REPORT ON DAMAGE TO SUGARCANE BY THE DYNASTID BEETLE, HETERONYCHUS LICAS (KLUG) ABSTRACT

Transcription

1 Entomology REPORT ON DAMAGE TO SUGARCANE BY THE DYNASTD BEETLE, HETERONYCHUS LCAS (KLUG) K. E. Cackett Rhodesia Sugar Association Experiment Station, Chiredzi, Rhodesia ABSTRACT Heteronych-us licas was considered a minor pest of non economic importance to the Rhodesian sugarcane industry until an unexpected increase in insect population led to extensive damage following a series of seasons with above-average rainfall from 1974 to Adults feed underground on shoots and stalks of all ages during the wet summer months, with larvae feeding on roots and crowns throughout late summer and winter. The extent of yield loss is associated with the degree of stand reduction and stool mortality, with severe damage necessitating early plowing and replanting. The life cycle of the insect has been discussed with preliminary results of some of the research work initiated to study the biology and control of the pest. Control with dieldrin has been suggested pending the results of long-term trials designed to compare various suitable insecticides and their residual activity under local conditions. NTRODUCTON The only insect pest known to cause economic damage to sugarcane in Rhodesia has been the Dynastid beetle Heteronychus licas (Klug), which feeds on cane in both the adult and larval stages. This insect was first recorded as a minor pest of sugarcane in Rhodesia in 1966, but it was not until 1974 that damage became more widespread and started causing concern. Although control measures were first introduced in 1977, when isolated outbreaks in young ratoon cane were treated, the magnitude of the problem was not fully appreciated until 1978, when damage to young ratoon canes late in the season showed that adult populations had reached alarming proportions, and that the insect posed a serious threat to the sugarcane industry. Detailed surveys in commercial cane revealed that adult damage was not confined to new shoots in young plant and ratoon cane, but that older cane was also being attacked and that the larvae were feeding on roots as well as on dead organic material.

2 1761 ENTOMOLOGY H. licas, more commonly known as the Black Maize Beetle, has been known as a pest of maize for many years, and it was first reported in Rhodesia by symesg following severe damage to maize crops in low-lying wet areas over the period 1912 to Small scattered outbreaks of damage to sugarcane were reported from Natal in the 1940fs, by which time H. licas had already been recorded as troublesome on cane in Mozambique (Carnegie et a3) Extensive damage to cane by H. licas in Swaziland in the 1960's led to a comprehensive study by Sweeney ', and it was subsequently found in adjoining areas of irrigated cane in Natal and Transvaal. t has also been recorded as a pest of sugarcane in Nigeria (~ardcastle~), and Tanzania (Le pelley5). Several other species of Heteronychus have been reported elsewhere as minor pests of cane, but records of H. licas appear to be confined to Africa. Williams et all0 record damage to cane by H. arator in Australia, but although this species is known to damage a wide range of crops in Southern Africa and elsewhere it was not recorded in sugarcane by sweeney 'in Swaziland, and has not been found in cane in Rhodesia. Several species of Heteronychus are known to damage cane in Madagascar, including H. rusticus, a species recorded in Swaziland and suspected by sweeney to be a cane pest. pemberton7 lists H. morator as a sugarcane pest in Java and the Philippines, and Avasthyl records H. robustus as causing damage to cane in ndia. The build-up of H. licas to serious proportion in the Rhodesian sugarcane industry occurred over a period of four years, all of which were characterized by abnormally high rainfall. This paper recounts the results of the preliminary observations and studies made after adult populations of H. licas increase dramatically following the first soaking rains of the season in November, 1978, resulting in widespread damage to sugarcane of all ages. LFE CYCLE Considerable information is available on the life cycle of H. licas from the work done by symesg on maize in Rhodesia, by sweeney8 on sugarcane in Swaziland, and from recent detailed records from trials in Rhodesia which are still in progress. The life cycle is closely related to seasonal climatic conditions, and Table 1 shows the life cycle pattern in relation to lonpterm mean meteorological data. (a) Eggs are laid during the main rainy season which extends from November to April. n Swaziland, Sweeney8 recorded eggs from October to February, but in Rhodesia eggs were found throughout March and April. The eggs are laid singly in moist soil among the roots and stools of the cane, mainly within 30 cm of the soil surface, and rarely more than 25 cm from the cane row. They are creamy-white and ovoid in shape when newly deposited, measuring 1.3 mm to 2.0 mm in length and

