A Wasp Parasitoid, Diadegma insulare (Cresson) (Insecta: Hymenotera: Ichneumonidae)1
Diadegma insulare is found in the United States and South America: New Hampshire west to southern British Columbia, south to Florida, Texas, and California; Hawaii, West Indies, and Mexico south to Venezuela.
These wasps are found in cruciferous crops, overwintering as a cocoon. The number of generations per year corresponds to the number of generations of its hosts: Hellula undalis (F.), Plutella armoracia Bsk., and P. xylostella (L.), the diamondback moth.
The egg is clear, and is hard to distinguish from the host's fat body when it is dissected. Unlike Cotesia plutella, another parasitoid of the diamondback moth, the egg is rounded rather than pointed and lacks a projection.
The larva of D. insulare is white and can be noticed more easily in a dissected host due to its extensive movements. It looks strongly segmented and bears a short (1/4 of the total length of the larva) narrow "tail".
The larva of D. insulare stays inside the diamondback moth larva until the latter spins a cocoon and is ready to pupate. Once emerged, D. insulare's larva spins its own cocoon inside the cocoon of the moth.
The length of this wasp does not exceed 6 mm. The female has a well defined ovipositor.
D. insulare is the most important naturally occurring enemy of the diamondback moth as it can parasitize up to 90% of its larvae. Not only the parasitized larvae die, but they also do less damage, consuming 80% less food than the non-parasitized individuals. D. insulare can be reared in captivity, but mass rearing presents many challenges. For instance, the wasps rarely parasitize larvae that are reared on artificial diet. Hence, the larvae have to be presented to the wasps while feeding on a cruciferous plant (e. g., cabbage).
D. insulare is highly susceptible to pesticides, which kill adult wasps through direct contact. The larvae inside the host also die when pesticides kill their hosts. Thus, major outbreaks of the diamondback moth are more likely in fields that are sprayed heavily, and to avoid such outbreaks one needs to conserve the Diadegma populations by planting its nectar sources and limiting the use of pesticides.
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Mitchell, E. R. et al. (February 2000). Stage by stage comparison of parasitoids important in biocontrol of cabbage pests. USDA. http://www.usda.ufl.edu/biocontrol/guide.htm (March 2000).