© 2008 Plant Management Network.
Adult Corn Earworm (Helicoverpa zea) Susceptibility to Methomyl
D. Ames Herbert, Jr., and Sean Malone, Tidewater Agricultural Research and Extension Center, Virginia Tech, Suffolk 23437; Thomas P. Kuhar, Eastern Shore Agricultural Research and Extension Center, Virginia Tech, Painter 23420; Hector E. Portillo and Joe P. Saienni, DuPont Stine Haskell Research Center, 1090 Elkton Rd, Newark DE 19714; and Robert W. Williams, DuPont Crop Protection, 13226 Ashford Park Drive, Raleigh, NC 27613
Herbert, D. A., Jr., Malone, S., Kuhar, T. P., Portillo, H. E., Saienni, J. P., and Williams, R. W. 2008. Adult corn earworm (Helicoverpa zea) susceptibility to methomyl. Online. Plant Health Progress doi:10.1094/PHP-2008-0312-01-RS.
Adult vial testing procedures were used to determine susceptibility of male corn earworm (Helicoverpa zea) to methomyl. Rate response bioassays using 0.39, 0.78, 1.56, 3.12, 6.25, 12.5, and 25 µg ai/vial were conducted using a laboratory colony. Results showed 10, 30, 70, 80, 80, 90, and 100% mortality at these rates. Field-collected moths from pheromone traps in Virginia were assayed at 0.25, 0.5, 1, 2, 5, and 10 times the 25 µg ai/vial rate (100% laboratory colony mortality rate). After 24 h exposure, 3.7 and 2.6% of moths survived the 12.5 and 25 µg ai/vial rates, with 100% mortality at all other rates. These results indicate that future assays should include lower rates in order to establish a baseline rate range for detecting susceptibility or resistance.
Corn earworm, Helicoverpa zea (Boddie), feeds on multiple host crops and the adult stage (moth) is capable of traveling long distances (9). Due to its potential for developing insecticide resistance (3,4,7,10), it is important to understand the level of susceptibility of H. zea to different insecticide chemistries. An increase in survivorship could cause a shift in regional integrated pest management (IPM) and resistance management strategies. In most field crops, proper application timing of a product with ovicidal and larval activity may reduce the number of treatments, since egg and early larval stages are when H. zea are most exposed. Field scouting and monitoring networks that use pheromone and blacklight traps assist with this timing. Switching, rotating, or avoiding insecticide classes by crop and/or time of year may improve control and slow resistance development. Synthetic pyrethroids, which are sodium channel modulators, have been a standard for H. zea control in field crops such as corn, cotton, and soybean, but reduced efficacy in this class has been reported since 2000 (7). Efforts to evaluate resistance development have also focused on the synthetic pyrethroid cypermethrin in the standard adult vial test (AVT) procedure (3,4,10). Adult vial tests have paralleled the field crop trend in decreased H. zea susceptibility to synthetic pyrethroids in the Midwestern and Southern United States (7) and to a lesser extent in the Northeast (5). This study assessed the susceptibility of male H. zea moths to methomyl (Lannate, E.I. DuPont de Nemours and Company, Wilmington, DE), a carbamate acetylcholine esterase inhibitor, in both laboratory and field experiments.
Methomyl Rate Response Bioassays
The standard AVT procedures of Plapp et al. (11) were used to assess the susceptibility of male H. zea to methomyl. To prepare 50 treated vials at each rate, 102 mg of technical grade (98% ai) methomyl were first dissolved in 2 ml of 100% reagent grade acetone, then another 198 ml of acetone was added for a total of 200 ml of 500 ppm ai methomyl solution. Serial dilutions from the stock solution were done with acetone at rates listed in Table 1 to obtain nine lesser concentrations. The interior surface of 20 ml borosilicate glass scintillation vials was treated with 0.5 ml of the appropriate methomyl solution to obtain 0.39, 0.78, 1.56, 3.12, 6.25, 12.5, 25, 50, 125, and 250 µg ai/vial. The uncapped vial was immediately placed on its side on an electric roller with no heating element (Star Manufacturing Company, St. Louis, MO) and rolled to provide an even coat until the acetone flashed off (about 15 min). Acetone-only vials were also prepared as above and used as controls. Vials were capped, labeled, and kept refrigerated at 5 to 7°C until used. Vials older than 2 weeks were not used in the AVT.
Table 1. Serial dilutions of a 500-ppm ai methomyl stock
* The stock solution was prepared by first dissolving 102 mg
A rate response bioassay was performed to determine the rates to be tested on field-collected moths. One-day-old male moths from a susceptible H. zea laboratory strain were used (Chesapeake Perl, Savage, MD). Moths were placed individually into vials with 0, 0.39, 0.78, 1.56, 3.12, 6.25, 12.5, and 25 µg ai of methomyl/vial, plus untreated (with and without acetone) vials, with ten replicates. Mortality was recorded at 1, 3, and 24 h. Results indicated that methomyl rates of 0.39 to 12.5 µg ai/vial did not have 100% mortality (Fig. 1), so 25 µg ai/vial, which provided 100% mortality at 1, 3, and 24 h after exposure, was selected as the diagnostic dose that controlled all susceptible individuals in the laboratory strain (Table 2). Prior research indicated that 2.5 µg ai/vial was sufficient to kill all susceptible H. zea (8).
Table 2. Mean percent mortality of male H. zea moths from a
* Ten vials per treatment and one moth per vial were used.