3 C Q c Q s E E So" 1.0 mm to 1.7 mm in width. During incubation they increase in size and become more spherical in shape, measuring 1.3 mm to 2.0 mm in diameter and becoming 1 ivory white just prior to hatching. sweeney8 reported that females laid between 30 and 80 eggs each, and symesg recorded 60 to 70 eggs per female. The incubation period varies from 8 to 21 days, depending on temperature and soil moisture. Eggs cannot survive exposure to the sun and are very susceptible to soil moisture stress, but they appear to be immune to the effects of excessive moisture. (b) The larvae are typical white grubs in sha~e and appearance, being dirty white in colour with a pale brown head and assur$ing a bluish-grey colour when '0

4 1763 ENTOMOLOGY actively feeding. There are three larval instars described by sweeney8 (Plate 1). Young first instar larvae first appear in late November, but the main hatching occur inclusively from December tq March. The larvae are active for 5 to 8 months, during which time they are capable of erratic movement in the soil, mainly in search of food or to burrow deeper as the soil- dries out between irrigations. An earthen cell is formed by the larvae towards the end of the third instar, which is normally in July/August, and although they may remain quiescent in these cells for several weeks without phange, they normally enter the pre-pupal stage witkin 9 week or two (sweeney89. (c) The pupal stage occurs from August to October following a period of 1 to 3 weeks as pre-pupae, bdi pupae tend to be most abundant in September and none have been found after mid-october. Cells are generally found in the vicinity of the stool at approximately the same depths as the feeding larvae, i.e. mainly within 30 cm of the soil surface. (d) The adult beetles are shiny black in colour, measuring 14 to 20 mm in length and 6 to 10 mm wide, but they are generally brown to dark brown soon after shedding their pupal skins. Most of them remain dormant in their cells until stimulated to emerge by the first rains of the season. Adults have been found feeding on cane on September, mainly in poorly drained areas, but it is.not clear whether these are newly emerged adults or whether they represent a small residual population which has successfully over-wintered. The main build-up of adult populations appears to be dependent on the occurrence of rainfall, irrespective of normal irrigation schedules, which ilndicatesthat pupal cells in the fields are not usually formed in the irrigation furrows. Adult damage DAMAGE TO SUGARCANE n young cane, the adults feed on new shoots just below the ground level, destroying the growing point and causing the central heart or the whole shoot to wither and die (Plates and ll). Such damage is readily visible in young ratoon cane 2 to 3 weeks after harvest, and the beetles are subsequently capable of completely destroying the primary stand within 2 to 3 months. Counts of "deadhearts" made in a field of NCo 376 in December, 1978, showed that shootsfha had been destroyed within 3 weeks of harvest, and that shoot mortality had increased to /ha after 12 weeks. Compensatory tillering is insufficient to replace such loss, with the result that stands are drastically reduced, gaps develop as whole stools are destroyed, and yields are soon reduced to a level necessitating plowing and replanting (Plate V). Visible symptoms of damage diminish as the shoots get older, because the growing points are then above the ground and the beetles rarely bore right through the stems causing the tillers to die back. For this reason it was at first believed that

