New Modes of Action of Insecticides

This journal is © The Royal Society of Chemistry 2002 Introduction Herbivorous insect and mite pests are a major threat in the continuous supply of food for human consumption. Synthetic insecticides and acaricides play a major role in modern agriculture and their chemical, environmental and toxicological properties have been improved considerably over the last six decades. Without the use of chemical insecticides dramatic yield losses in many cropping systems worldwide would occur (Oerke and Steiner, 1996). In 2001 a total of 7.56 billion US Dollars was spent in order to protect crops from damage by invertebrate pest species (Beckmann and Haack, in press). The world market for insecticides is still dominated by compounds irreversibly inhibiting acetylcholinesterase (AChE), an important enzyme in the central nervous system of insects, responsible for the cleavage of the neurotransmitter acetylcholine. However, as shown in Table 1 the market share of these AChE inhibitors, i.e. organophosphates and carbamates, decreased from 71% in 1987 to 51% in 1999. Together the AChE inhibitors and those insecticides acting on the voltagegated sodium channel (vgSCh), in particular the pyrethroids, account for approx. 70% of the world market. Here we would like to give an overview of the active ingredients introduced more recently and their biochemical modes of action. Insecticides acting on ligandand voltagegated ion channels Nicotinic acetylcholine receptor agonists The insect nicotinic acetylcholine receptor (nAChR) is one of the insecticidal targets which came in vogue after the introduction of the neonicotinoid (chloronicotinyl) insecticide imidacloprid by Bayer CropScience in 1991, and was reviewed very recently (Nauen et al., 2001). Insect nAChR are thought to be pentamers of two identical alphasubunits and three beta-subunits making the transmembrane protein with a central cation-permeable ion channel found in post-synaptic membranes. Since the launch of imidacloprid (the top-selling insecticide world-wide) neonicotinoids have developed into a major class of insecticides with several other active ingredients introduced over the last 7 years; Figure 1 gives a few examples of the latest additions to this class of chemistry. All neonicotinoid insecticides act as agonists of the insect nAChR located in the central nervous system. They are effective against sucking insects such as aphids, whiteflies and planthoppers, beetles and some lepidopteran pests and can be used as foliar sprays, seed treatments and via soil application. Another very interesting agonist of the insect nAChR, the macrocyclic natural compound spinosad (Figure 2), was commercialized by DowAgroscience in 1997 (Thompson and Hutchins, 1999) and does not belong to the NEW MODES OF ACTION OF INSECTICIDES