Effect of soil moisture and host plants on behaviour and survival of the common cutworm, agrotis segetum (Denis & Schiffermüller) (Lepidoptera: Noctuidae)

[摘要] English: The objectives of this study were to investigate the effect of light, soil moisture and hostplants on behaviour of Agrotis segetum (Denis & Schiffermuller). A marking techniquefor larvae was developed to study behaviour at night. The marker consisted of al: 1mixture of Humbrol paint and fluorescent powder. It had no adverse effect on larvalbehaviour and survival. Dusting of larvae with fluorescent powder also proved useful intracking movement for short range studies. Powder trails could be detected for a distanceof up to two meters.The effect of illumination with different light sources i.e incandescent, infrared and UVlight on activity of larvae was evaluated. Larvae were starved for 24, 48, 72 and 96hours. Significantly more larvae that had been starved for 72 and 96 hours remained onthe soil surface in search of food after an hour of illumination with infrared and UV light.Light and dark adaptation periods of 24,48 and 72 hours had an effect on larval activity.Phototaxis of A. segetum was therefore influenced by their degree of starvation and darkadaptation. Larvae moved towards the light source when illuminated with UV lightunder field conditions possibly perceiving it as a sign of open space.To evaluate the feeding behaviour of A. segetum larvae under wet and dry conditions, agreenhouse trial with Chenopodium carinatum R. Br., Portulaca oleracea L. and Zeamays L. was conducted. Significantly more above ground plant sections of all the threeplant species were damaged under both dry and wet soil conditions. Larvae survived forapproximately one week at soil moisture levels 10, 20, 40, 60, 80 and 100 % in theabsence of food. To evaluate survival in the presence of food at various moisture levels,Amaranthus hybridus L. and P. oleracea were buried in separate pots at soil moisturelevels 0, 10 and 80 %. All larvae remained alive for up to 10 days at the 80 % moisturelevel for each weed species. Larval survival was lowest at ° % soil moisture in both theabsence and presence of food. Larval development was supported until pupation on A.hybridus at 80 % soil moisture.Agrotis segetum moths were given a choice between six plant species for oviposition.Eggs were laid on all plant species. Oviposition was however significantly higher on A.hybridus and 1. purpurea. Significantly more eggs were laid on stems than on the leaves.When laid on leaves, the abaxial was preferred over the adaxial surface. There was nosignificant difference in the number of eggs laid when given a choice between Vetiveriazizanioides (L.) Nash and Pennisetum purpureum (K.) Schumach. Dry leaves of P.purpureum and V. zizanioides were more preferred for oviposition than fresh plantsections.Plants selected by moths for oviposition are not always appropriate hosts for newlyemerged larvae. Larval survival was low on some of the plants preferred for oviposition.First instar larvae were notable to feed on the grasses, P. purpureum, V. zizanioides andZ. mays and died while in the first instar. Survival and mass gain of first instar larvae fedon A. hybridus and C. album was significantly higher than on the rest of the plant species.Presence of these plant species in uncultivated maize fields seems to contribute to highpopulation levels due to their suitability as hosts. Leaf trichome density on the plantspecies evaluated could however not account for the differences in survival anddevelopment of A. segetum larvae. The type rather than density of trichornes could beinhibitory or stimulatory to oviposition and larval survival of A. segetum.

[发布机构] University of the Free State