Some bacteria found on leaf surfaces can kill aphids and other insects with seemingly no impact on other species. There is therefore the potential to use them as a directly applied biological control agent or to manipulate the crop environment to favour their development. There is a hope that such agents could be used as an alternative to conventional chemical insecticides.
A range of bacteria were isolated from 11 different plant species, inoculated into aphid colonies and assessed for their lethality to aphids. Of the 140 that were isolated, nine were found to be effective against six different aphid species - peach-potato aphid, black bean aphid, cabbage aphid, lupin aphid, carrot-lettuce aphid and glasshouse-potato aphid.
After numerous assessments, including dose response and timing of killing, the most potent bacteria that killed aphids were Pseudomonas poae, P. fluorescens and Citrobacter werkmanii. All the bacteria were taken up by aphids from surfaces as well as from liquids, indicating that they may be suitable for foliar application.
The genomes of the three bacteria studied were sequenced and a range of insecticidal toxin genes in each of the genome sequences were discovered. Feeding assays with Lepidopteran caterpillars were also tested with the strains, but none were able to kill the insects, providing hope that the toxicity is highly specific.
Small-scale trials indicated that plants treated with a bacterial suspension of P. poae as opposed to water were less favourable for aphids. This experiment will need to be repeated with more plants but if the result proves to be realistic then the bacterium may be useful as a deterrent agent.
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