No abstract

The oviposition behaviour of Episyrphus balteatus DeGeer (Diptera: Syrphidae) was investigated in four-choice experiments in screen cages and in experiments with direct behavioural observations. Females laid hardly any eggs in the absence of aphids and preferred larger aphid colonies to small ones. Honeydew attracted the females via olfaction and elicited oviposition. Females were able to perceive the presence of conspecific eggs and oviposited less often in aphid colonies in which eggs were present. The oviposition deterring stimulus was also active when the eggs were removed, hence, the deterrent did not act visually, but probably olfactorily.

Host-finding behaviour of Prostephanus truncatus was evaluated in a four-choice olfactometer for reactions to odours of maize, cassava chips, wheat, cowpea (a non-host plant), and woody plant species in which reproduction has been observed, and of organisms associated with maize stores. P. truncatus reacted in general to odours from starchy commodities and to some of the woody plants. Beetles did not respond to volatiles from cowpea or organisms associated with maize stores, but did react to the aggregation pheromone produced by single male P. truncatus (secondary attraction). P. truncatus raised on cassava and emigrants from maize responded as strongly to maize odours as beetles raised on maize, whereas P. truncatus trapped with pheromone traps showed reduced or no reactions. Stored commodities seem to elicit short-range arrestment, but not primary attraction at long range. However, stored products might only be attacked facultatively by P. truncatus, co-evolved primary attraction could possibly exist to the native host-plant complex.