Diamondback Moth
ID
444-007 (ENTO-581NP)
Introduction
Diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae), is the most important pest of brassica crops worldwide. Its common name is derived from the white diamond shapes formed when the wings are closed by the adult moth (Fig. 1). In Virginia, it is active from May to November undergoing multiple generations. The larvae feed on the leaves of brassica weeds and crops such as broccoli, collards, cabbage, and kale (Fig. 2). Large populations can cause significant damage that affects both crop yield and marketability. DBM is a significant problem mostly due to insecticide resistance. Populations of DBM have developed resistance to most insecticides used against it, making it one of the most challenging pests to control.
Identification
DBM life cycle from egg to adult requires ~283 degree days (usually less than 30 days), with 50°F as base temperature. It can have multiple generations per season. Eggs are oval-shaped, cream colored and approximately 0.4 mm long. They are deposited on leaves in a cluster of 2-8 eggs (Fig. 3). Eggs hatch in a few days. There are 4 instars of DBM larvae.
Early instars are pale in color while later instars are green and can reach up to 11 mm in size (Fig. 4).
When touched, DBM larvae will twitch frequently as a defense mechanism to deter predator feeding.
Larvae also may drop on a thread of silk to escape. Adult moths are 6-12 mm in length and are brown with a light-colored diamond pattern along the back of their wings. Adults have a short lifespan of ~12- 16 days.
Natural Control
Biological control of DBM plays a role in keeping pest densities in check. In Virginia, the hymenopteran parasitoids Diadegma insulare and Oomyzus sokolowskii are the primary natural enemies of DBM. They attack larvae. Among the two species, D. insulare is found more commonly in Virginia (Fig. 5). D. insulare larvae crawl out from inside the DBM pupa and use its casing for protection when pupating (Fig. 6). The D. insulare pupa is cylindrical and brown in color, while the DBM pupa is green and misshapen. High rates of parasitism of DBM (>75%) were found in multiple locations around Virginia in 2022 indicating the impact of this parasitoid.
Monitoring
Sampling for DBM can include visual observations for larvae and feeding injury (Fig. 7) as well as adult moth trap captures. Thresholds for monitoring may vary by state and crop type but often all lepidopteran larvae are counted and grouped together for action thresholds. Sampling is recommended once a week with a minimum sample size of 50 plants. Treatment for cabbage in Virginia is recommended if at least > 20% of plants contain 1 or more lepidopteran larvae before the heading stage, or > 5% of plants if 1 or more larvae is found between heading and harvest.
DBM moth activity can be monitored in fields using sticky panel type traps, such as Delta or wing traps, baited with a commercially-available DBM sex pheromone lure. Traps should be checked weekly and stick liners replaced as needed. There is not a set threshold currently for trap captures of DBM.
Chemical Control
A wide range of insecticides are registered for control of DBM. However, resistance levels in local populations often impact the efficacy of various products. Since the mid 2000s, the best control has been achieved with Bacillus thuringiensis products, various diamide insecticides, spinosyns, various insect growth regulators, emamectin benzoate, and tolfenpyrad. To reduce resistance development, it is strongly recommended to rotate sprays of insecticide groups by their mode of action throughout the growing season, or to seek other pest management strategies such as mating disruption or rotating away from brassica crops for a period, to break the life cycle of DBM.
References Cited
Baker, P.B., Shelton, A.M., Andaloro, J.T., 1982. Monitoring of Diamondback Moth (Lepidoptera: Yponomeutidae) in Cabbage with Pheromones. J Econ Entomology 75: 1025-1028.
Cordero, R.J., Bloomquist, J.R., Kuhar, T.P., 2006. Susceptibility of two diamondback moth parasitoids, Diadegma insulare (Cresson) (Hymenoptera; Ichneumonidae) and Oomyzus sokolowskii (Kurdjumov) (Hymenoptera; Eulophidae), to selected commercial insecticides. Science Direct. Biological Control 1: 48-54.
Hill, T.A., Foster, R.E., 2000. Effect on insecticides on the diamondback moth (Lepidoptera: Plutellidae) and its parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae). Journal of Economic Entomology 93: 763-768.
Munir, S., Dosdall, L.M., O’Donovan, J.T., 2014. Evolutionary Ecology of Diamondback Moth, Plutella xylostella (L.) and Diadegma insulare (Cresson) in North America: A Review. Annual Research and Review in Biology 5(3): 189-205.
Philips, C. R., Fu, Z., Kuhar, T. P., Shelton, A. M., Cordero, R. J., 2014. Natural History, Ecology, and Management of Diamondback Moth (Lepidoptera: Plutellidae), with Emphasis on the United States. J. Integ. Pest Mngmt. 5(3): 1-11.
Riley, D., Smith, H., Bennett, J., Torrance, P., Huffman, E., Sparks, Jr, A., Gruver, C., Dunn, T., Champagne, D., 2020. Regional Survey of Diamondback Moth (Lepidoptera: Plutellidae) Response to Maximum Dosages of Insecticides in Georgia and Florida. J. Econ. Entomol.113(5): 2458-2464.
Talekar, N.S. and Hu, W.J., 1996. Characteristics of Parasitism of Plutella xylostella (Lep: Plutellidae) by Oomyzus sokolowskii (Hym., Eulophidae). Entomophaga 41: 45-52.
Talekar, N.S., and Shelton, A.M., 1993. Biology, Ecology, and Management of the Diamondback Moth. Annual Review of Entomology 38: 275- 301.
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Publication Date
February 23, 2024
Diamondback moth is one of the most important pests of brassica crops worldwide including Virginia primarily because of its ability to develop resistance to various insecticides. This fact sheet provides basic information on the biology and management of this pest.
