New insecticides get ready for market

The ongoing arms race between farmers and the voracious bugs that tuck into their crops is about to step up a gear. Agrochemicals giants Bayer, Dow and BASF are all registering new compounds that promise to tackle the pests already resistant to today's insecticides, while avoiding harm to the bugs' natural predators.  

Spinetoram, a mixture of 3'-O-ethyl-5,6-dihydro spinosyn J (left) and 3'-O-ethyl spinosyn L (right)

The companies presented their new products in October at the 16th International Plant Protection Congress in Glasgow, UK. Of the new products, German firm BASF's metaflumizone is the furthest through the regulatory process, being already approved for sale in several countries, including Austria and Germany. The compound works on the nervous system of target insects, blocking sodium channels to gradually induce paralysis. 

Previous sodium channel-targeting insecticides, while incorporating similar nitrogen-based functionality in their structures, had to be metabolised into their active forms by the insect. Metaflumizone works directly, and is 'very active' against the Colorado potato beetle, a major pest that has built up resistance to the current crop of insecticides, say BASF. 

Resistance is futile 

Dow's spinetoram, currently being evaluated by the US Environmental Protection Agency, is derived from spinosyn, a natural product that has insecticidal activity. The Dow team employed a new tactic to optimise the compound's activity - an artificial neural network.  

This algorithm uses a feedback loop to fine-tune the structure-activity relationships of the family of compounds, helping chemists to design increasingly potent derivatives. 'These studies . led to a major breakthrough in the chemistry,' Thomas Sparks, advisor at Dow AgroSciences in Indianapolis, Indiana, told Chemistry World. 

Bayer's latest insecticide is also a member of a known insecticide family, structurally modified to boost its properties. Where Bayer's first two tetronic acid-based insecticides, spirodiclofen and spiromesifen, coat the leaf surface, new family member spirotetramat is actually taken up by the plant.  

The compound is then carried around the plant, including into roots and newly-forming shoots, giving sucking insect pest like aphids and whitefly an unpleasant surprise. The compounds work by inhibiting lipid biosysnthesis, affecting reproduction in adults and especially juveniles. 

The companies stress the importance of using the new insecticides in rotation with others to avoid resistance developing. But Graham Moores, a biological chemist from Rothamsted Research, Harpenden, UK, who is examining an alternative approach to the perennial resistance problem, still questioned the long-term efficacy of the new pesticides. 'Looking at the new compounds being presented at Glasgow, some incorporate ester bonds - I wonder if they will be open to attack from the insects' non-specific esterase enzymes?' Unfortunately, resistance is inevitable, he added: 'Historically, new compounds work well only for a short time.'  

James Mitchell Crow