February to 9 % in early March with an overall mean of 2 % for all sampling dates. Pupal parasites D. collaris: This ichneumonid was the pupal parasite most frequently reared from our samples (Table 1). Although two specimens were reared from 33 DBM pupae collected on 14 February 1990, most individuals emerged from cocoons collected rather late in the growing season in March. Parasitism of pupae sampled ranged from 9 to 10% in February to 9-31 % in March with the highest percentage recorded from the 22 March sampling of potted plants at Maejo. The overall mean parasitism for all cocoons sampled was 9 %. The fact that only one specimen was reared from BCMK was probably due to more frequent application of broad-spectrum insecticides at that location. Many adults of this species were observed in the field at Maejo in March of both years. D. collaris, like M. orientale and B. excarinata appears to occur in significant numbers only late in the dry season when temperatures are rising and DBM is relatively scarce. This species was previously recorded from the Khao Khor highlands of north-central Thailand where parasitism was 23% and 63% in February of 1985 and 1986, respectively (Keinmeesuke and Vattanatangum 1986).

12 562 Rowell. Jeerakan and Wimol Brachymeria excarinata Gahan: Specimens identified as the subspecies Brachymeria excarinata plutellae (Joseph et al. 1972) were reared from DBM cocoons collected from late February until the last sampling of potted plants at Maejo on 22 March Parasitism of pupae collected ranged from 4 to 9% in early-mid March to 25% from the last sampling date. Overall parasitism for all cocoon samples was 6%. Although Brachymeria sp. parasites of DBM were imported to Thailand in 1965, they did not survive in the laboratory and none were released (Rao et al. 1971). Brachymeria excarinata has been recorded in southern India both as a primary parasite with 15-60% parasitism of DBM pupae (Cherian and Basheer 1939) and as a hyperparasite of C. plutellae (Nagarketti and Jayanth 1982). lsotima sp. (Hymenoptera: Ichneumonidae): A single female emerged on 22 March 1989 from a group of DBM pupae collected at BCMK. At least two Isotima species have been reared from lepidopterous rice pests in Malaysia (van Vreden and Ahmadzabidi 1986); they had not previously been recorded as parasites of DBM. This specimen readily attacked new DBM pupae in the laboratory, inserting the ovipositor through the top of the cocoon and repeatedly flexing its abdomen. This parasite probably has a wide host range and perhaps attacks DBM only when other hosts are not readily available. Summary Cotesia plutellae appears to be the dominant larval parasite at the two locations sampled, destroying 45-50% of DBM larvae in February and March. Macromalon orientale was present in low numbers in March but did not parasitize more than 9% of larvae. A total of about 60% of DBM larvae were parasitized by the two species at Maejo in early March. Diadromus collaris appears to be an important pupal parasite late in the growing season, parasitizing about 10% of DBM pupae in February and up to 30% in March. Although numbers of B. excarinata were negligible in February, this species parasitized 25 % of the pupae sampled at Maejo on 22 March From 27% to almost 60% of all DBM pupae sampled were destroyed by a combination of these two species at Maejo in March The main gap in the occurrence and activity of DBM parasites appears to be the critical period early in the dry season in December when farmers are most likely to treat with broad-spectrum insecticides. It is during this period when the careful choice of more selective products should greatly increase the chances of effective natural control occurring later in the season. Further development and application of basic pest management principles is urgently needed to reduce dependence on pesticides and to preserve the diversity of natural enemies associated with crucifer crops in northern Thailand. Acknowledgments The authors would like to express their appreciation to Dr. T.C. Narendran, University of Calicut, Kerala, for the identification of chalcidid parasites and of Apanteles sp. (glomeratus group); the help and advice of Dr. Banpot Napompeth, Mr. Kosol Charernsom, and Ms. Amporn Winotai of the National Biological Control Research Center, Bangkok, is gratefully acknowledged. The authors would also like to thank Church World Service, New York for providing financial support for this project. References Adalla, C.B., Hoque, M.M., Rola, A.C., Stuart, T.H.,and Sumayao, B.R Participatory technology development: the case of integrated pest management extension and women project. First Asia-Pacific Conference of Entomology, 8-13 Nov. 1989, Chiang Mai, Thailand, Abstract volume, 210 p.

13 Crucifer Pest IPM in Thailand 563 Cartwright, B., Edelson, J. V., and Chambers, C Composite action thresholds for the control of lepidopterous pests on fresh-market cabbage in the lower Rio Grande valley of Texas. J. Econ. Entomol., 80, Chacko, M Macromalon sp., a parasite of Plutella maculipennis Curt. in Assam (India). Tech Bull. Commonw. Inst. Biol. Control, 19, Chalfant, R.B., Denton, W.H., Schuster, D.J., and Workman, R.B Management of cabbage caterpillars in Florida and Georgia by using visual damage thresholds. J. Econ. Entomol., 72, Chelliah, S., and Srinivasan, K Bioecology and management of diamondback moth in India. In Talekar, N.S., and Griggs, T.D. (ed.) Diamondback moth management: Proceedings of first international workshop, Asian Vegetable Research and Development Center, Taiwan, Cherian, M.C., and Basheer, M Brachymeria excarinata Gahan. (Family Chalcididae), a pupal parasite of Plutella maculipennis Curt. in South India. Proc. Indian Acad. Sci., 7, Goodell, G Challenges to international pest management research and extension in the third world: do we really want IPM to work? Bull. Entomol. Soc. Amer., 30, Joseph, K.J., Narendran, T.C., and Joy, P.J New species of Brachymeria Westwood in the collections of Commonwealth Institute of Biological Control, Bangalore, India. Tech. Bull. Commonw. Inst. Biol. Control, 15, Keinmeesuke, P., and Vattanatangum, A The effects of natural enemies on diamondback moth populations in the Khao Khor highlands. J. Entomol. Zool., 4, (in Thai). Kirby, R.D., and Slosser, J.E A sequential sampling plan for three lepidopterous pests on cabbage. Southwest. Entomol., 6, Morisak, D.J., Simonet, D.E., and Lindquist, R.K Use of action thresholds for management of lepidopterous larval pests of fresh-market cabbage. J. Econ. Entomol., 77, Nagarkatti, S., and Jayanth, K.P Population dynamics of major insect pests of cabbage and their natural enemies in Bangalore district (India). In Proceedings of International Conference on Plant Protection in the Tropics, Malaysian Plant Protection Society, Kuala Lumpur, Malaysia, O Neil, R.J., Andrews, K.L, Barfield, C.S., and Sobrado, C.E Sampling program for fall armyworm in maize. J. Econ. Entomol., 82, Rao, V.P., Ghani, M.A., Sankaran, T., and Mathur, K.C A review of the biological control of insects and other pests in South-East Asia and the Pacific region. Tech. Comm. No. 6. Commonw. Inst. Biol. Control, Trinidad. Rushtapakornchai, W., and Vattanatangum, A Control threshold for diamondback moth and cabbage looper on cabbage. Annual Research Report for Entomology and Zoology Division. Dept. of Agriculture, Bangkok, Thailand, 9 p. (in Thai with English summary). Smith, E.H Integrated pest management (IPM)--specific needs of developing countries. Insect Sci. Applic. 4, van Vreden, G., and Ahmadzabidi, A.L Pests of rice and their natural enemies in peninsular Malaysia. Pudoc, Wageningen, 230 p. Workman, R.B., Chalfant, R.B., and Schuster, D.J Management of the cabbage looper and diamondback moth on cabbage by using two damage thresholds and five insecticide treatments. J. Econ. Entomol., 73,