Choice of feeding sites, growth and survival by Crocidolomia pavonana (F.)

Abstract

The large cabbage moth, Crocidolomia pavonana (F.) (Lepidoptera: Crambidae), is one of the most significant pests of Brassica crops and has re-emerged as a serious pest of crucifer crops in tropical and sub-tropical Asia because of the control used against the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). The damage caused by this pest can be significant because larvae devour young tissues of the plant at different stages and bore into the centre of the cabbage head at maturity and can damage the whole cabbage plant. Despite its economic importance the interaction of this pest with its host plant is not well understood. The current study showed that larval feeding by C. pavonana was influenced by the presence of cabbage in artificial diet, in laboratory bioassays. Of the four diets developed and tested, neonates survived and completed developmental stages (81%) on diet containing cabbage (Brassica oleracea cv sugarloaf) more so than on base diet (diet without cabbage), diet containing wheat grass or spinach powder. Neonates were able to locate diet with cabbage powder but not when cabbage was absent (i.e. base diet). Similarly, in an olfactometer test, neonates were strongly attracted to the odour of diet with cabbage compared with diet without cabbage. More than twice the number of feeding events was observed on filter paper disks impregnated with diet containing cabbage powder than on paper disks with base diet. The behavioural responses of neonates to diet with cabbage are linked to the glucosinolates present on cabbage plants. Sinigrin elicited feeding when added to diet without cabbage and also when applied to a non-host plant, Gossypium hirsutum (Sicot 71RRF). Neonates’ response to allyl isothiocyanate in a two-arm olfactometer was stronger than to sinigrin, which would explain larval attraction to damaged cabbage plants. These results indicate that sinigrin and allyl isothiocyanate provide important feeding and olfactory cues for C. pavonana larvae. Larval feeding and moth oviposition behaviour of C. pavonana was further studied using whole cabbage plants of two different ages (6 and 8-leaf stage) and this was compared to the response of a well-studied crucifer specialist, P. xylostella. Plant age did not significantly affect oviposition and feeding preference by C. pavonana whereas P. xylostella preferred younger plants for oviposition and larval feeding. The intra-plant distribution of eggs and feeding sites was similar for both species, but egg distribution showed a pattern different from feeding sites. Neonates that hatched from eggs deposited on lower leaves moved towards the apical portion of the plant and established a majority of feeding sites on younger leaves. Preference for younger leaves was also observed on excised leaves, indicating architectural position was not the factor driving choice. Younger leaves are known to have a higher concentration of nitrogen content and proteins suitable for larval development, whereas lower leaves are considered not palatable because of fibrous content and low nutritional quality. Therefore, leaf age is an important factor affecting choice of feeding sites by the crucifer specialists. Sinigrin content, inferred from a separate study, did not explain feeding choice. The significant increase in the concentration of total aliphatic and indole glucosinolates with reduced leaf age does correspond with larval preference for younger leaves. Leaf wax and leaf toughness brought about by leaf age were studied as possible factors involved in leaf choice. Wax morphology, observed using scanning electron microscopy, differed between old and young leaves. The removal of leaf surface wax did affect C. pavonana neonates’ feeding choice, with greater preference for leaf disks with wax in situ, compared against leaf disks with wax removed, in a dual choice assay. Leaf wax extracted from either old or young leaves also induced feeding when added to artificial diet without cabbage. Leaf toughness, was not found to be a significant factor in feeding choice. The effect of sinigrin (allyl glucosinolate) and its hydrolysis product, allyl isothiocyanate on growth and survival of C. pavonana was investigated using artificial base diet and diet containing cabbage leaf powder, and compared with the response of P. xylostella. Consistent with other findings in this study, sinigrin promoted larval feeding for both crucifer specialists. Response to allyl isothiocyanate though, differed between C. pavonana and P. xylostella. Allyl isothiocyanate was found to be toxic and lethal for P. xylostella but C. pavonana neonates fed and survived, and when assessed at 24 hours, had gained weight. Similarly, varying the concentrations of allyl isothiocyanate (1 to 10 μmol/g diet) did not significantly affected survival of C. pavonana neonates whereas 1 μmol/g diet caused maximal mortality for P. xylostella neonates. The very high survival of C. pavonana neonates on diet containing allyl isothiocyanate indicates that C. pavonana handles glucosinolate hydrolysis products in a way that differs from P. xylostella, and this finding points to a new or different mechanism for glucosinolate metabolism.