5 K. E. CACKETT 1764 ', T ' PLATE 3

6

7 K. E. CACKETT 1766 damage was confined to young tillers in late-season crops, i.e. those harvested from September until the end of the cutting season early in December. Detailed surveys in revealed that beetles were actively feeding on cane of all ages, boring into both young and mature stalks and leaving a hole 1.0 to 1.5 cm in diameter at the base of the stalk leading into an enlarged chamber (Plates V V, V). Cane damaged in this way does notdie, but the injury causes a serious growth check and loss of uniformity, which is generally the only indication of beetle activity until the stools are inspected below ground level. There is considerable evidence to show that adults prefer to feed on young cane and that they only move into older cane when less young tillers are available i.e, towards the end of the cutting season. However, the pattern of infestation and damage lacks consistency, a fact which was also noted by sweeneys in Swaziland, and it is common to find a heavily infested field adjacent to one almost entirely free from infestation. The adults tend to favour heavier soils, but although in they were more widespread on heavy black montmorillonite clays they were certainly not confined to them, causing damage on lighter textured soils but avoiding gravely and stony situations. Larvae damage While the main food of the first instar larvae is organic matter in the soil, the second instar, as it grows, feeds progressively more on green plant material, while the third instar feeds voraciously on the host plant (sweeney8). The main food item is roots of all sizes, but later stage larvae will tunnel into the stool and crown, severing 'roots but also feeding on more solid material including the dead crown of the previous crop. Damage is most obvious from February to August, and it can be devastating when populations are high. Extreme injury causes wilting and yellowing of leaves symptoms which are identical to those caused by soil moisture stress. Under such circumstances whole stools can easily be pulled out of the ground with little effort, because the root system has been mostly destroyed. However, there can be a high level of infestation without any obvious superficial effect to the cane, as individual stalks or stools seldom die. Regular DSTRBUTON N THE SOL fortnightly samples were taken from a heavily infected field during the summer to study the vertical and lateral distribution of adults, larvae, and eggs. Typical counts are shown in Table 2, each value being the mean of 12 samples expressed as a percentage. Data shown in Table 2 clearly indicate thatathe distribution of adults and larvae is closely associated with their feeding habits. The adults are present mainly in the vicinity of the stool, with relatively few being found in the interrow. n the, 2 3,, if

8 1767 ENTOMOLOGY TABLE 2. Percentage distribution of adults, larvae and eggs Month Adults Larvae Depth (cm) nrow nter row nrow nter row nrow nterrow Jan QO Feb 0-15 " Mar APr O. 0 case of larvae, 70 to 80% were found in the top 30 mm around the stool, with the rest scattered throughout the profile feeding on roots which are more widely distributed than the preferred food of the adults. Eggs were found mainly in the surface horizon of the cane row where adults were most numerous. The distribution of adults and larvae in the soil varies markedly in relation to soil moisture conditions, with larvae tending to burrow deeper as the soil dries out between irrigations, and adults tending to migrate to wetter areas when the soil gets too dry near the surface. EFFECT ON YELD t has not been possible to quantify the effects of H licas damage on yields of sugarcane in Rhodesia, simply because the problem has not been studied for a sufficient length of time. However, records from certain fields which were treated in 1976 after severe adult damage to young ratoons showed that yields dropped by 30 to 40% as a result of the damage before being restored to normal levels two years after treatment. Many other commercial fields, whose yield potential dec lined to 60-70% in recent years have been found to be heavily infested, although fl. licas may not have been the sole cause of the problems. t has also been apparent that smut (Ustilago scitaminea Syd). levels have been considerably higher than normal in heavily infected fields, largely due to the increased susceptibility sf late-formed tillers, and the general unthriftiness of the crop which makes it more prone to disease attack.

