Evaluation of potential Pyrethrum Synergists on agriculturally significant insect species

Abstract

The indiscriminate use of pesticides in the control of agricultural and public health pests has led to the development of insecticide resistance in many insect pest species, posing a large threat to the continued success of chemical control measures. Resistance towards pesticides can be due to a number of mechanisms, but enhanced detoxification of the insecticide by metabolic enzymes, such as esterases, glutathione-S-transferases (GSTs) or monooxygenases, plays a large role in insecticide resistance. Synergists, such as piperonyl butoxide (PBO) can be used to overcome metabolic insecticide resistance. Synergists are capable of inhibiting enzymes involved in insecticide resistance, thus restoring a level of susceptibility. By virtue of its enzyme inhibitory qualities, PBO is often used to enhance the efficacy of pyrethrum (an organic insecticide). In some countries, however, PBO is no longer certified for organic use and this study explored the use of natural plant oils or extracts as potential organically-certifiable pyrethrum synergists. From a commercial viewpoint, an effective natural synergist could potentially allow development of new pyrethrum formulations into higher value niche markets, such as organic agriculture. Synergised pyrethrum could also offer a new control option against insects resistant to synthetic pesticides in mainstream agriculture. The efficacy of various natural plant oils and extracts as pyrethrum synergists were examined for synergistic potential towards pyrethrum with several agriculturally significant insect pest species, Helicoverpa armigera, Frankliniella occidentalis, Myzus persicae, Meligethes aeneus and Musca domestica, a serious public health pest, using discriminating dose bioassays. PBO proved to be the most effective pyrethrum synergist in all species tested, however, a number of natural plant oils also showed efficacy. The most promising compounds were dill apiole oil and parsley seed oil, synergising pyrethrum in all the species tested. Dill apiole oil and parsley seed oil contain the methylenedioxyphenol ring structure that is characteristic of PBO. Enzyme inhibition studies were used to examine the ability of the natural plant oils and extracts to inhibit the enzymes involved in metabolic insecticide resistance (esterases, GSTs and monooxygenases). In general, inhibition of esterases and GSTs was not correlated to synergism of pyrethrum in vivo. However, in H. armigera and M. persicae, with esterase mediated pyrethroid resistance, PBO, dill apiole oil and parsley seed oil inhibited non-specific esterases. In M. aeneus and M. domestica, where resistance to pyrethroids has been linked to monooxygenases, PBO, dill apiole oil and parsley seed oil showed significant inhibition of monooxygenases. However, not all compounds that inhibited esterases or monooxygenases synergised pyrethrum. The lack of correlation between enzyme inhibition and synergism could be due to a number of factors, including the ability of the synergist to penetrate the insect cuticle, speed of distribution of the synergist through the insect, metabolism of the compounds in the insect and affinity for the target site.