FEEDING RATES OF DIFFERENT LARVAL INSTARS OF A SUGARCANE GRUB, LlGYRUS SUBTROPICUS BLATCHLEY (COLEOPTERA: SCARABAEIDAE)' ABSTRACT

Transcription

1 FEEDING RATES OF DIFFERENT LARVAL INSTARS OF A SUGARCANE GRUB, LlGYRUS SUBTROPICUS BLATCHLEY (COLEOPTERA: SCARABAEIDAE)' R. H. Cherry University of Florida Institute of Food and Agricultural Sciences Everglades Research and Education Center P.O. Box 8003 Belle Glade, Florida ABSTRACT feeding rates of the three lilival instars of Ligyrus subtropicus Blatchley were measured under simulated field temperatures. First and second inslnrs consumed raw carrot at an avcrngc of 0.03 g/grub/wk and 0.26 g/grub/wk respectively. Third instars consumed raw carrot at varying rates during the 9 rna that they naturally occur under field conditions. Mean monthly consumption of raw carrot by third insists ronged from l.ol to 1.93 g/grub/wk. Data in this study show that the appearance of L. subtropicus damage in September in Florida sugarcane fields is partially explained by increasing populations of the large and voracious third instars at this time. These data further emphasize the tremendous feeding capacity of each L. sublopicus third instar under field conditions found in Florida sugarcane fields. Lastly, lhese data emphasize the importance of correct timing of nooding for grub control to reduce sugarcane destruction by the voracious third instal's. Key Words: Insecta. sugarcane. grubs, Scarnbaeidae. Coleoptera, Ligyrns subtropicu... J. Agric. Entomol. 8(3): (July 1991) Sugarcane is FlOl'ida's most valuable field crop and is primarily grown in the Everglades area of southern Florida. Since 197], several species of Scarubaeidae have been noted causing significant damage to Florida sugarcane. Of these pests, the white grub Ligyrtls subtropicus Blatchley is the species of primary economic imporlance (Gordon and Anderson 1981). Currently no chemical control is known for these pests and flooding of sugarcane fields is sometimes used for control (Cherry 1984a). Feeding damage by L. subtropicus to the sugarcane plant is mainly through larval feeding on the plant roots and underground stem. Miller and Bell (1983) reported a study in which L. subtropicus adults were placed in buckets containing sugarcane plants. As the larvae developed, for each gram increase in lan'al weight, there was a decrease in plant root weight of ]2.8 g. Sosa (1984) showed L. subtropicus reduced sugar yield by 39% in a commercial sugarcane field in Florida. The effect of L. subtropicus third-instal' feeding on sugarcane root:shoot relationships has been reported by Coale and Cherry (1989a). In their study, root and stool (underground stem) dry matter were reduced linearly by increased grub infestation and root:shoot dry matter ratios decreased as the number of grubs/plant increased. These authors also reported on the impact of third instal' feeding of L. subtropicus I Recei\'ed for publiclltion 16 Mo)' \990; accepted 2 February

