Studies on potential of companion crops in managing the diamondback moth, Plutella xylostella (L.) (Lepidoptera: plutellidae) in cabbage/kale cropping systems in Kenya

Abstract

The diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae) is a major pest and threat to brassica production in many parts of Africa. The adverse ecological and environmental effects as well as the cost of chemical pesticides that are commonly used in its control, besides the increasing resistance to chemicals makes it necessary to explore alternative control methods. Integrated Pest Management (IPM), being the emerging pest control paradigm, offers a number of options out of which biocontrol, companion cropping among others, have great potentials. This study investigated the potential of utilising companion vegetable crops as trap or repellent crops in managing the diamondback moth in cabbage/kale cropping systems. Crop preferences for oviposition and suitability for survival and development of P. xylostella were assessed in the laboratory on seven different test plants: cabbage (Brassica oleracea var capitata L.), kale (Brassica oleracea var. acephala L.), rape seed (Brassica napus L. subvar. oleifera), Ethiopian mustard (Brassica carinata B.), Cleome (Cleome gynandra L.), tomato (Lycopersicon esculentum L.) and coriander (Coriandrum sativum L.). In addition, the effect of the test plants on P. xylostella adult olfactory responses, longevity and fecundity were investigated. In the field, each of the latter five test plants was intercropped with cabbage at Jomo Kenyatta University of Agriculture I' and Technology (JKUAT) farm and kale at Kenya Agricultural Research Institute (KARI-Matuga) and the P. xylostella infestation levels and damage assessed. Another field trial with different planting patterns of both cabbage and coriander was conducted at JKUAT to evaluate the most appropriate planting pattern for reducing P. <0.05) xylostella infestation. The effect of the companion crops on nontarget insects and important plant diseases was also evaluated on cabbage. Laboratory choice and no-choice tests indicated that Ethiopian mustard was found to be a preferred host for oviposition by P. xylostella while tomato was the least preferred for oviposition, as compared to other test plants. Choice tests also revealed that the number of eggs laid by the moths were significantly fewer on treatments of cabbage or kale combined with tomato, coriander or Cleome, when compared to those laid on sole-cabbage or kale. Larval percent survival was significantly lower ((P<O.05) on mustard and Cleome, thus recording a prolonged developmental period while coriander and tomato did not support larval development beyond first instar. Moths fed on different test plant flowers showed no significant differences in the adult male and female longevity and fecundity, except that adult longevity of JKTJAT culture males was significantly on mustard than on other test plants flowers, while fecundity was significantly higheron coriander and Cleome. A significantly higher percentage of moths showed upwind orientation to mustard than all other test plants. Field trials, however, revealed that the unsprayed plots of cabbage and kale grown alone supported significantly higher infestation levels of P. xylostella as compared to the intercropped plots and thus resulted in higher P. xylostella damage scores and reduced marketable produce. The Dipel@ sprayed plots of sole-cabbage and sole-kale had the lowest P. xylostella infestation, although not significantly different from infestation on cabbage or kale interplanted with tomato. The results also indicated that a planting pattern of 2 rows of cabbage to a row of coriander (where coriander was scattered throughout the hills within a plot) was found to be promising for successful management of P. xylostella. The test plants also significantly affected infestation levels of other insects and plant diseases on cabbage. These results indicate that Ethiopian mustard can be successfully utilized as a trap crop and tomato as a repellent crop for the management of P. xylostella in Kenya.