Diadegma insulare
(Hymenoptera: Ichneumonidae)

Diadegma insulare is a Lepidoptera parasitoid that occurs naturally in eastern North America to the plains states and in southern California.

Appearance

Diadegma insulare is a small (6 mm long) ichneumonid wasp with reddish-brown legs and abdomen. It pupates inside the cocoon made by the mature diamondback moth larva replacing the host pupal covering with its own cocoon which may have a distinctive white band. Diamondback moth cocoons are white inside (green when the larvae first form the cocoon); D. insulare wasps are visible as dark bodies inside the cocoon, before the adult D. insulare emerges. Adults can be seen searching in the crop foliage.

Habitat (Crops)

Brassica crops.

Pests attacked

Diamondback moth.

Life cycle

D. insulare overwinters in crop debris as a pupa within the cocoon of its host. Immediately upon emerging, mating takes place, and the female, using her antennae, seeks a host larva (small to medium-sized larvae are selected). When a suitable host has been located, the female flexes her abdomen beneath her thorax and legs and oviposits one egg in each moth larva. She does not sting and paralyze her prey, and larger moth larvae sometimes escape by thrashing about.

After 10 to 15 days, the parasitoid larva emerges from the host cocoon and spins its own cocoon within that of its host. There are 4 to 6 generations per year, corresponding to the number of host generations.

Relative Effectiveness

Diadegma insulare is the most important parasitoid of the diamondback moth in North America and Canada. Native populations of D. insulare have parasitized up to 70% of diamondback larvae in field trials in New York and from 50% to almost 90% in Wisconsin.

In one study, diamondback moth larvae placed in agricultural crops (cole crops and a wide variety of other crops including corn, dry beans, alfalfa, tomatoes, and apples), weedy areas, and woodland edges were parasitized 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.

Conservation

Limiting insecticide use and using Bacillus thuringiensis (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.

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. nsulare, 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.

Pesticide susceptibility

D. insulare is very sensitive to insecticides. Adults can be killed by contact with insecticide deposits, and larvae die inside the host, if the host larva is killed by an insecticide, even Bt. However, D. insulare may partially avoid insecticide exposure because its pupae are protected inside the diamondback moth cocoon, and the wasps are highly mobile and recolonize 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 are 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.

Commercial availability

Available commercially (see the off-site publication, Suppliers of Beneficial Organisms in North America).

Taken from

Edward J. Grafius (1997) Know Your Friends: Diadegma insulare, Parasite of Diamondback Moth, Midwest Biological Control News Online. Vol.IV, No.1.

Other references:

Harcourt, D.G. (1960) Biology of the Diamondback Moth, Plutella maculipennis (Curt.) (Lepidoptera: Plutellidae), in Eastern Ontario. III. Natural Enemies. The Canadian Entomologist, 92: 419-428.

Hoffmann, M.P. and Frodsham, A.C. (1993) Natural Enemies of Vegetable Insect Pests. Cooperative Extension, Cornell University, Ithaca, NY. 63 pp.

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