2 164 J. Agric. Entomol. Vol. 8, No.3 (1991) on sugarcane nutrition (198gb). This latter study showed that leaf nitrogen, phosphorus, and potassium concentrations decreased linearly as the level of grub infestation increased. The objective of this study was to determine larval feeding rates of the three different larval instal's of L. subtropicus under simulated field temperatures. These data were previously unknown and will provide a better understanding of the seasonal feeding impact of the insect expected under field conditions. MATERIALS AND METHODS A thermograph was used to determine soil temperatures in Florida sugarcane fields at which L. subtropicus larvae would normally be expected to feed. A temperature sensor was placed 10 em deep beneath a sugarcane plant located in a sugarcane field at Belle Glade, Florida. The location of the sensor was chosen since this is the habitat where third instal's, the longest enduring stage of L. subtropicus, are found in highest density (Cherry 1984b) and other life stages of the species are also commonly found. Soil temperatmes were recorded continuously on the thermograph from July 1986 to July The mean monthly soil temperatures I'anged from a low of 18.3 C in January to a high of 28 C in July with the overall mean yearly soil temperature being 24.5 C ± 3.3 S.D. as reported by Cherry and Boucias (1989). Mean monthly soil temperatures from this thermograph from July 1986 to May 1987 were used in this study. July to May are the months during which larvae of L. sublropicus normally are present in sugarcane fields (Cherry 1985) and these monthly temperature data were not reported in the Cherry and Boucias (1989) study. In feeding tests, the same general procedure was used to test the three different instal's. Grubs (larvae) were collected by digging under sugarcane plants in commercial sugarcane fields in southern Florida from July 1987 to May To simulate field conditions, larval instal's were collected and tested during months when each instal' naturally occurred. After collection, grubs were brought to a laboratory and used in a feeding test a few hours later. Only healthy appearing grubs were used in tests since some grubs appeared diseased or were bruised during collection. Feeding experiments were conducted by weighing each grub and then placing the grub into a metal can which was one-half filled with moist sand (10% W;W). Moist sand was used as a substrate, instead of the highly organic muck soil in which these grubs naturally occur, to ensure that no nutritional value could be gained from ingestion of soil. Thereafter, a piece of fresh carrot was weighed and placed into the container. This carrot piece had previously had the softer, center piece (stele) removed since this part of the carrots decomposed rapidly under test conditions. It. would have been preferable to directly measure consumption of sugarcane roots by the larvae. However, this would have been extremely difficult, if not impossible because of the numerous, fragile roots involved. Raw caltot appeared to be a readily acceptable food to the grubs and was easy to use in the feeding experiments. Raw carrots have been used in other scarab feeding studies such as Smith et al. (1975) and King et al. (1981). Smith et al. (1975) also note that until larval consumption of grass roots can be measured directly, data on carrot consumption represent the best estimate of scarabaeid intake available for root feeding species. Cans with carrot pieces, but no grubs were used as controls since carrot pieces alone normally gained weight over the

3 CHERRY: Sugarcane Grub Feeding Rates 165 lest period. This weight gain was used later to more accurately calculate carrot consumption. Cans were then placed in a temperature cabinet and held for 7 d at the mean monthly soil temperature previously determined from the thermograph data (Table 1). At the end of the 7-d period, carrot pieces were removed, rinsed with water to remove sand, towel-dried, and weighed. Sand in the cans was examined microscopically to determine if carrol I>ieces had been shredded. rather than consumed. This shredding occurred infrequently with the large third instars and data from these grubs were discarded. Data were also not recorded if the grub molted or died during the 7-d test. Mortality and/or molting occurred more frequently with the smaller first and second instars and very infrequently with the larger third instars. Two days each month. a group of 25 grubs was collected from the fields and tested along with 3 carrot controls. The mean weight change of the 3 controls was used to adjust carrot weight changes for each group of 25 grubs. The initial weight of each carrot fed to each grub was multiplied by the expected weight change from the mean of the 3 controls. The mean percentage of weight gain of the control carrots in all tests ranged from about 3 to lo%. Table 1. Consumption of raw carrot by third instar Ligyrus subtropicus under simulated field temperatures. Mean Mean Monthly Mean Consumption Month Soil Temp. ("C) Grub Wt. (g)* (gfgrubfwk) August September October November December January February March April WI. -...eight of gmbs collecled directly from the field and before the feeclin~ test. l Specific test conditions were modified for each of the three larval instars to adjust for the different sizes of the instars and the mean monthly soil temperatures at the time the instal' was present in the field. Fifty first instars were collected during July Each of these lalvae was placed into 8 1- by 4-cm diameter can containing 8 O.I-g carrot piece and held at 28 C. Fifty second instars were also collected during July Each of these larvae was placed into a 1.5- by 5-cm diameter can containing a O.5-g carrot piece and also held at 28 C. From late August through April, third instars are the predominant stage of L. sllbtropicus found in Florida sugarcane fields. Fifty third instars were thus collected and tested each month from August 1987 through April Each of these larvae was placed in a 2- by 6-cm diameter can containing a 3.0-g carrot piece. The third instal's collected in any month were then tested at the mean monthly soil temperature for that month (Table I).

