Boletín de la AeE

Parasitoids of leafminer insects are faced with the difficult task of fínding their concealed host. It is well known that chemical cues play an irnportant role in the foraging of parasitoids. Various cues can be used by forging parasitoids to localise a host infested plant and furthermore to fínd their preferred host stage. The response to chemical cues can be modifíed by previous contact or oviposition experience with the host. Recently y-tube olfactometer studies have shown that long range volátiles emitted from apple plants infested with Phyllonorycter pomonella 2. attract the braconid Pholetesor bicolor N. In further experiments it has aiso been observed that there is a stronger long range attraction towards plants infested with the tissue-feeding host, compared to plants infected with íhe sapfeeding host (Lengwiler et al., unpubl.). In the laboratory, this braconid is only able to parasitize the sap-feeding stage of the leafminer, even though the wasp wi)l attempt to parasitize the tissue-feeding hosts. ín order to ascertain if parasitoid females can discrimínate between plants infested with sap-feeding hosts, plants infested with tissue-feeding hosts and, non-infested plants, behavioural observations in Plexiglás cages were performed. P. bicolor females were given an experience fíve hours before the experiment on plants infested by sap-feeders (Sfemales), by tissue feeders (T-females) and no-experience (N-females). For each treatment, groups of three female wasps where released in sepárate Plexiglás cages. The experiment was replicated ten times, for a total of 30 females/treatment The Plexiglás cage contained three plants: one infested with sap-feeding hosts, one with tissue-feeding hosts and one uninfested. Within a two hour period the behaviour and location of parasitoids was recorded by sean sampling (every 5 min.). Results show that parasitoids rarely visited the uninfested plants, regardless their previous experience. S-females were recorded more frequently on sap-feeders infested plants and overall appeared more active than the T- and N-females. T- and N-females made no discrimination between sap-feeders and tissue-feeders infested plants. The foraging strategy of the parasitoids and the occurrence of host age discrimination during host location are discussed. Further studies are underway to detennine the source of contact cues which guide the parasitoid to the host after landing onto an infested plarit.