Diadegma insulare

J.C. Lee, M.A. Carrillo, and W.D. Hutchison
Department of Entomology, University of Minnesota

Diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is a major pest of cruciferous plants worldwide and is quickly developing resistance to many insecticides. Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) is one of the most important parasitoids attacking diamondback larvae in North America. Diadegma insulare is native to Central America and it also attacks other lepidopteran pests such as the cabbage webworm, Hellula undalis (F.) (Pyralidae), and Plutella armoricae (Busck) (Plutellidae). Diadegma insulare is often abundant in fields not treated with insecticides. From 1994 to 2003, D. insulare parasitized 62-82% of diamondback moth collected in cabbage plants in Minnesota. Parasitism rates have been recorded to be greater than 90% in South Carolina. Diamondback larvae parasitized by D. insulare initially feed at the same rate as unparasitized larvae. As parasitized diamondback larvae mature, they become sluggish and feed less. Therefore, encouraging D. insulare populations not only reduces pest populations but should also reduce the per capita feeding by diamondback larvae and subsequent damage to the crop.

This ichneumonid wasp is small, ¼ inch long, slender and black.  It has dark brown and yellow striped legs. The upper side of the abdomen is dark but the underside is yellow-green. Females have a distinctive “tail” at the end of the body. This “tail” is the ovipositor which serves to lay eggs into the host larva. Characteristic of ichneumonids, the forewing has a cell in the shape of a horse head (see image, left).

Based on the outward appearance, a parasitized larva is indistinguishable from an unparasitized larva. However, the parasitoid egg or larva is easily found in the host by dissection. The parasitized and unparasitized diamondback at the early pupal stage are also indistinguishable; they are soft, green and encased in a mesh cocoon. However, parasitized diamondback remain in the early pupal stage for 2-5 days, whereas unparasitized hosts develop within a day or two (J.C. Lee, personal observations). At the later stage, the moth and parasitoid pupae are distinct. The moth pupae is tapered and changes from pale to dark as it matures. Diadegma insulare pupae are eliptical, initially pale, and then dark brown with a characteristic white band around the middle.

Female D. insulare are fecund and lay up to 800 eggs in their lifetime. Females lay a single egg into diamondback larvae, preferably in 3rd or 4th instars. The egg hatches inside and the new parasitoid larva feeds within the host larva. The host larva will survive long enough to spin its cocoon. A few days later, the D. insulare larva kills the host and makes its own pupal case in the host cocoon. In about one week, the parasitoid adult emerges. At 77°F, parasitoid development from egg to adult takes about 16 days, whereas it can take up to 36 days at 59°F. This parasitoid overwinters in the pupal stage within the remnants of the crop.

Currently D. insulare is not available commercially; however, it is being reared for research purposes. Diadegma insulare has been reared in field cages and in the laboratory with hosts raised on artificial diet.  The parasitoid pupa can be stored at 39.2°F for up to 3 weeks. Storage for longer periods results in 50% or lower emergence rates. D. insulare has complementary sex determination which causes a highly male biased population when inbreeding occurs. Many laboratory colonies have crashed due to male bias. Several steps can be taken to avoid inbreeding and male bias. For instance, wild D. insulare should be introduced in a colony as often as possible. In addition, if diamondback hosts are reared on plants, the plants should be well fertilized. A higher percentage of females emerge from hosts raised on well-fertilized plants than unfertilized plants. If possible, female D. insulare used to parasitize host larvae should be 2-7 days old, because this is the time when they produce the highest female biased ratio. Wasps in the parasitizing arena should not be crowded.

A flowchart / instruction sheet is available for rearing D. insulare on a small scale to produce about 50-100 D. insulare pupae a week. Jana Lee has reared them successfully with this method for two years. Materials/space needed: 1) a greenhouse or separate area to grow plants, 2) 2-3 small cages (1 x 1 x 1 ft) for diamondback moth adult to lay eggs, 3) large cage or several small cages for diamondback larvae to develop, 4) one small cage for diamondback moth production, and 5) a D. insulare parasitizing arena- a large cage or a growth chamber (16L:8D, 26°C, 75 R.H.). Bug dorms or plastic containers with mesh sides work well as cages. Mustard seed is cheap, grows quick and the host and parasitoid develop well on them. 

