Visual control of host pursuit in the parasitoid fly Exorista japonica

SUMMARY The tachinid fly Exorista japonica is a parasitoid of many kinds of lepidopterous larvae. After encountering a suitable host, the fly pursues the crawling larva on foot using visual cues to guide it. To investigate the visual control of host pursuit, we observed and videotaped pursuits of a host, the common armyworm Mythimna separata, for frame-by-frame analysis. Observation was performed in sunlight and under illumination from a fluorescent lamp. The fly pursued hosts discontinuously with a repeated stop-and-run motion. During a run, its movements consisted of rotation, forward translation and sideways translation. Rotation during a run was positively correlated with the angular position of the host’s head. The direction of translation depended on the angular position of the host’s head. Forward translation was negatively correlated with the visual angle subtended by the host. These results suggest that the fly orients and walks towards the leading edge of a moving target. There was little difference in the results between sunlight and illumination from a fluorescent lamp.

Arom atic Amino Acid and Amine Levels in the Hemolymph of Parasitized and Unparasitized Larvae of Mythimna separata

The levels of aromatic amino acids and amines in the hemolymph of larvae of the common armyworm, My­ thimna separata, were analyzed after parasitization by the braconid wasp, Apanteles kariyai. Methyldopa, homovanillic acid, tryptophan and tyrosine remained unchanged in unparasitized and parasitized larvae. Dopamine and DOPA showed characteristic changes at pupation that were associated with the hardening and darkening of the cuticle; Dopamine in the hemolymph of unparasitized last instar larvae increased before pu­pation. Large increases in kynurenine and octopamine were detected in parasitized larvae. The increased level of octopamine in the hemolymph may be stress related.

Effects of water stress on curly Mitchell grass, the common armyworm and the Australian plague locust

Temporal water and nitrogen relations of curly Mitchell grass, Astrebla lappacea (Lindl.) Domin, in response to single waterings of 0, 20, 60 and 180 mm were studied. Effects on the survival and development of the common armyworm, Mythimna convecta (Walker), and the Australian plague locust, Chortoicetes terminifera (Walker), were recorded. Glasshouse trials were conducted to ascertain the level of rainfall required for insect survival. More than 20 mm of rainfall was needed for insect survival. Even at 60 mm, development was retarded in some instances. Plant nitrogen was positively correlated (R2 = 0.51) with plant moisture. Insect survival was better correlated with plant moisture (R2 = 0.61) than with plant nitrogen (R2 = 0.41).