Evolution of Resistance by Helicoverpa zea (Lepidoptera: Noctuidae) Infesting Insecticidal Crops in the Southern United States.

We created a deterministic, frequency-based model of the evolution of resistance by corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), to insecticidal traits expressed in crops planted in the heterogeneous landscapes of the southern United States. The model accounts for four generations of selection by insecticidal traits each year. We used the model results to investigate the influence of three factors on insect resistance management (IRM): 1) how does adding a third insecticidal trait to both corn and cotton affect durability of the products, 2) how does unstructured corn refuge influence IRM, and 3) how do block refuges (50% compliance) and blended refuges compare with regard to IRM? When Bt cotton expresses the same number of insecticidal traits, Bt corn with three insecticidal traits provides longer durability than Bt corn with two pyramided traits. Blended refuge provides similar durability for corn products compared with the same level of required block refuge when the rate of refuge compliance by farmers is 50%. Results for Mississippi and Texas are similar, but durabilities for corn traits are surprisingly lower in Georgia, where unstructured corn refuge is the highest of the three states, but refuge for Bt cotton is the lowest of the three states. Thus, unstructured corn refuge can be valuable for IRM but its influence is determined by selection for resistance by Bt cotton.

Oviposition preferences, Bt susceptibilities, and tissue feeding of fall armyworm (Lepidoptera: Noctuidae) host strains.

BACKGROUND: The fall armyworm, Spodoptera frugiperda (Smith), is a pest of many economically essential crops across several continents. Documentation of resistance to Bt toxins has caused growing concern in agricultural communities regarding the ability to keep fall armyworm populations below economic thresholds. The existence of two host strains referred to as the 'rice' and 'corn' strains is a complicating and under-researched factor of fall armyworm biology and management. It is essential to characterize the differences between the host strains, as well as their rice/corn hybrid offspring, to elucidate their contributions to field-evolved resistance. RESULTS: Corn was a preferred oviposition host for both rice and corn strain fall armyworm, and a suitable larval host plant for each of the four populations tested. Corn strain females displayed a significant preference towards oviposition on plants that lacked mechanical damage. The rice strain population was generally less tolerant to Cry1F corn tissue than the corn strain and hybrid populations, which performed in a similar way to one another. CONCLUSION: The preference for corn as an ovipositional host may have an impact on resistance management when coupled with differential host strain Bt tolerances, though more studies are needed. Hybrid tolerance to Bt toxins could possibly contribute to the evolution of Bt resistance. This is the first study to compare the larval fitness and survival of rice/corn hybrid fall armyworm to that of pure host strains using a tissue-based approach.

Cry1 Bt Susceptibilities of Fall Armyworm (Lepidoptera: Noctuidae) Host Strains.

The fall armyworm, Spodoptera frugiperda (Smith; Lepidoptera: Noctuidae), is a highly polyphagous, multivoltine pest of commercial crops including corn (Zea mays L.), cotton (Gossypium spp. L.), rice (Oryza sativa L.), and pasture grasses. Fall armyworm has become a growing concern in agricultural communities across the Americas as field populations in many locales have evolved resistance to several Cry1 toxins derived from the bacterium Bacillus thuringiensis Berliner (Bt). An often overlooked aspect of fall armyworm biology is the existence of two host strains, the 'rice' and 'corn' strains. There has been little research devoted to the characterization of fall armyworm host strains, although there is evidence that the rice and corn-strains may differ in their tolerances to Bt toxins expressed by transgenic plants. In this study, diet-based bioassays were conducted to compare the susceptibilities of one rice-strain, two corn-strains, and one rice-corn hybrid population to Cry1Ab, Cry1Ac, and Cry1F protein. Results indicate that the corn-strains and hybrid populations are more tolerant to the Bt toxins, especially to Cry1F, than the rice-strain population. Results from this study, when combined with existing techniques for host strain identification, may aid in the development of regional insect resistance management programs for fall armyworm.

Disruption of mating in codling moth (Lepidoptera: Tortricidae) by chlorantranilipole, an anthranilic diamide insecticide.

The influence of the anthranilic diamide insecticide chlorantranilipole (DPX-E2Y45; Rynaxypyr against the adult stage of codling moth, Cydia pomonella L., was examined. Insecticide residues in plastic cups sprayed with 56 mg AI L(-1) (equivalent to the recommended field rate) had a minimal effect on adult survival or fecundity, but significantly fewer female moths were mated in treated than in untreated cups. Similar results were found in field studies with adults placed in screened cages on treated apple foliage for up to 3 weeks. Mating was disrupted when either sex was topically dosed with technical material (0.01-0.1 microg). Males were more sensitive to the disruptive effects of chlorantranilipole than females. A 2 h exposure significantly reduced male activity (walking and wing fanning), but not female calling. A 25 h exposure significantly reduced the activity of both sexes. The proportion of males landing near a sex pheromone source in a flight tunnel was significantly reduced following exposures of 2-4 h and completely eliminated following a 17 h exposure. Male moths partially recovered from either topical or residual exposure to chlorantranilipole, but the extent of their recovery was dose and time dependent. Male response to sex pheromone recovered within 2 h following a 4 h exposure, but following a 17 h exposure only a partial recovery occurred after 48 h. Mating by moth pairs exposed to chlorantranilipole residues for 24 h did not significantly increase during recovery periods of 24-96 h, except in cups treated with 10% of the standard concentration. The proportion of field-collected female codling moths caught in pear ester-baited traps that were mated was significantly lower in apple orchards treated season-long with chlorantranilipole compared with females caught in either untreated orchards or those treated with sex pheromones to cause mating disruption.

Rynaxypyr: a new insecticidal anthranilic diamide that acts as a potent and selective ryanodine receptor activator.

Rynaxypyr is a highly potent and selective activator of insect ryanodine receptors with exceptional activity on a broad range of Lepidoptera. A strong correlation between insecticidal activity and ryanodine receptor activation is observed along with selective activity against insect over mammalian receptors. The synthesis and biological results are presented.

Insecticidal anthranilic diamides: a new class of potent ryanodine receptor activators.

A novel class of anthranilic diamides has been discovered with exceptional insecticidal activity on a range of Lepidoptera. These compounds have been found to exhibit their action by release of intracellular Ca2+ stores mediated by the ryanodine receptor. The discovery, synthesis, structure-activity, and biological results are presented.