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How to identify it
- Adults are small, slender and black, about 1/4 inch long; UF/IFAS puts the length at no more than 6 mm.[2][1]
- The legs are striped dark brown and yellow, and the abdomen is dark above and yellow-green underneath.[2]
- Females carry a distinct tail at the tip of the abdomen, the ovipositor they use to lay eggs in caterpillars. As in other ichneumon wasps, the forewing has a cell shaped like a horse's head.[2][1]
- A parasitized caterpillar looks like a healthy one from outside. The difference shows at the pupal stage: the wasp's pupa is elliptical, pale at first and then dark brown with a white band around the middle, while the moth's pupa is tapered.[2]
- The wasp larva is white and strongly segmented, with a short, narrow tail about a quarter of its length, and it spins its own cocoon inside the moth's cocoon.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Cotesia plutella | Another parasitoid of the diamondback moth. Its egg is pointed, while the Diadegma egg is rounded and lacks a projection, a difference that shows only when a host is dissected.[1] |
Where it lives
Found from New Hampshire west to southern British Columbia and south to Florida, Texas and California, and also in Hawaii, the West Indies, and from Mexico south to Venezuela. It lives in crucifer crops.[1]
Biology and life cycle
Besides the diamondback moth, it attacks two other caterpillars, Plutella armoracia and the cabbage webworm Hellula undalis, and it has as many generations a year as its hosts do.[1][2]
A female can lay up to 800 eggs in her lifetime, placing one egg in each diamondback larva and preferring third- and fourth-stage caterpillars.[2]
The wasp larva feeds inside the caterpillar, which lives long enough to spin its cocoon. A few days later the wasp larva kills it and spins its own cocoon inside the host's, and the adult wasp emerges about a week after that.[2][1]
At 77°F, development from egg to adult takes about 16 days; at 59°F it can take up to 36 days.[2]
It spends the winter as a pupa in its cocoon, in the remains of the crop.[1][2]
Parasitized caterpillars feed normally at first but grow sluggish and eat less as they mature; UF/IFAS reports they eat 80 percent less than unparasitized larvae.[2][1]
Damage and signs
- None; it is a beneficial insect. VegEdge reports it parasitized 62 to 82 percent of the diamondback moth larvae collected from Minnesota cabbage from 1994 to 2003, and rates above 90 percent have been recorded in South Carolina.[2]
- It is often abundant in fields not treated with insecticides, and UF/IFAS notes that major diamondback moth outbreaks are more likely in heavily sprayed fields.[2][1]
Health risk
A beneficial parasitic wasp. Neither UF/IFAS nor VegEdge mentions any risk to people or pets; the female's ovipositor is used to lay eggs in caterpillars.[1][2]
Control
Prevention
Plant nectar sources. In feeding studies compiled by VegEdge, adults lived 27 days on sweet alyssum, 21 on buckwheat, 20 on wild mustard, 18 on wild carrot, 14 to 20 on yellow rocket and 15 on kale.[2][1]
Biological control
Let it work. VegEdge reports the wasp is not available commercially, and UF/IFAS advises conserving existing populations by planting nectar sources and limiting pesticide use.[2][1]
Pesticides
Avoid broad-spectrum insecticides. Adults are very susceptible to most insecticides used against the diamondback moth, such as permethrin, carbaryl and methomyl, and spinosad has been reported to kill all exposed adults within 8 hours. Wasp larvae also die when an insecticide kills their host.[2][1]
Bacillus thuringiensis var. kurstaki and tebufenozide have been reported to cause little harm to adult wasps. Bt eaten by a parasitized caterpillar can still affect the wasp inside, but Bt breaks down quickly, and because parasitized caterpillars eat less they may not take in a lethal dose.[2]
Common questions
- What is the small black wasp around my cabbage plants?
- If it is slender, black and about 1/4 inch long, with dark brown and yellow striped legs, it may be Diadegma insulare, a parasitoid of diamondback moth caterpillars. Females have a tail-like ovipositor at the tip of the abdomen.[2]
- Can I buy Diadegma insulare to release?
- VegEdge reports it is not available commercially and has been reared for research. The practical approach is to conserve wild wasps with nectar plants and careful insecticide choices.[2][1]
- Which sprays are easiest on this wasp?
- VegEdge reports that Bacillus thuringiensis var. kurstaki and tebufenozide caused minimal mortality to adults, while permethrin, carbaryl, methomyl and spinosad are harmful to them.[2]
Sources
- University of Florida IFAS Extension (Featured Creatures), “Wasp Parasitoid, Diadegma insulare, EENY-124 (A. Sourakov and E. Mitchell; archival copy).” journals.flvc.org/edis/article/download/108388/103652/150317 (accessed 2026-11-08)
- University of Minnesota Extension (VegEdge), “Diadegma insulare, VegEdge beneficial insect profile (J. C. Lee and others).” vegedge.umn.edu/beneficial-insect-profiles/diadegma-insulare (accessed 2026-11-08)
- iNaturalist, “Diadegma insulare taxon page (order and family).” www.inaturalist.org/taxa/298986 (accessed 2026-11-08)