Movement and host finding of Trichogramma brassicae on maize plants

Direct observation of searching patterns and residence times of Trichogramma brassicae Bezdenko on maize plants were made at 18 degrees C and 25 degrees C. Temperature had a strong effect on the residence times: parasitoids spent an average of 44.9 minutes on the plant at 18 degrees C and 20.8 minutes at 25 degrees C. Observations on single plants showed that parasitoids mainly walked from one leaf to another. The leaf part closest to the stem was the most often visited and longest searched leaf area. At 18 degrees C, many parasitoids went to the lower leaf side and stopped searching. Leaf level, leaf side or size of the leaf where the parasitoid landed had no effect on residence time. Although about 20% of total searching time was spent in following the leaf edge and mid rib, Ostrinia (Hubner) egg masses touching the mid rib were not found more often than those away from the mid rib. Host finding was also not influenced by the position of egg masses relative to the stem, or by the leaf height. Naturally laid egg masses were found twice as often as artificially placed egg masses. The scales that O. nubilalis adults left in small patches on the plant seemed to be as effective a cue as host cues artificially confined to a small area around the host egg mass. When maize plants were covered with fine netting and a number of O. nubilalis kept close to the plant for one night in such a way that the moths were unable to touch the plants, host finding and residence times of T. brassicae were significantly higher on these plants than on untreated plants. This indicates that volatile cues left by O. nubilalis had an arresting effect and were used in host finding by T. brassicae.