9 K. E. CACKETT Severe losses in Swaziland have been attributed by sweeney8 to be due mainly to larval damage, with the most serious effects occurring in 3rd to 5th ratoons. He measured a mean loss of 55 tonslha from 9 fields in the third ratoon, and recorded losses of almost 100 tonslha in more seriously infected fields. The evidence in Rhodesia indicates that such losses are possible, but only in areas which have been heavily infected for at least 2 to 3 years, and certainly not over the industry as a whole. Biological and cultural control CONTROL MEASURES Diseases and predators of Heteronychus spp. were studied by sweeney8 who concluded that it was unlikely that any form of biological control wourd give permanent success because endemic insect parasites and predators were too scarce in cane fields. Similarly, cultural practices cannot be expected to control the pest, although they could effectively reduce damage in susceptible areas. For example, damage tends to be more severe in poorly drained areas, where adults emerge early in the season to start feeding, and improved drainage in such areas would reduce insect damage in addition to providing other yield benefits. Drying out after harvest by reducing the frequency of irrigation late in the seasoncwould have the effect of reducing adult activity, but the degree of desiccation required to kill adults and larvae would cause considerable stress to the cane. Eggs are more susceptible to desiccation, but the beetles will not lay eggs in dry soil, and rainfall during the main egg-laying period would reduce the chance of prolonged periods of drying-out. Other cultural practices, such as avoidance of alternate hosts, rotations, deep cultivations, light traps, etc., would all be beneficial, but none would be effective in significantly reducing the overall population. Chemical control The only immediate prospect of effective control of Heteronychus licas is by the use of insecticides. White grubs of various insect species have long been troublesome throughout the sugarpane-growing world and have been classified as major pests in many countries, with the result that a considerable amount of research has been conducted on methods of controlling these pests with insecticides. n S. Queensland, the larvae of Rhyparida spp. beetles cause dead-heart symptoms similar to those caused by the adult H. licas and they are effectively controlled with aldrin of BHC (Molter and ~ungomer~~). White grubs of several Melolonthid species also attack sugarcane in Queepsland, and are generally control- $( '") ' i

10 1769 ENTOMOLOGY led with BHC, as dieldrin and aldrih have been shown to be ineffective (~ilson' 1. n ndia, BHC has also been shown effective against grubs of Holotrichia spp. (~vasthy' ), while ~ates~ reported that in the Americas white grubs were no longer considered as major pests since the advent of chlorinated hydrocarbons, particularly BHC. sweeney8 recommended dieldrin for the control of H. licas in Swaziland, in preference to any other insecticide, but he did not conduct trials to compare the relative effectiveness of different compounds. H. licas completes its life cycle underground and both adults and larvae are subterranean feeqiers. t is thus an extremely difficult pest to control because of the problem of getting a suitable insecticide into the soil where the insect is active. Soil treatment at the time of planting is the only practical means of applying insecticide to the main feeding zone, i.e. underground in the vicinity of the stool. Trials have been initiated in Rhodesia to compare various concentrations of a range of insecticides for pre-plant treatment, but it will be several years before these trials provide useful desults. n the meantime, dieldrin is being used throughout the industry, applied either as a 50% W. P. or 15% E. C. at the rate of 2 kglha a.i. sprayed in a band over the planting furrow before the setts are covered back. t has been shown by many workers that dieldrin incorporated in the soil has a residual effect of up to 4 years, so that treatment at planting should provide control for the plant crop and 2 to 3 ratoons. However, the Rhodesian crop cycle is 6 to 8 years and often longer, so that effective reapplication of insecticide to the established crop becomes necessary. Considerable progress has been made by local farm equipment suppliers in developing suitable implements for applying insecticide below the surface for H. licas control, but in many cases such methods cannot be used and surface application is necessary. Work in Swaziland has shown that when dieldrin is sprayed on the soil surface it has sufficient residual activity to prevent serious larval damage for at least 1 to 2 years. Surface spraying will not prevent adult damage, however, even though it will eventually kill the beetles, because the adults are capable of inflicting considerable damage to the cane between the time of contact with the dieldrin on the soil surface and eventual death. Sweeney8 found that adults contacting the soil surface after surface spraying 1 to 3 months previously, died within a few days; if sprayed 5 to 9 months previously the beetles took 10 or more days to die; and when surface spraying was done 1% to 2 years previously, mortality was erratic with only a 50% kill in 10 to 14 days. Trials were initiated in Rhodesia in 1978 to study the effectiveness of surface applications of dieldrin and aldrin for control of H. licas, and results revealed valuable short-term effects. Trial. The second ratoon of a commercial field was harvested in November,,

