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

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.