It is well known that insecticides can disrupt the natural equilibrium between pests and natural enemies. Adult D. insulare are very susceptible to most chemical insecticides (e.g., permethrin, carbaryl, azinphosmethyl, and methomyl) used to control it host the diamondback moth. In addition, D. insulare can also be negatively affected by the actinomycete-derived insecticide spinosad, which has been reported to cause 100% mortality after only 8 h of exposure. On the other hand, insecticides such as tubefenozide and Bacillus thuringiensis (Bt) var. kurstaki have been reported to cause minimal mortality to adult D. insulare. Immature stages of the parasitoid can be affected by both contact and ingested insecticides by its host.  Hosts exposed to contact insecticides will die as will the immature parasitoid. In addition, immature parasitoids may still be indirectly affected if the host ingests these pesticides, including Bt (Idris and Grafius, 1993b). However, Bt degrades quickly reducing the risk to the parasitized host. Since parasitized host feed slower, the amount of pesticide ingested may not kill the host and the parasitoid.As in many cases, an integrated pest management (IPM) program to control diamondback moth is the most efficient way to keep pest population densities below economically damaging numbers. In addition, an IPM program will slow the development of resistance of the diamondback moth. The combinations of natural enemies (e.g., D. insulare), biologically-derived products (e.g., Bt), and harmless chemical insecticides (e.g., tebufenozide) to control diamondback moth, and the use of proven non-lethal fungicides (e.g., chlorothalonil) for disease control is recommended as part of and integrated crop management.

Fox, L.R., K.M. Kester, and J.Eisenbach. 1996. Direct and indirect responses of parasitoids to plants: sex ratio, plant quality and herbivore diet breadth. Entomologia Experimentalis et Applicata 80: 289-292.

Gourdine, J.S., G.S. McCutcheon, A.M. Simmons, and G.L. Leibee. 2003. Kale floral nectar and honey as food sources for enhancing longevity and parasitism of Diadegma insulare (Hymenoptera: Ichneumonidae), a parasitoid of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae). Journal of Agricultural and Urban Entomology 20: 1-6.

Hill, T.A., and R.E. Foster. 2000. Effect of insecticides on the diamondback moth (Lepidoptera: Plutellidae) and its parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae). Journal of Economic Entomology 93: 763-768.

Hu, G.Y., E.R. Mitchell, D.H. Sieglaff, and J.S. Okine. 1998. Field production of two species of parasitoids of the diamondback moth (Lepidoptera: Plutellidae) Florida Entomologist 81: 526- 534.

Idris, A.B., and E. Grafius. 1993a. Differential toxicity of pesticides to Diadegma insulare (Hymenoptera: Ichneumonidae) and its host, the diamondback moth (Lepidoptera: Plutellidae). Journal of Economic Entomology 86: 529-536.

Idris, A.B., and E. Grafius. 1993b. Pesticides affect immature stages of Diadegma insulare (Hymenoptera: Ichneumonidae) and its host, the diamondback moth (Lepidoptera: Plutellidae). Journal of Economic Entomology 86: 1203-1212.

Idris, A.B., and E. Grafius. 1995. Wildflowers as nectar sources for Diadegma insulare (Hymenoptera: Ichneumonidae), a parasitoid of diamondback moth (Lepidoptera: Yponomeutidae). Environmental Entomology 24: 1726-1735.

Johanowicz, D.L., and E.R. Mitchell. 2000. Effects of sweet alyssum flowers on the longevity of the parasitoid wasps Cotesia marginiventris (Hymenoptera: Braconidae) and Diadegma insulare (Hymenoptera: Ichneumonidae). Florida Entomologist 83: 41-47.

Lee, J.C., G.E. Heimpel, and G.L. Leibee. 2004. Comparing floral nectar and aphid honeydew diets on the longevity and nutrient levels of a parasitoid wasp. Entomologia Experimentalis et Applicata. (In press).

Muckenfuss, A.E., and B.M. Shepard. 1994. Seasonal abundance and response of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), and natural enemies to esfenvalerate and Bacillus thuringiensis subsp. kurstaki Berliner in coastal South Carolina. Journal of Agricultural Entomology. 11: 361-373.

Okine, J.S. E.R. Mitchell, and G.Y Hu. 1996. Low temperature effect on viability of Diadegma insulare (Hymenoptera: Ichneumonidae) pupae and effect of this parasitoid on feeding rate of diamondback moth larvae (Lepidoptera: Plutellidae). Florida Entomologist 79: 503-509.

Sieglaff, D.H., E.R. Mitchell, and G.Y. Hu. 1998. Evaluation of rearing methods for Diadegma insulare (Hymenoptera: Ichneumonidae) an endoparasitoid of the diamondback moth (Lepidoptera: Plutellidae). Florida Entomologist 81: 578-582.

Wold-Burkness, S.J., W.D. Hutchison, J.C. Lee, R.L. Hines, P.C. Bolin, and G.E. Heimpel.  2005.  A long-term survey of parasitoid species composition and parasitism of Trichoplusia ni (Lepidoptera: Noctuidae), Plutella xylostella (Leipidoptera: Plutellidae), and Pieris (=Artogeia) rapae (Lepidoptera: Pieridae) in Minnesota cabbage.  Journal of Entomological Science. 40: 211-221.