Development of a rapid resistance monitoring bioassay for codling moth larvae.

BACKGROUND: The codling moth, Cydia pomonella (L.), is one of the most important pests of apple worldwide. Use of insecticides for management of this insect has been extensive and has resulted in resistance development. There are a number of different bioassay methods to monitor for codling moth resistance; however, many are not applicable to new insecticides and most are time consuming. A novel 16-well plasticware bioassay plate containing lyophilized diet was developed for rapid resistance monitoring of codling moth. RESULTS: The contact insecticides acetamiprid and azinphosmethyl were significantly more toxic to neonates than to fourth instars. However, there was no significant difference in LC(50) values between neonates and fourth instars to the ingestion insecticides chlorantraniliprole, methoxyfenozide, novaluron and spinetoram. Field colonies of codling moth were significantly more resistant to methoxyfenozide than susceptible populations. A diagnostic dose of 20 µg mL(-1) (LC(99) ) was established to monitor for codling moth resistance to methoxyfenozide. CONCLUSIONS: The results presented here demonstrate that a novel and rapid bioassay can be used to monitor for codling moth resistance to methoxyfenozide. The bioassay method is relevant to both ingestion and contact insecticides, but a single diagnostic dose, regardless of larval age, is only relevant to ingestion insecticides. Age-dependent diagnostic doses are likely necessary for contact insecticides.

Life stage toxicity and residual activity of insecticides to codling moth and oriental fruit moth (Lepidoptera: Tortricidae).

The codling moth, Cydia pomonella (L.), and oriental fruit moth, Grapholita molesta (Busck), are two key pests of apple (Malus domestica Borkh.) in North Carolina. Growers extensively relied on organophosphate insecticides, primarily azinphosmethyl, for > 40 yr to manage these pests. Because of organophosphate resistance development and regulatory actions, growers are transitioning to management programs that use new, reduced-risk, and OP-replacement insecticides. This study evaluated the toxicity of a diversity of replacement insecticides to eggs, larvae, and adults, as well as an assessment of their residual activity, to codling moth and oriental fruit moth. Laboratory-susceptible strains of both species were used for all bioassays. Fresh field-harvested apples were used as a media for assessing the ovicidal activity of insecticides. For larval studies, insecticides were topically applied to the surface of lima bean-based diet, onto which neonates were placed. Toxicity was based on two measures of mortality; 5-d mortality and development to adult stage. Ovicidal bioassays showed that oriental fruit moth eggs were generally more tolerant than codling moth eggs to insecticides, with novaluron, acetamiprid, and azinphoshmethyl having the highest levels of toxicity to eggs of both species. In contrast, codling moth larvae generally were more tolerant than oriental fruit moth to most insecticides. Methoxyfenozide and pyriproxyfen were the only insecticides with lower LC50 values against codling moth than oriental fruit moth neonates. Moreover, a number of insecticides, particularly the IGRs methoxyfenozide and novaluron, the anthranilic diamide chlorantriliprole, and the spinosyn spinetoram, provided equal or longer residual activity against codling moth compared with azinphosmethyl in field studies. Results are discussed in relation to their use in devising field use patterns of insecticides and for insecticide resistance monitoring programs.

On-plant survival and inheritance of resistance to Cry1Ab toxin from Bacillus thuringiensis in a field-derived strain of European corn borer, Ostrinia nubilalis.

BACKGROUND: The high dose plus refuge is one of the major components of the resistance management plan mandated for transgenic corn expressing Cry toxins from Bacillus thuringiensis Berliner (Bt) that targets the European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae). This strategy was based on assumptions such as functional recessive inheritance, which has not been previously tested for O. nubilalis. The authors used a field-derived resistant strain of O. nubilalis to define the nature of resistance to Cry1Ab toxin by examining the inheritance and on-plant survival of susceptible and resistant insects and their F(1) progeny. RESULTS: The resistant strain exhibited > 800-fold resistance to Cry1Ab. Resistance was primarily autosomal and controlled by more than one locus or multiple alleles at one locus. The degree of dominance D calculated on the basis of LC(50) values was - 0.45(h' = 0.27), indicating that resistance was incompletely recessive. No survivors were found on vegetative-stage Bt corn, although both resistant larvae and their F(1) progeny were able to survive on reproductive corn 15 days after infestation. CONCLUSIONS: A field derived O. nubilalis strain exhibited high levels of resistance to Cry1Ab and survived on transgenic corn by feeding on tissues with low Cry1Ab expression. The Cry1Ab resistance was primarily autosomal, incompletely recessive and polygenic. Tissue and on-plant survival data indicated that dominance varies depending on plant stage.

Efficacy of neonicotinoid seed treatments to reduce soybean aphid populations under field and controlled conditions in Nebraska.

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is native to Asia and was recently (2000) detected in North America. Since then, it has become a significant threat to U.S. soybean production. Although neonicotinoid insecticide seed treatments, imidacloprid and thiamethoxam, have been suggested as a method of control, the season-long efficacy is still uncertain. Therefore, the use of imidacloprid and thiamethoxam seed treatments to control soybean aphid in Nebraska were examined. Soybean aphid populations were monitored weekly in an irrigated field study planted during the later half of the typical Nebraska planting window during 2005 and 2006. Imidacloprid and thiamethoxam were quantified by leaves through time, and leaf specific bioassays were conducted. In 2005, aphid populations were very low; however, in 2006 aphid numbers were significantly higher than 2005, reaching approximately 1,200 aphids per plant in the untreated plots. Aphid injury significantly reduced yield and individual seed size in 2006. Imidacloprid significantly reduced aphid densities in 2006 but not below the economic threshold. In 2006, thiamethoxam held aphid densities below the economic threshold. Leaf specific bioassays and leaf specific imidacloprid and thiamethoxam quantification indicated that thiamethoxam was present in the plant at higher concentrations and for a longer period.

Desenvolvimento e Reprodução do Predador Podisus distinctus (Stal) (Heteroptera: Pentatomidae) Frente a Doses Subletais de Permetrina / Development and Reproduction of the Predator Podisus distinctus (Stal) (Heteroptera: Pentatomidae) Exposed to Sublethal Doses of Permethrin

Development and reproduction were studied in the predator Podisus distinctus (Stal) subjected to sublethal doses of the insecticide permethrin under laboratory conditions. Third instar nymphs were subjected to topical application of permethrin at the doses of 0, 0.0172, 0.172, 1.72, 17.2 e 172 ppb. The only parameter statisticaly different was egg viability. This parameter had the gratest increase at the dose 0.172 ppb provinding evidence for the phenomenon of hormesis.

Desenvolvimento e reprodução foram estudados no predador Podisus distinctus (Stal) submetido a doses subletais do inseticida permetrina em condições de laboratório. Ninfas de 3° instar foram submetidas a aplicação tópica de permetrina nas doses de 0; 0,0172; 0,172; 1,72; 17,2 e 172 ppb. O único parâmetro significativamente afetado foi a viabilidade de ovos. Esse parâmetro teve um máximo de estímulo na dose 0,172 ppb provendo evidência do fenômeno de hormese.