No abstract

Concerns about the failure of insecticides to control the diamondback moth (DBM) Plutella xylostella in the Sydney region of New South Wales, Australia, necessitated the current investigation to establish the susceptibility of four field populations of the DBM to six insecticides. These include two each of organophosphates (OPs), and synthetic pyrethroid insecticides as well as two new products with different modes of action, spinosad and indoxacarb, currently recommended for DBM control in the region. Topical application of the insecticides to the third-instar larvae showed high resistance to pyrethroids (permethrin and esfenvalerate) of 35.0-to 490.0-fold. Resistance to the OPs (methamidophos and chlorpyrifos) and indoxacarb ranged from 12.1-to 36.2-fold and from 11.4-to 34.6-fold, respectively. However, the field populations were susceptible to spinosad (resistance factors only twoto threefold compared with the susceptible strain). A 2 h pre-treatment of the esfenvalerate-resistant strain with the synergists piperonyl butoxide and diethyl maleate increased the toxicity of esfenvalerate by 30.0-and 1.9-fold, respectively, suggesting the involvement of esterases and/or monooxygenases as the key mechanism(s) of insecticide resistance with glutathione S-transferases playing a minor role.

The tropical warehouse moth Ephestia cautella (Walker) is an important storage pest of maize grain. The control of this major moth pest relies heavily on the use of synthetic insecticides. We assessed the biological efficacy of a commercial neem oil (Calneem oil) against E. cautella in the laboratory, using the common grain protectant Actellic as a positive control. Contact toxicity, repellency, effects on oviposition, development and grain damage were assessed. The assays were conducted by treating 100 g of maize grains with Calneem oil concentrations of 3, 5 and 7 ml/l; and Actellic at 2 ml/l. Calneem oil significantly (P, 0.05) reduced oviposition rate, development of eggs and immature stages, and emergence rate of E. cautella progeny in maize grains. Grains treated with Calneem oil had significantly (P, 0.05) reduced grain damage in storage, and the oil had a repellency of 53 -70% compared with the negative control. Calneem oil protected maize grain against damage by E. cautella for 10 weeks, compared with 20 weeks protection by Actellic. There was no resistance development to the oil for three generations of E. cautella. The insects that survived treatment with the LC 90 concentration (12.2 ml/l) of the Calneem oil laid fewer eggs, and some of their progeny were deformed. Calneem oil provided a good level of protection to maize grains against infestation by E. cautella, and may therefore be a suitable substitute for Actellic.

Following the detection of resistant diamondback moth (DBM) populations to synthetic pyrethroid, organophosphorus and indoxacarb insecticides in the Sydney Basin, a study of the major biochemical mechanisms was conducted to determine the type of resistance in these populations. The activity of cytochrome P450 monooxygenases increased two-to sixfold when compared with the susceptible strain. Up to a 1.9-fold increase in esterase activity in resistant strains compared with the susceptible strain was observed. In vitro inhibition studies showed that profenofos, methamidophos and chlorpyrifos strongly inhibited the esterases while permethrin and esfenvalerate resulted in less than 30% inhibition. Qualitative analysis of the esterases using native polyacrylamide gel electrophoresis showed four bands in both the susceptible and resistant individuals with more intense staining in the resistant individuals. The development of these bands was inhibited by methamidophos and chlorpyrifos pretreatment of the protein extract while permethrin and esfenvalerate did not exhibit this effect. Glutathione S-transferase (GST) activity was significantly higher in two field populations compared with the remaining populations. Overall, the study showed that the mechanisms of insecticide resistance in the DBM populations in the area studied were due to cytochrome P450 monooxygenases, esterase and GSTs, and possibly other non-metabolic mechanisms that were not investigated in the present study.

Weeds are rapidly developing resistance to synthetic herbicides, and this can pose a threat to the ecosystem. Exploring allelopathic species as an alternative weed control measure can help minimize the ecological threat posed by herbicide-resistant weeds. In this study, we aimed to evaluate the contribution of some polyphenols to the allelopathy of rosemary (Rosmarinus officinalis L.). The phytotoxic effects of rosemary (leaves, roots, inflorescences, and stems) crude extracts were tested on lettuce (Lactuca sativa L.). Soils incorporated with dried rosemary leaves were also tested on test plants. Reversed-phase high-performance liquid chromatography (HPLC) analysis was used to determine the content of some polyphenols (caffeic, ferulic, gallic, rosmarinic, carnosic, and chlorogenic acids) in rosemary. The specific activity and total activity of crude extracts and individual compounds were evaluated using lettuce. The crude extract of rosemary leaves showed the highest growth inhibitory effect among the rosemary organs tested. Soil amended with rosemary leaf debris reduced the dry matter and seed emergence of lettuce. Carnosic acid was the main compound detected in rosemary leaves and had a high specific activity when tested on lettuce. During the seed germination period, there was observed filter paper coloration among the test plants treated with carnosic acid (250 μg/mL). The high concentration and strong inhibitory effect of carnosic acid could explain the inhibitory activity of the rosemary leaf extract. Hence, we conclude based on the total activity estimation that carnosic acid among the other tested compounds is the major allelochemical in rosemary leaves.