W. D. Hutchison, P. C. Bolin, and R. L Hines
Department of Entomology, University of Minnesota
Introduction
The diamondback moth, DBM (Plutella xylostella), is the single most destructive pest of cabbage and leafy greens worldwide. It was introduced from Europe in the nineteenth century, and is now widely distributed throughout North, Central and South America, Hawaii, and Asia. Because of its pest status worldwide, DBM has also developed resistance to numerous insecticides, including several Bacillus thuringiensis (Bt) products.
Biology & Life Cycle
Adult DBMs are brown to grayish in color and have a maximum wingspan of about 3/4 of an inch. At rest, the folded wings present an image of light-colored diamond shapes along the backs of the wings where they meet.
Interestingly, this is one of the few Lepidoptera that overwinters as an adult moth. After mating in early spring the females deposit small, almost round, yellowish-white eggs singly or in small groups on both sides of leaves of host plants. Eggs are nearly impossible to see without a handlens. Larvae hatch after a few days and begin feeding just under the surface of the leaf tissue (difficult to see without a microscope). After several days of feeding, mid-instar larvae exit the leaf and feed on the leaf surface. Larvae cease to feed when the temperature drops below 50 F. As with other insects development through all life stages is delayed during cool temperatures. In contrast, populations can increase dramatically when temperatures rise above 80 F. The life cycle may be as long as 50 days at low temperatures and as short as 15 days at high temperatures. Mature late-instar larvae are pale green and about 1/3 of an inch long.
Pupae are green and encased in a delicate, netlike cocoon on leaves or in soil debris. Four to six generations occur annually depending on summer temperatures. Adults overwinter in plant debris or in the soil.
Damage
Initial damage results in small incomplete holes caused by young larvae and larger complete holes caused by mature larvae. The entire plant may become riddled with holes under moderate to heavy populations. Larvae also feed in the developing heads of cabbage, causing deformed heads and encouraging soft rots. However, because of their small size, relative to cabbage looper (CL) and imported cabbageworm (ICW), it takes approximately 20 DBM larvae to defoliate as much cabbage as 1 cabbage looper, or about 2 ICW larvae.
Management Options
Biological Control
In the upper Midwest, DBM is usually suppressed by a very effective native parasitoid, Diadegma insulare (Family: Ichneumonidae). This beneficial wasp consistently parasitizes 70-80% of larva populations (at least for mid-and late-season cabbage in MN and WI; we have less data for early plantings, which can also be hit hard by DBM). Therefore, the level of parasitism should be accounted for when making control decisions. The graph below illustrates the high levels of parasitism of DBM. For more information on Diadegma insulare, view the VegEdge insect fact sheet.
Insecticide Options
When control is needed, and DBM is the dominant caterpillar pest present, we encourage Bt use to help conserve the beneficial insect complex (see also management tips for cabbage looper, where Bt is less effective). Bt is much easier on the beneficial insects and D. insulare than conventional insecticides.
For more information, see an overview article on Bt by Dr. Rick Weinzerl and T. Henn, University of Illinois, and the the VegEdge Diadegma insulare fact sheet by Lee et al.
Action Thresholds for Insecticide Use
| 1. Where DBM is primary insect pest (usually early-season): |
|---|
| Before Cupping |
| 50% plants infested with 5 or more larvae each. |
| Cupping to harvest. |
| 10% plants infested with 1 or more larvae each. |
| 2. Where ICW and/or CL are primary pests (usually mid- and late-season): |
|---|
| Transplant to cupping (leaf stages only) |
| 30% plants infested with 1 or more medium-large imported cabbageworm larvae and/or 1 or more cabbage looper eggs or larvae. |
| Cupping to harvest |
| 10% plants infested with 1 or more medium-large imported cabbage worm larvae and cabbage looper eggs or larvae. |
NOTE: To ensure proper use of insecticides, refer to the most recent edition of the Midwest Vegetable Production Guide (BU-7094-S).
References
Cornell University. 2003. Vegetable Disease ID and Management. http://vegetablemdonline.ppath.cornell.edu/.
Grafius, E.J., and A. B. Idris. Know your friends. Diadegma insulare, parasite of diamondback moth. 1997. Midwest Biological Control News. http://www.entomology.wisc.edu/mbcn/kyf401.html
Hard, C.G., 1991. Growing Cole Crops (FO-1862).University of Minnesota Extension Service. University of Minnesota, St. Paul, MN. http://www.extension.umn.edu/garden/yard-garden/vegetables/growing-cole-crops-in-minnesota-home-gardens/
Lee, J.C., M.A. Carrillo, and W.D. Hutchison. 2003. Diadegma insulare. VegEdge Fact Sheet, University of Minnesota Extension Service. University of Minnesota, St. Paul, MN. See Diadegma insulare page.
Mahr, D.L, and N.M. Ridgway. 1993. Biological control of insects and mites. North Central Regional Publication 481.
Pfleger, F.L., and S.L. Gould. 1999. Diseases of Cole Crops (FS-1169). University of Minnesota Extension Service. University of Minnesota, St. Paul, MN. http://www.extension.umn.edu/garden/yard-garden/diseases/diseases-of-cole-crops/
Rice Mahr, S.E., R.A. Cloyd, D.L. Mahr, C.S. Sadof. Biological Control of Insect Pests of Cabbage and Other Cruicifers. North Central Regional Publication 471.
University of Minnesota Extension Service BU-7094-S. Midwest Vegetable Production Guide for Commercial Growers. To order: https://ag.purdue.edu/btny/midwest-vegetable-guide/Pages/default.aspx