Illinois Natural History Survey - University of Illinois

Influence of Intercropping and Trap Cropping on Diamondback Moth and Its Natural Enemies

Insect pests attracted to crops grown as monocultures can sometimes be sidetracked by planting noncrop or alternate-crop plants nearby. These alternative plantings may also favor natural enemies that attack the pest insects. Trap cropping and intercropping are production practices in which this "neighborhood alteration" effect can be used as a pest management tool to reduce insecticide applications to the main crop. With trap cropping, plants particularly attractive to the target pest are grown in a small part of the field to lure the pests away from the main crop. Intercropping involves planting two or more crops simultaneously or sequentially in various row arrangements within a field. Specific plants may be chosen as intercrops because they restrict or inhibit the pest insects' ability to locate and colonize the main crop.

Intercropping and trap cropping were evaluated as ways to reduce populations of the diamondback moth (Plutella xylostella) on cabbage in a three-year study by University of Illinois Department of Entomology graduate student Angel Gonzalez, working with Survey entomologist Cathy Eastman. The diamondback moth, a worldwide pest of crucifers, such as cabbage, is resistant to several insecticides and is often the most numerous caterpillar attacking crucifers in Illinois. For the experiment, cabbage was grown alone, in plantings with border rows of mustard and sweet alyssum as trap crops, or in various row-intercrop arrangements with cabbage as main crop and tomato, sweet corn, soybean, safflower, or sweet alyssum as intercrops. Populations of diamondback moth and important natural enemies were monitored to determine if the presence of trap crop or intercrop plants would decrease pest establishment and increase numbers of natural enemies. The study was conducted at the University of Illinois Vegetable Crops Farm in Champaign.

Diamondback moth larvae dining on cabbage.

Because mustard and sweet alyssum were more attractive than cabbage for egg laying by diamondback moths in laboratory tests, they were selected as the trap crops. When planted as trap crop borders along rows of cabbage, mustard and sweet alyssum reduced diamondback moth populations on cabbage within those plots compared to those in the monoculture. But this was only effective when the trap crops were sprayed with insecticides at 14- to 18-day intervals. If not controlled, diamondback moth populations developing on the trap crops spilled over into the cabbage grown within those plots, resulting in pest numbers equal to those in the monoculture plots.

The presence of soybean and safflower as intercrops did not influence diamondback moth numbers, but sweet alyssum as an intercrop increased their numbers on cabbage within the plots. Cabbage intercropped with sweet corn had fewer caterpillars compared to the monoculture during one year but not in the second year. Tomato reduced diamondback moth populations in some intercrop arrangements but not others. It was most effective when planted in close association with cabbage.

Six species of parasites attacked diamondback moth in this study. Although parasitism by the major species (Diadegma insulare) was as high as 95% in some plots, parasitism by this insect was not increased significantly in intercrop or trap crop treatments. Parasitism by the second most common species (Microplitis plutellae), however, was increased in the sweet alyssum-safflower-sweet alyssum-cabbage treatment.

The lady beetle Coleomegilla maculata was the most common predator. Its numbers were considerably higher on cabbage in treatments that contained corn in the pollen-shedding stage. This predator was probably attracted to both aphids and pollen in the corn-intercrop treatments because these are important food sources.

The overall goal of this research is to improve cultural practices as components of pest management programs for diamondback moth in cruciferous vegetables. This study provided valuable clues to diamondback moth response to altered planting arrangements and possible effects on important natural enemies.

Angel Gonzalez, University of Illinois, and Cathy Eastman, Center for Economic Entomology

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