A study of insect pests of brussels sprouts in Canterbury; their phenology and the resultant crop losses

Abstract

The relationships between insect pests and damage in Brussels sprouts were studied for three seasons in Canterbury. Each season the major pest species, white butterfly (Artogeia rapae L.), diamondback moth (Plutella xylostella (Curt.)) and cabbage aphid (Brevicoryne brassicae L.), were monitored on unsprayed areas within the crop. White butterfly numbers followed the same pattern each season reaching a peak of between 2.9-3.4 larvae/plant in late March of each year. Diamondback moth was more variable between seasons but each year there was a shift in the population from the leaves into the developing sprouts as these became available from mid March onwards. Cabbage aphid was the least regular in both timing and intensity of attack. Peak numbers in 1979, 6.1 colonies/plant, were 15 times greater than in 1980, the year of the lowest attack.

Larvae of white butterfly and diamondback moth were confined in clip-cages for feeding tests on Brussels sprout leaves. For both species over 85% of the feeding occurred in the final instar.

Mean leaf area consumed at 20° C was 46.1 cm² for each white butterfly larva and 2.9 cm² for each diamondback moth larva. When tested over a range of temperatures it was found that pupae were lightest after development at the higher temperatures and that, for white butterfly, consumption dropped markedly at these temperatures.

The effects of artificial defoliation, simulating pest attack, on Brussels sprout plants were studied for two seasons in field plots. Greatest yield loss occurred following defoliation in the mid season. The plants were able to compensate for defoliation in the early stages of growth and to a limited extent in the mid season. In the late stages the plants were unable to compensate for defoliation but yield losses were low due to much of the yield already being formed. Removal of leaves from the mid zone of the plant caused the greatest yield loss.

Insecticides were used in two seasons to provide a range of pest numbers for damage assessment. In the first season the selective insecticides carbaryl and demeton-Smethyl were used to produce low numbers of lepidoptera and aphids respectively. The greatest marketable yield of sprouts was recorded from the carbaryl treatment and the weights of damaged sprouts/plot were positively correlated with diamondback moth larval numbers. In the second season the insecticide dichlorvos was used to control pests in early, mid or late periods of Brussels sprout growth. Resultant yields indicated that omission of sprays in the mid period resulted in the greatest loss of total sprout yield and that the greatest numbers of damaged sprouts followed mid and late spray-free periods.

A range of densities of larvae were confined in cages over individual plants at stages throughout the crop's growth. Yield was negatively linearly related to the estimated percentage defoliation caused by white butterfly feeding. There was no relationship between diamondback moth and total yield but a positive linear relationship was found between larval numbers and numbers of damaged sprouts in the mid season.

When sprouts were examined on the stems, greatest numbers of damaged sprouts were found in the middle zone. Sprouts were not entered by the larvae until the sprouts were more than 7 mm diameter. Some reduction of damage occurred among the lower sprouts by shedding of the outer leaflets.

The relationship between white butterfly feeding and Brussels sprout growth was examined by using a simulation model. The model indicated that the cumulative effect of white butterfly feeding was greater than would be expected from a single defoliation and that 56% of yield loss could be prevented by use of a single insecticide spray 12 weeks after transplanting.

As a result of these investigations a rational spray programme is proposed utilizing pest thresholds and the size of the developing sprouts as indicators of the need for insecticides to be applied.