4 166 J. Agl'ic. Entomol. Vol. 8, No.3 (1991) Means ± standard errors are presented for weights and carrot consumed of first and second instal's. Linear correlation coefficients were determined for several parameters related to feeding of the third instal' grubs. RESULTS AND DISCUSSION First and second instal's collected during July weighed an average ± SE of 0.14 ± 0.04 g and 0.54 ± 0.08 g,!'espectively. Feeding data were collected on 22 first instal's and 35 second instal's which did not die 01' molt during the 7-d feeding test. First and second instal's consumed raw carrot at an average ± BE of 0.03 ± 0.03 g/grub/wk and 0.26 ± 0.05 g/grub/wk, respectively. During August 1987, about half of the larval population in sugarcane fields had molted into the large third instal's. Third instal's collected during August weighed an average 2.06 g and were now consuming raw carrot at an average 1.01 g/grub/wk (Table 1). \Vilson (1969) reported that sugarcane damage is generally greatest when third inst.ar grubs are abundant since this stage is extremely voracious. More specifically, Sosa (1984) has noted that L. subtropicus grub damage becomes noticeable during early September in Florida sugarcane fields. Data presented here corroborate these studies by showing that the appearance of L. subtropicus damage in September is partially explained by increasing populations of the large and voracious third instal's at this time. From October to April, third instal's of L. subtropicus are the predominant. (> 95%) stage found in Florida sugarcane fields (Cherry 1985). These data further emphasize the importance of correct timing of flooding to kill larvae of L. subtropicus (Cherry 1984a) to avoid the rapid increase in grub feeding which starts to take place during August and will continue for several months thereafter. Cherry (1984a) has shown that grub mortalit.y from flooding in Florida sugarcane is dependent on several variables such as duration of flooding, water temperature, insect stage, and gl'tlb species. Data presented in that study in conjunction with available weather data (Casselman 1970) on seasonal temperatures and rainfall are useful for selecting the most effective methods of flooding for grub control in Florida sugarcane. Feeding data during the 9-mo period from August to May when third instal's are commonly found in Florida sugarcane fields are shown in Table 1. Mean monthly soil temperatures ranged from a low of ls.3 C in January to a high of 27.7 C in August. Field-collected third instal's weighed least in August and September. F!'Om October to May, the third instal's ranged in mean monthly weight f!'om 3.37 to 3.78 g/grub. These field data indicate continuous feeding by the third instal s in Florida sugarcane fields during the entire 9-mo period since field collected grubs had either grown or essentially maintained weight during the period. These field data are corroborated by the laboratory feeding study which also showed third instal' feeding during the entire 9-mo period at simulated field temperatures. Mean monthly consumption of raw caltot ranged from 1.01 to 1.93 g/grub/wk and no seasonal trends in feeding were apparent. Taken together, the preceding field and laboratory daro show that the third insrol's reed during the entire 9-mo period including the cooler winter months. Linear correlation coefficients fol' parameters related to feeding of third instal's of L. subtropu:us are shown in Table 2. Smith et al. (1975) reported that consumption of carrot by third instal's of the grub, Sericesthis nigrolirzeata Boisduval increased between 4 and ao c. Consumption of L. subtropicus was not significantly correlated

