Crucifer host plant suitability for bertha armyworm (Mamestra configurata) and diamondback moth (Plutella xylostella)
Ulmer, Bryan James
Crucifer host-plant suitability for bertha armyworm, Mamestra configurata Walker, and diamondback moth, Plutella xylostella L., was examined using cultivars and breeding lines of Brassica napus L., B. juncea L., B. rapa L., B. carinata L. and Sinapis alba L. Larval growth, development and survival, as well as feeding and oviposition preferences, were examined on physically and chemically distinct cruciferous plants using choice and no-choice experiments. In choice and no-choice experiments with entire plants, single leaves and leaf discs, the B. juncea lines (AC Vulcan and H-Allyl) and the S. alba lines (AC Pennant and L-GS) were the poorest host plants in terms of bertha armyworm larval weight gain, development and feeding preference. The results indicate that specific foliar glucosinolates such as sinigrin, which is predominant in B. juncea, and sinalbin, which is abundant in S. alba, may provide crucifer crops with some protection from bertha armyworm feeding. Bertha armyworm oviposition preferences were examined on representative cultivars from B. juncea, B. carinata, B. napus and S. alba. S. alba AC Pennant received the greatest number of eggs despite being relatively resistant to larval feeding. Oviposition was substantially greater on B. napus than on B. juncea in all choice and no-choice experiments. Full-flower B. napus plants were significantly preferred for oviposition over plants in pre-flower or pod stages. Bertha armyworm laid most eggs in the upper portion of the crop canopy on the underside of leaves. The effect of conspecific eggs on bertha armyworm oviposition site selection was also examined. In choice experiments, females showed a strong preference for plant material with conspecific eggs over plants without eggs. Females showed a much stronger preference for plants with eggs of a different female than for plants with their own eggs. Gravid females also showed a preference for leaves which had been treated with a methanol egg wash over control leaves, indicating that the source of attraction may be chemical. There were no consistent differences in diamondback moth larval growth, development, survival or fourth-instar feeding preference among the crucifer lines tested. However, there were differences in oviposition and first-instar feeding preferences on 'Glossy' and 'Waxy' lines of B. rapa. Although females laid more eggs on 'Glossy' plants, there was a strong preference among first-instar larvae for' Waxy' plants in a choice situation. The results indicate that B. rapa expressing the glossy leaf wax characteristic showed some resistance to diamondback moth, similar to that observed previously with glossy B. oleracea. The resistance appears to have a behavioural basis and is expressed against early-instar larvae.