Moth Collections and Vial Assays in Virginia in 2007
For the field experiment, we used 250 µg ai/vial (ten times the laboratory diagnostic rate) as the highest rate for the field collected bioassays. Rates chosen to be tested in the field experiment began with 50, 125, and 250 µg ai/vial, plus an acetone control. Due to high mortality at these methomyl rates, 6.25, 12.5, and 25 µg ai/vial were used in assays conducted in mid and late August. Male H. zea moths were collected during July and August 2007 using wire cone Hartstack traps (6) baited with H. zea pheromone lures (Product 100337, Hercon Luretape, 1.25 mg ai/unit, Hercon Environmental, Emigsville, PA) in Accomack Co., Southampton Co., and the city of Suffolk, VA. Moths were held in the laboratory with a sugar water source for 24 h in 8.5-cm diameter × 18-cm high cardboard tubes fitted with bridal veil cloth tops. Moths that had a healthy appearance (wing scales intact) were placed individually into vials and caps were loosely fit. Mortality was recorded after 24 h for tests conducted 31 July-7 August; for 14-21 August, mortality was noted at both 3 and 24 h. A total of 745 moths were tested.
Of the 547 moths exposed to methomyl in the field experiment, none survived the rates of 250 µg ai/vial (n = 99), 125 µg ai/vial (n = 99), or 50 µg ai/vial (n = 149) (Fig. 1). However, one moth survived 25 µg ai/vial (n = 50) and 3 survived 12.5 µg ai/vial (n = 100) after 24 h of exposure. None survived the rate of 6.25 µg ai/vial (n = 50) (Table 3). When mortality was recorded at both 3 and 24 h (14-21 August), only 2 (of 250 tested) additional moths were dead at 24 h. The mode of action of methomyl is by binding to the enzyme acetylcholine esterase, impeding the breakdown of the neurotransmitter acetylcholine, thus disrupting normal nervous system function very rapidly. Two other factors may explain the quick knockdown observed with methomyl: fast cuticular penetration of the highly soluble active ingredient and possible penetration of methomyl vapor via the moth’s spiracles. This suggests that 3 h may be an appropriate time for mortality assessment at the rates tested. Additionally, 3 h readings would reduce mortality in the control — an important consideration if Abbott’s formula (1) is used to adjust for control mortality. In this experiment, at least ten control moths died between 3 and 24 h (Table 3).
Table 3. Susceptibility of pheromone-trapped adult male H. zea to methomyl.
× Date collected.
y Adjusted for control mortality using Abbott’s formula.
z Vials were not checked at 24 h as all moths were dead at 3 h.
The moths that survived the vial test were captured in the city of Suffolk, but their origin cannot be determined since pheromone trapping indiscriminately catches both local and immigrant moths, either of which could carry the genetics for resistance. To pinpoint the origin of the moths would require collecting and rearing larvae from each location tested. For example, in previous work, Accomack Co., VA had nearly twice as many cypermethrin-resistant H. zea moths reared from field-collected larvae when compared to those collected from pheromone traps (5). Hutchison et al. (7) suggest that such increases may result from collecting larvae from pyrethroid-treated fields, where selection pressure has already eliminated part of the susceptible population. It is also plausible that the higher mortality (i.e., apparent lower susceptibility) of field-collected moths is affected by stress factors and age that may reduce their robustness as compared to those reared from field-collected larvae and maintained under laboratory conditions. For example, in our laboratory bioassay H. zea moth mortality at 24 h with rates of 6.25 and 12.5 µg ai/vial was 80 and 90%, respectively, whereas the mean corrected mortality of field-collected moths at those same rates was 100 and 96.3%, respectively (Tables 2 and 3). In contrast, Kanga et al. (8) found laboratory-strain H. zea moths more susceptible to methomyl than field-collected moths (LC50 = 0.51 and 0.30 µg ai/vial, respectively).
Summary and Suggestions for Future Investigations
In summary, pheromone-trapped adult male H. zea, regardless of collection location, were highly susceptible to the methomyl concentrations tested (6.25 to 250 µg ai/vial). Exposure to 12.5 and 25 µg ai/vial resulted in 96.3 and 97.4% corrected mortality at 24 h after exposure; all remaining dosages resulted in 100% mortality at 24 h. The laboratory strain had survivorship up to and including the rate of 12.5 µg ai/vial at 24 h, but had 100% mortality at 25 µg ai/vial after 1 h. Future assays should include rates < 6.25 µg ai/vial in order to establish a baseline rate range for detecting susceptible and resistant moths, as has been done for cypermethrin (10). The surface area of the vials used is about 40.2 cm², thus 6.25 to 250 µg ai of methomyl per vial rates translates to a field rate of 15.5 to 621.9 g ai/ha. As a reference, a low- to medium-range label rate of Lannate LV for H. zea control in soybean, cotton, and sweet corn is 0.88 liter/ha (253 g ai/ha; ¾ pt/acre). This is two times the rate of the 50 µg ai/vial rate (124 g ai/ha) used in this study that resulted in 100% mortality. As another reference, the field exposure rate of 253 g ai/ha applied in 30 gpa water is equivalent to 450 µg ai/vial. The high mortality observed at rates as low as 6.25 µg ai/vial indicate that there may be a very low frequency of resistance alleles to methomyl in the H. zea populations tested from Virginia. The close proximity in diagnostic rates for pyrethroids (5 to 10 µg ai/vial) and methomyl (6.25 to 25 µg ai/vial) as observed in the populations tested in this study, indicate that methomyl may be an effective tool for resistance management because of its known mortality to H. zea adults and some ovicidal activity (2). However, due to short residual activity in the field (< 2 days), other insecticides will continue to be needed for effective larval control. In order to fully assess insecticide resistance, adult females should also be assayed. Testing adults reared from field-collected larvae could also be important in determining resistance levels in local populations.
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