Most fifth instar Argina cribraria female larvae reared in paired groups on fresh sunnhemp leaves practised cannibalism on the male. A potential cannibal pounced on the victim and severed its dorsal vessel, causing profuse bleeding of haemolymph, immobility and contraction. The cannibal fed on victim's carcass and might consume 1–2 larvae and 1–3 pupae. More females emerged from an experiment having larval cannibalism. The cannibals were lighter, but the relative titres of sugars, proteins and phenols were higher in cannibals than those in control female caterpillars.

Factors contributing to host plant resistance in sorghum to Chilo partellus (Swinhoe) were studied. Oviposition on the plants in the field started 3 weeks after germination and peaked 2–6 weeks later. Egg batches (96.1%) were laid on the leaf blades compared with 3.9% on the leaf sheaths. On the leaf blades, significantly more egg batches were laid on the upper (57.2%) than on the lower leaf surfaces. In young plants, leaves attached to the upper and lower segments were preferred for oviposition, while leaves in the upper segments more or less maintained their share of egg batches over time; there was a shift from the leaves in the lower to those in the middle segments with the plant age. Differences among sorghum lines for oviposition were significant and these lines were divided into three categories based on their levels of oviposition.

Sorghum plants reacted to insect feeding in the stem by production of a red staining compound which did not significantly affect the development of the insect. Four damage symptoms were observed and these were related to different plant ages. Plants compensated damage by the production of extra tillers, some of which produced panicles that were ready by approximately the time of harvest. Plants infested 2 weeks after germination produced maximum tillers and juvenile panicles; and also had the least effective tillers, plant heights and grain yields. Grain yields were depressed in the infested plants for the cultivars tested, except in LC 119/80–3 where there were actual grain yield gains.

Leaf-feeding and ‘dead heart’ scores, which were related to reduced plant heights and yields in some of the cultivars tested, are proposed as rational indices for susceptibility ratings and monitoring of crop losses.

Insect-free tubers of four potato varieties, namely Kenya Baraka, Anett, Kerrs Pink and Roslin Gucha were exposed to adult Phlhorimaea operculella for 14 days in uncontrolled conditions in storages. On examination of the tubers 16 days after exposure, the percentage of infested tubers was determined: Kenya Baraka—62.50, Anett—77.50, Kerrs Pink—85.00 and Roslin Gucha—86.25%. The larvae made the following mean tunnel lengths: 44.65, 95.33, 88.33 and 104.23 cm in Kenya Baraka, Anett, Kerrs Pink and Roslin Gucha varieties, respectively. Although there were no statistically significant differences in the number of tubers infested (per cent), there were significant differences in the mean length of tunnels made by the larvae in Kenya Baraka and the other three varieties at P < 0.05. The mean number of larvae which survived in 10 tubers were 6.75, 11.25, 15.75 and 18.00 for Kenya Baraka, Anett, Kerrs Pink and Roslin Gucha varieties, respectively. There were statistically significant differences between Kenya Baraka and Kerrs Pink; Kenya Baraka and Roslin Gucha; and Roslin Gucha and Anett varieties (P < 0.05). Thus Roslin Gucha was the least resistant of the four varieties tested, whereas Kenya Baraka emerged as the most resistant variety. The mechanism of resistance in this case was most probably antibiosis.