| dc.description.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 . | en |
