Know Your Friends

Diadegma insulare, Parasite of Diamondback Moth

Diadegma insulare is the most important parasitoid of the diamondback moth in North America and Canada. It is a small ichneumonid wasp and attacks small to medium-sized diamondback moth larvae. D. insulare pupates inside the cocoon made by the mature diamondback moth larva. The black colored developing wasp can be seen inside the cocoon, in place of the light colored diamondback moth pupa. Parasitism is often >75% in the field. D. insulare is very sensitive to insecticides. Adults can be killed by contact with an insecticide deposit. Larvae are killed inside the host, if the host larva is killed by an insecticide, even by Bacillus thuringiensis (Bt). In the field, D. insulare may partially avoid insecticide exposure because its pupae are protected from insecticide inside the diamondback moth cocoon and the wasps are highly mobile, recolonizing treated areas.

Severe diamondback moth outbreaks often involve intensive insecticide use, combined with insecticide resistance in diamondback moth and sensitivity in D. insulare. Diamondback moths in the north central U.S. are susceptible to insecticides or only moderately resistant. However, diamondback moths that are highly resistant to insecticides and also resistant to Bt can be brought into the field on Brassica transplants. A grower can quickly get into a downward spiral of increased insecticide use with minimal effect on the diamondback moth larvae followed by reduced parasitism. Thus, the first step in conserving D. insulare populations is to carefully inspect transplants for diamondback moth larvae and use insecticides, including Bt, only as needed.

D. insulare females require nectar sources. A nectar source can increase D. insulare female longevity from 2-5 days to more than 20 days. Numbers of diamondback moth larvae parasitized increases from zero per D. insulare female, with a poor nectar source, to more than 150 per female with an optimal nectar source. A number of wildflowers can serve as nectar sources for D. insulare, including members of the cabbage family and Queen Anne's lace. Other flowers, including oxeye daisy, perennial sow thistle and common lambs-quarters are poor nectar sources. The best nectar sources were flowers with wide or shallow corollas, where D. insulare can easily reach the nectar. However, D. insulare also can push between sepals at the base of the flower or chew holes in the sepals behavior previously reported for bumble bees, but not for ichneumonid wasps. D. insulare also can obtain nutrition from aphid honeydew, although it is a poorer food source than flower nectar.

In Michigan, D. insulare are found in a wide variety of habitats. Diamondback moth larvae placed in agricultural crops (corn, dry beans, alfalfa, tomatoes, apples, and Brassica crops), weedy areas, and woodland edges were parasitized. This was in spite of very low (often undetectable) natural numbers of diamondback moth. This suggests that D. insulare may have alternate hosts or is a highly effective searcher.

D. insulare can be easily monitored by examining diamondback moth cocoons: diamondback moth cocoons will be white inside (green when the larvae first form the cocoon); D. insulare wasps will be visible as dark bodies inside the cocoon, before the adult D. insulare emerges. Alternatively, large diamondback moth larvae can be collected and reared until they form cocoons. D. insulare wasps can also be seen searching in the crop foliage. D. insulare can be reared on diamondback moth larvae in the laboratory, but a large proportion of wasps will be males, unless conditions are ideal. With the high natual rates of parasitism, rearing and mass releases.

Limiting insecticide use and using Bt where possible, allowing wildflowers (especially wild brasiccas) to grow around crop fields, and allowing diamondback moth to colonize wild brassicas and crops such as canola will increase the abundance and effectiveness of D. insulare for management of diamondback moth.

- Edward J. Grafius, Michigan State University and A. B. Idris, Department of Zoology, Universiti Kebungsaan, Malaysia

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