Lethal effect of Goniozus legneri on Cactoblastis cactorum: A potential biocontrol agent for inundative releases.

Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae), the cactus moth, is native to South America with a widespread distribution in Argentina. The larvae consume the interior of Opuntia spp. (Cactaceae) plants. The moth was used as a biocontrol agent against invasive non-native Opuntia spp. in many countries around the world. The cactus moth arrived unintentionally in Florida, USA, expanded its range and threatened Opuntia-based agriculture and natural ecosystems in southern North America. The insect is also a pest of cultivated O. ficus-indica L. in Argentina. An endemic South American parasitoid, Goniozus legneri Gordth (Hymenoptera: Bethylidae), is used in inundative biological control programmes against lepidopteran pests. The goal of this work was to evaluate G. legneri as a biocontrol agent to be used in inundative releases against C. cactorum. Mortality of C. cactorum by G. legneri was assessed at different spatial scales, as well as the interactions with Apanteles opuntiarum Martínez & Berta (Hymenoptera: Braconidae), a common Argentine natural enemy of C. cactorum. The ability of G. legneri to paralyse, parasitise and kill C. cactorum was confirmed. The paralysis inflicted on C. cactorum larvae reduced larval damage to the plants by 85%. Using two parasitoid species increased the mortality of C. cactorum larvae, but it was highly dependent on the order of their arrival. The combined mortality caused by both parasitoids was higher than a single one, in particular when G. legneri arrived first (56 ± 1%), suggesting asymmetric competition due to the preference of G. legneri attacking previously parasitised larvae. Goniozus legneri has potential as an inundative biocontrol agent of C. cactorum, but its interaction with the classical biocontrol agent A. opuntiarum needs to be considered.

Toxicological and biochemical response to azinphos-methyl in Cydia pomonella L. (Lepidoptera: Tortricidae) among orchards from the Argentinian Patagonia.

BACKGROUND: Azinphos-methyl is the main insecticide used to control codling moth on apple and pears in Northern Patagonia. The aim of this study was to evaluate the toxicological and biochemical response of diapausing larvae of codling moth in orchards subjected to different insecticide selection pressure. RESULTS: Dose-mortality assays with azinphos-methyl in diapausing larvae of Cydia pomonella L. showed significant differences between the LD(95) from a population collected in one untreated orchard (2.52 microg moth(-1)) compared with that in a laboratory-susceptible population (0.33 microg moth(-1)). Toxicity to azinphos-methyl in field populations of diapausing larvae collected during 2003-2005 was evaluated by topical application of a discriminating dose (2.5 microg moth(-1)) that was obtained from larvae collected in the untreated orchard (field reference strain). Significantly lower mortality (37.71-84.21%) was observed in three out of eight field populations compared with that in the field reference strain. Most of the field populations showed higher esterase activity than that determined in both the laboratory susceptible and the field reference strains. Moreover, there was a high association between esterase activity and mortality (R(2)=0.64) among the field populations. On the other hand, a poor correlation was observed between glutathione S-transferase activity and mortality (R(2)=0.33) among larvae collected from different orchards. CONCLUSIONS: All the field populations evaluated exhibited some degree of azinphos-methyl tolerance in relation to the laboratory susceptible strain. Biochemical results demonstrated that esterases are at least one of the principal mechanisms involved in tolerance to this insecticide.

Effects of chlorpyrifos on enzymatic systems of Cydia pomonella (Lepidoptera: Tortricidae) adults.

The control program of codling moth (Cydia pomonella L.) in the Río Negro and Neuquén Valley is intended to neonate larvae. However, adults may be subjected to sublethal pesticide concentrations generating stress which might enhance both mutation rates and activity of the detoxification system. This study assessed the exposure effects of chlorpyrifos on target enzyme and, both detoxifying and antioxidant systems of surviving adults from both a laboratory susceptible strain (LSS) and a field population (FP). The results showed that the FP was as susceptible to chlorpyrifos as the LSS and, both exhibited a similar chlorpyrifos-inhibitory concentration 50 (IC50 ) of acetylcholinesterase (AChE). The FP displayed higher carboxylesterase (CarE) and 7-ethoxycoumarine O-deethylase (ECOD) activities than LSS. Both LSS and FP showed an increase on CarE activity after the exposure to low-chlorpyrifos concentrations, followed by enzyme inhibition at higher concentrations. There were no significant differences neither in the activities of glutathione S-transferases (GST), catalase (CAT) and superoxide dismutase (SOD) nor in the reduced glutathione (GSH) content between LSS and FP. Moreover, these enzymes were unaffected by chlorpyrifos. In conclusion, control adults from the FP exhibited higher CarE and ECOD activities than control adults from the LSS. AChE and CarE activities were the most affected by chlorpyrifos. Control strategies used for C. pomonella, such as rotations of insecticides with different modes of action, will probably delay the evolution of insecticide resistance in FPs from the study area.

Geographic variability in response to azinphos-methyl in field-collected populations of Cydia pomonella (Lepidoptera: Tortricidae) from Argentina.

BACKGROUND: Resistance to insecticides has been related to application history, genetic factors of the pest and the dynamic within the treated area. The aim of this study was to assess the geographic variation in azinphos-methyl response and the role of esterase and cytochrome P450 monooxygenase enzymes in codling moth populations collected within different areas of the Río Negro and Neuquén Valley, Argentina. RESULTS: Diapausing field-collected populations showed resistance ratios at the LC(50) that were 0.7-8.7 times higher than that of the susceptible strain. Mean esterase (EST) and cytochrome P450 monooxygenase activities (expressed as α-N min(-1) mg(-1) prot(-1) and pg 7-OHC insect(-1) min(-1) respectively) were significantly correlated with LD(50) values from the field-collected populations. In addition, azinphos-methyl response was associated with the geographic area where the insect population was collected: populations from isolated and more recent productive areas presented significantly lower resistance ratios in comparison with populations from older and more intensive productive areas. CONCLUSION: The populations assayed presented different resistance levels to azinphos-methyl. The response was highly correlated with the orchard's geographic location. EST and ECOD activities were involved in azinphos-methyl response in the given region.

Codling moth males do not discriminate between pheromone and a pheromone/antagonist blend during upwind flight.

Discrimination between conspecific and heterospecific signals is a key element in the evolution of pheromone-mediated communication in insects. Pheromone antagonists prevent heterospecific attraction. They are typically pheromone synergists in other species and enable specific communication in closely related species, using partly the same chemicals. In codling moth, Cydia pomonella, as in other moths, upwind flights to a pheromone/antagonist blend were slower and more convoluted than to pheromone. However, this deteriorated flight behaviour did not account for the strong decrease in male attraction. The pheromone/antagonist blend blocked instead the onset of upwind flight. This was corroborated by placing a separate source of pheromone close by, which restored male attraction. Males flying upwind did not discriminate between pheromone and the adjacent pheromone/antagonist blend, and landed on either source. This indicates differences in the neural regulation for initiating and maintaining a behavioural response to pheromone.