Corn Rootworm Larvae Show Insecticide Resistance

Bob Wright, Extension Entomologist, SCREC, Clay Center;
Lance Meinke and Blair Siegfried, Assoc. Professors of Entomology, UNL.

During the last few years, reports of insecticide control failures for adult western corn rootworms have increased in parts of Nebraska where beetle spray programs have been used for many years. Recently, we compared susceptibility of adult rootworms collected from throughout Nebraska to an organophosphate (methyl parathion, Penncap M®), a carbamate (carbaryl, Sevin®), and a pyrethroid (bifenthrin, Capture®) insecticide to determine if resistance might be evolving in areas where control failures were reported.

Beetles collected from areas where adult control problems were reported were generally 10-15 times more tolerant to methyl parathion, 5-10 times more tolerant to carbaryl, and 2-3 times more tolerant to bifenthrin than in other areas of the state. This is based on differences in LD₅₀ values (i.e., the amount of insecticide required to kill 50% of the population). These results in combination with reports of control failures suggest strongly that resistance has developed to both organophosphate and carbamate insecticides as a result of intensive selection on adult rootworms. Because of the importance of soil insecticides throughout the cornbelt and the potential for adult resistance to impact efficacy of soil insecticides, susceptibility of rootworm larvae reared from resistant and susceptible parents was compared to the active ingredients of four soil insecticides; tefluthrin (Force®), chlorpyrifos (Lorsban®), terbufos (Counter®), and carbofuran (Furadan®) and one adult insecticide; methyl parathion (Penncap M®).

Corn Rootworm Larva

Western corn rootworm beetles were collected from Clay County in August 1994 and determined to be susceptible to all of the insecticides tested. Beetles collected from York County in August 1995 were identified as being resistant to methyl parathion and carbaryl. Collections from both sites were obtained and laboratory colonies were established at the USDA Northern Grain Insects Research Lab at Brookings, S.D. The larval offspring of the two colonies were used in bioassays of the five insecticides listed above. Larvae were allowed to develop for approximately 10 days after hatching (third instars) before being sent to the University of Nebraska for bioassays. Technical grade insecticide was dissolved in acetone and different concentrations of each insecticide were applied to individual rootworm larvae. Control larvae were treated with acetone only. Each insecticide concentration was tested against 10 insects per replication with three replications per insecticide. Mortality was recorded 24 hours after treatment. Each compound was tested individually on the resistant and susceptible strains, and therefore, direct comparisons of resistance levels among the compounds should be avoided.

Results from bioassays of larvae obtained from the resistant (York County) and susceptible (Clay County) western corn rootworm colonies indicated consistently higher LD₅₀ values (at least 2.5 greater) in the York County colony for all compounds tested. Methyl parathion was the only compound that was tested on both field-collected adults and larval offspring, and results of these bioassays indicate that the larvae were perhaps even more resistant than the adults (9 times greater for adults and 15 times greater for larvae).

Since methyl parathion is not used as a soil insecticide, it seems likely that the resistance seen in larvae is a result of selection pressure on the adult rootworms. There were only small differences in LD₅₀ s between resistant and susceptible populations when the organophosphate insecticides terbufos and chlorpyrifos were tested (2.5-3.5 times). This suggests that there is not a general response of the methyl parathion resistant rootworm population to all organophosphate insecticides, and one cannot assume that if an adult control failure occurs with methyl parathion, all other aerial or soil applied organophosphate compounds also will fail.

LD₅₀ s of the resistant population for compounds other than organophosphates were approximately five times higher for tefluthrin (a pyrethroid) and 16 times for carbofuran (a carbamate). It is not clear, however whether the differences in susceptibility are the result of selection with insecticides used in adult management programs (i.e., methyl parathion) or if these differences resulted independently from selection by soil insecticides. These results may indicate that the mechanism conferring resistance is relatively non-specific and results in cross resistance to a variety of insecticide classes.

It should be stressed that results from this investigation are indicative of larval susceptibility under standard laboratory conditions and for only a single stage of development and are not indicative of product performance under field conditions. Furthermore, results obtained thus far represent only a single resistant collection site and may not be indicative of all the resistant rootworm populations that have been identified. The methods used in these bioassays provide preliminary data on larval susceptibility to soil insecticides and indicate that differences in susceptibility do exist between populations. These results suggest that slight decreases in larval susceptibility potentially could occur in areas where adult control problems have been detected. However, numerous other factors, in addition to insect susceptibility, can influence efficacy of soil insecticides including application timing, weather, calibration, rootworm population pressure, and microbial degradation.

Additional experiments will be conducted to evaluate whether resistance detected in adult rootworms affects the efficacy of soil insecticides. A number of soil insecticide trials were conducted during 1996 in areas of Nebraska where adult resistance has been detected. However, rootworm population pressure was too low to obtain meaningful results. Experiments to test the efficacy of soil insecticides under field conditions and development of bioassays that more directly reflect field conditions will continue in 1997 in order to confirm the impact of adult resistance on larval control strategies.