Improving secondary pick up of insect fungal pathogen conidia by manipulating host behaviour

Abstract

It is often assumed that efficient application of a mycoinsecticide involves hitting the target pest insect directly with a lethal dose of conidia. However, secondary pick-up of conidia from surrounding vegetation may be a more important source of inoculum. We have investigated ways of increasing conidia acquisition by enhancing host movement. The aphid alarm pheromone, E-β-farnesene, significantly increased mortality among peach potato aphids. Myzus persicae Sulzer, that were exposed for 24 h to discs of green pepper leaf sprayed with conidia of Verticillium lecanii (Zimmerman) Viegas then transferred to fresh untreated discs to allow disease development. A more practical approach to increasing conidia pick-up appears to be the use of sub-lethal doses of the chloronicotinyl insecticide imidacloprid. One percent of the recommended dose, applied systemically, dramatically increased aphid movement; quantified by image analysis of videotaped aphid behaviour. This resulted in greater mortality from mycosis in experiments where aphids were exposed to insecticide-treated leaf discs that had been sprayed with fungal conidia. A comparison with results from an experiment where conidia were sprayed directly onto aphids which were feeding on insecticide-infused pepper discs established that synergy was due to an indirect effect of the insecticide, i.e. through increased movement, rather than a direct effect viz. predisposition of insecticide-weakened insects to disease.