11 K. E. CACKETT TABLE 3. Effect of treatments on adult and larvae populations and on shoot mortality (percentages) Treatment Adults Larvae Dead shoots i Control kglha a.i., 2.5 kglha a.i., 4.0 kglha a.i. 1.0 kglha a.i #, 2.5 kglha a.i iij " 4.0 kglha a.i Banded over cane row Banded over inter-row Full cover spray , and within 2 weeks extensive shoot mortality was evident as a result of adult beetle activity. A trial was conducted on the site, with treatments applied 3 weeks after harvest and comprising 3 levels each of dieldrin and aldrin (1,O; 2,5; and 4,O kg/ha a.i.) applied either as (a) a band over the cane row, (b) a band over the interrow, or (c) a full cover spray. mmediately after application the inter-row was cultivated and reshaped for furrow irrigation. Weekly sampling for insect counts, and weekly records of dead shoots were taken to provide a measure of insect activity. Results after 3 inonths are shown in Table 3, where cumulative totals have, been expressed as percentages of control values. The most noteworthy short-term effects were that (a) aldrin provided better control than dieldrin, (b) 1 kg/ha was less effective than 2.5 or 4.0 kglha. (c) interrow application was less effective than the other methods, irrespective of insecticide treatment, (d) banding over the cane row was no less effective than a full cover spray at twice the cost, and (e) control of larvae was better than control of adults. Trialll. Four levels of dieldrin were compared in a second trial, with the insecticide applied as a banded surface spray over the cane row in an area where damage was so severe that four months after the trial was started it had to be abandoned because of the loss of stand caused by H. licas damage. Table 4 shows cumulative counts for the first three months after treatments were applied, expressed as percentages of control values. The results from both hese trials clearly show that surface applications of dieldrin and aldrin, even at low concentrations, rquce egg-laying and larval populations more effectively than they reduce adult popubtion,s even though the larvae t \

12 ENTOMOLOGY TABLE 4. Effect of treatments on insect populations and shoot mortality (percentages) Treatment Adults Larvae Eggs Dead shoots Control kg/ha a.i kglha a.i. \ 64, 44, ' 3.0 kglha a.i kglha a.i. L&) do not come into contact with the insecticide. Sweeney8 found out that egg-laying by fertile adult females was inhibited even by sub-lethal doses of dieldrin, and that any progeny produced survived for only a very short period. t must also be recognized that adult counts did not accurately reflect their response to treatments, because adults are constantly moving and many that have contacted insecticides will die elsewhere and be replaced by others. DSCUSSON H. licas is an endemic insect in Rhodesia, as it is in Swaziland where environmental conditions are very similar, and under natural conditions it feeds in wet areas on grasses, sedges, reeds, and bulrushes, provided these plants are not waterlogged. However, the normal semi-arid conditions prevailing in the natural bush restrict the excessive multiplication of the insect because suitable breeding sites dry out during the winter months and seriously interfere with the life cycle during the larval phase. The available evidence clearly shows that the build-up of H. licas from 1974 to 1978 was closely associated with a series of seasons with abnormally high rainfall, following a period of several years with rainfall below average. This is shown in the following table in which seasonal totals and deviations from the long-term (35-year) mean of 586 mm are presented. Season Rain Dev. from season Rain Dev. from (mm) mean (mm) (mm) mean (mm)

