The biology and ecology of the diamondback moth plutella xylostella (l.) with special reference to susceptibility of six cabbage varieties grown in Kenya

Abstract

The biology and ecology of the diamondback moth Plutella xylostella (L.) was studied using cabbage varieties; Copenhagen maket, Sugar loaf, Gloria F1 hybrid, Prize drumhead, Red acre arid Chinese cabbage. The studies were carried out at the National

Horticultural Research Center (NHRC) Thika, from May 1992 to March 1993. The following were the objectives of the study; to evaluate susceptibility of the commonly grown cabbage varieties to diamondback moth infestation; to determine the effects of different host plants on the life history of the insect and to observe the abundance of diamondback moth parasitoids in a cabbage field. To achieve the first objective, field experiments were carried out using the six cabbage varieties, planted in a two-factor completely randomised block design (CRBD) replicated four times. Diamondback moth immature numbers were recorded from each variety to estimate population build-up on the six different cabbages. Leaf damage due to diamonback moth infestation was recorded from each cabbage variety and finally the marketable weight was recorded to evaluate the percent marketable weight loss due to diamondback moth infestation. Data was processed through analysis of variance (ANOVA) for a Factorial ANOVA. Separation of means was done by Duncans multiple range test (DMRT) at a significance level of 0.05. The diamondback moth infestation contributed to a significant marketable weight loss among all the cabbage varieties. Price drumhead recorded the highest percent marketable weight loss of 44.97% followed by sugarloaf 41.90%, while varieties chinese cabbage, Red acre, Gloria f1 hybrid and Copenhagen market lost 30.00%, 26.49%,

18.50% and 18.38% respectively. The cabbage 'II 1rieties Red acre and Gloria f1 hybrid, were the least damaged by the insect pest. The two varieties also had the least pest population of 1.18 ± 0.13 and 1.39 ± 0.14 insects per plant, respectively, while cabbage variety Chinese cabbage, Price drumhead, Sugarloaf and copenhagen market recorded 1.49 ± 0.24, 1.69 ± 0.31, 2.13 ± 0.52 and 2.71 ± 0.12 insects respectively. To achieve the second objective the diamondback moth was reared in the laboratory using clean leaf of the six cabbage varieties grown in an insect proof screen house to determine the effect of different host plants on the insect's life cycle. The experiment layout was a completely randomised design replicated five times. Data was collected on the insect's development period, the amount of leaf eaten throughout the growing period and the resultant weight of the insect. Data was analysed through two way ANOVA and separation of means by DMRT at significance level of 0.05. The host plants demonstrated significant variations in their effect on the insect's life history. The diamondback moth completed development on all the six cabbage varieties. Insects reared on Red acre and Gloria) 1 hybrid took the longest time of 19.2 ± 0.3, 19.0 ± 0.2 da IS to develop respective I¥. On these varieties, insects ate significantly less leaf area of 139.9 ± 8.29 and 140.:3± 4.84 rnrn". Insects reared on Price drumhead, Sugarloaf, Chinese cabbage and Copnnhagen market took 19.0, 18.6± 0.2, 18.0 ± 0.1 and 18.2 ± 0.2 days, and ate, 215.0 ± 4.8, 235.8 ± 10.75, 248.4 ± 13.96 and 249.6 ± 19.05 rnrn", respectively. Slight deviations in susceptibility trend and host plant effects recorded within the same variety were attributed to evaluation methods used to analyse susceptibility. The studies also revealed that, parasitism of the diamondback

moth in the field accounted for 7.6 % to 11.8 % insect mortality. The identified parasitoids were within the families, ichneumonidae, eulophidae, chalcididae and braconidae. The most abundant was the eulophid Tetrastichus sokolowskii Kurdj .