5 CHERRY: Sugarcane Grub Feeding Rates 167 Table 2. Linear correlation coefficients for parameters related to feeding of third instars of L. subtropicus. t Consumpt.ion Grub wt. Soil Temp. Time Consumption Grub wt. I -0.51* 0.59* Soil Temp * Time 1 t Coni>umption - rfiw carrot (g)/gmh/...k. Gnlb wi. \~1.(g). Soil temp. melln monthly soil tcinperat.\lre (OCl. Time =months of third illf'otlll" duration. i.e. August = J. September == 2. etc. Significant (P < 0.05) correlutioll. with soil temperature in this study. However, this lack of correlat.ion between consumption and temperat.ure is not surprising since grubs of various ages and weights were used, and a more narrow temperature range (18 to 28 C) was tested in this study than in the Smith et al. (1975) study. The three significant con'elations found in Table 2 are also easily explained. Grub ''''eight increased over time since newly molted grubs are small and these grubs ate and generally continued to grow during the 9-mo period. Soil temperature decreased over time since the soil was warmest in August and generally decreased in the winter~spring period. Grub weight was negatively correlated with soil temperature because grubs gained weight during the 9-mo period in spite of soil temperatures decreasing during this period. As a last note, it is interesting to compare data found in this study to similar study of Smith et al. (1975). Smith et al. measured consumption of raw carrot by thil'(l instar grubs of S. nigrolineata at different temperatures. This small scarabaeid causes pasture damage in southeastern Australia. The mean weight of the third instal's tested ranged from 0.26 to 0.29 g and each grub consumed ca. 0.2 g of raw carrot/wk at 20 C. Consumption also decreased drastically in the cold winter months due to cold soil temperatures. In this study, third instal's of L. suhtropicus were much larger than S. fligrolineata with a mean monthly weight ranging from 2.06 to 3.78 g. As expected, third instal's of L. subtropiclls ate much more than S. fligrolineata with each grub consuming a mean J.20 g of raw carrot/wk at 20 C. Lastly, feeding by L. sublropicus continued during the winter months since the grubs were not limited by cold soil temperatures. The comparison of the Smith et al. (1975) study with data from this study emphasizes the tremendous feeding C81>8ciiy of each L. subtropicus third instal' under the field conditions found in Florida sugarcane fields. AC1,,'lOWLEDGMEKT This work was supportcd by a grant from the 1?lorida Sugar Cane League. Florida Agricultural 8xpcrimcnt Station Journal Scrics No

6 168 J. Agric. Eowmo!. Vol. 8, No.3 (1991) REFERENCES CITED Casselman, T The climate of the Belle Glade area. Florida Agricultural Experiment Station Circular 8-205, Univ. of Florida, Gainesville, 17 pp. Cherry, R. 1984a. Flooding to control the grub Ugyms sllbtropicus (Coleoptera: Scarnbaeidae) in Florida sugarcane. J. Eeon. EntomoL 77: b. Spatial distribution of white grubs (Coleoptera: Scarabaeidae) in Florida sugarcane. J. Eean. Entomol. 77: Seasonal phenology of white grubs (Coleoptera: Scarabaeidae) in Florida sugarcane fields. J. Eeon. EntomoL 78: Cherry, R, and D. Eaucins Incidence of Bacillus popil/ioe in different life stages of Florida sugarcane grubs (Coleoptera: Scarabaeidae). J. Entomol. Sci. 24: Coale, F., and R. Cherry. 1989a. Effect of white grub (Ugyrus subtropicus IBlatchleyJ) infestations on sugarcane root:shoot relationships. J. Am. Soc. Sugar Cane Techno!. 9: b. Impact of white grub (Ligyrus subtropicus IBlatchley]) infestation on sugarcane nutrition. J. Plant Nutr. 12: Gordon, R. t and D. Anderson The species of Scarnbneidne (Coleoptera) associated with sugarcane in south Florida. Fla. Entomo!. 64: King, P., C. Mercer, and J. r.,'leekings Ecology of the black beetle, Heterollyclws arator (Coleoptera: Scarabaeidae) - innuence of temperature on feeding, growth, and sulvival of the larvae. New Zealand Journal of Zoology. 8: Miller, J. and M. Bell Life cycle of the white grub and its effect on sugarcane. J. Am. t Soc. Sugar Cane Technol. 2: 85. Smith, T., M. Porter, and A. Furnival Effects of temperature and developmental stage on feeding by larvae of Sericesthis nigro/if/cola (Coleoptera: Scarabaeidae). Entomol. Exp. Appl. 18: Sosa, D., Jr Effect of white grub (Coleoptera: Scarahaeidae) infestation on sugarcane yields. J. Econ. Entomol. 77: Wilson, G White grubs as pests of sugarcane, PP' Itl J. R. Williams, J. R. Metcalfe, R. W. Mungomery, and R. Mathes leds.l, Pests of sugar cane. Elsevier Publishing Co., New York, 568 pp.