13 K. E. CACKETT f he increase in H, licas populations is attributed, therefore, to (a) the series of wet seasons which created a greater number of suitable habitats and breeding sites in the natural bush, (b) many of these breeding sites remaining wet throughout! the winter without the adverse effect of a drying cycle, and (c) the development of 1 ever-increasing populations in cane fields which offer the insect optimum conditions all the year round. The result of these factors was an increase in the reproductive rate far in excess of normal, causing a minor pest to assume major pest status in a relatively short period of time. The Rhodesian sugarcane industry was not affected by any serious insect pest before the advent of H. licas During the years when rapid expansion of cane production took place ( , it was customary on virgin soils to apply dieldrin or aldrin in the planting furrow for termite control,'but this practice was discontinued with the progress of time. n recent years there have been no insecticides applied to the cane fields, and the sudden increase in H. licas has been unchecked by residual chemicals. t has thus been necessary to initiate control measures from scratch, with economic considerations largely dictating the extent to which they are adopted. Growers have been strongly advised to treat all newly planted fields, and in the case of established fields to give priority to areas of poor drainage, to heavier soil types, and to young ratoons. Chemical methods cannot be expected to eradicate an endemic pest, and the research projects which have now been initiated are aimed at reducing populations in,order to minimize damage. n time, results of these trials will serve to reinforce or modify the basic control measures already advised, all of which are soundly based on experience gained elsewhere on the control of insects with similar feeding habits. REFERENCES 1. Avasthy, P. N. (1965). The problem of white grubs of sugarcane in ndia SSCT Proc. 12: Bates, J. F. (1965). Pest Control in sugarcane in the Americas SSCT Proc. 12: Carnegie, A. J. M. Dic, J., and Harris R. H. G. (1974). nsects and nematodes of South African sugarcane. Entomology Mem. No. 39, Dept. Agric. Techn. Serv., Rep. 5. Afy., p Hardcastle, J. Y. (1963). Outbreaks and new records, Nigeria FA0 P. Prot. Bull. 11 (5): Le Pelley, R. H. (1959). Agricultural insects of East Africa. E. Afr. High Comm. Nairobi, Kenya. 0,,, * 6. Moller, R. B., Mungomery, R. W. (1962).18Some1 insects pests adversely influencing ratooning in South Queensland. SSGT Proc.' 1 :

14 ENTOMOLOGY 7 Pemberton, C.F. (1967). nsect pests affecting sugarcane plantations within the Pacific. SSCT Proc. 11 : Sweeney C. (1967). The Scarabaeoidea associated with sugarcane in Swaziland. An account of premilinary investigations into the bionomics and control. Res. Bull No. 16, Swaziland Min. of Agric., pp Symes, G. B. (1925). The Black Maize Beetle (Heteronychus licas Klug). Observations on life history and control. Rhod. agric. J. 22(1): 83-98, (2): \ 10. Williams, J. R., Metcalfe, J. R. Mungomery, R. W., and R. Methes, (1969). Pests of sugarcane. Elsevier, Amsterdam. 11. Wilson, G. (1959). Screening soil insecticides against white grubs. SSCT Proc. 10: NFORME SOBRE EL DAN0 QUE OCASONA EL ESCARABAJO* DNASTDO, HETERONYCHUS LCAS (KLUG.) EN CANA DE AZUCAR K. E. Cackett RESUMEN Heteronychus licas siempre habia sido considerada una plaga menor sin importancia economics para la industria azucarera de Rhodesia, hasta que un inesperado aumento de poblaci6n del insecto result6 en un daiio significative que coincidi6 con un aumento de la precipitacion entre 10s &os de 1974 y Los adultos se alimentan bajo tierra de va'stagos y tallos de todas las edades durante 10s rneses humedos del L verano, m~entras que las larvas lo hacen de raicillas y yemas subterrineas a fin'es del verano y durante el invierno. Las pdrdidas estan en acorde con las "fallas" en siembras y mortalidad de cepas habiendo la necesidad de "voltear" el campo y resembrarlo cuando el dafio es muy severo. El trabajo trata tambien sobre el ciclo de vida del insecto, y se ofrecen resultados preliminares de investigaciones que se han iniciado sobre el estudio biologic0 ye1 control de la plaga. Se aconseja el uso de Dieldrin por el momento hasta que se tengan 10s resultddos de pruebas comparativas con otros insecticidas y sus efectos residuales en condiciones locales. *Mayate, Chicharon, Gallego, etc.