The diamondback moth (DBM) is the most destructive pest of cabbage and other cruciferous vegetables worldwide, and one of the planet’s worst agricultural pests, wreaking billions of dollars’ worth of havoc for growers annually. From causing major problems for the Australian canola industry in recent years, to causing a short supply of Brussels sprouts in the U.K. this past holiday season due to a termed “biblical” migration of the pest, to becoming increasingly hard to control in the U.S., the DBM has growers globally concerned.
The DBM is a perennial problem in the southern U.S., the region that leads North America in the production of cabbage and collard crops. DBM can overwinter and breed continuously in the warmer region producing as many as 15 generations per year.
The caterpillars of this pest moth are extremely efficient at developing resistance to all classes of insecticides, and are highly dispersive and adaptive to new environments, making outbreaks of the pest unpredictable. Insecticide resistance and the lack of natural enemies are believed to be the reasons why DBM is increasingly hard to control.
Crop Damage and Economic Losses
Crop damage is caused by DBM caterpillars feeding on leaf tissue. Caterpillars consume the tissue from the underside of the leaves, causing irregular patches that appear window-like, as they leave the upper tissue and leaf veins intact. Economic losses also can occur for crops such as broccoli when total rejection of shipments occurs from caterpillars infesting the florets. The latest total worldwide estimate for DBM management is $4 billion to $5 billion dollars annually, and $150 million to $200 million annually in the U.S.
Host Plant Resistance Recognition
DBM caterpillars feed exclusively on crucifers, or plants in the Brassicaceae family, including the head and stem crops such as cabbage, broccoli, cauliflower, and Brussels sprouts; the leafy crops such as Bok choy, collards, kale, and mustards; and the root crops, such as radish, turnip, and rutabaga. Plants in this family contain biochemicals called glucosinolates that are used as egg-laying stimulants by the DBM. Although the moths are crucifer specialists, crop species are not equally preferred, and DBM has a strong preference for collard plants. Mustards, turnips, and kohlrabi are the least preferred.
Leaf color, wax content, head compactness, and levels of glucosinolates all affect a plant’s resistance to the DBM. Shiny green or glossy leaves reportedly cause DBM caterpillars to spend more time searching for food, and less time feeding, which also might improve predation of the pest by natural enemies.
What Growers Can Do
Implementing an integrated strategy is critical to manage the DBM in cruciferous vegetable production systems. Pest management programs must focus not only on DBM, but also the entire crucifer pest complex, which includes numerous other caterpillar species, aphids, harlequin bugs, stinkbugs, and leaf beetles. This is because the presence of other early season pests is generally the motivation for use of preventative sprays in cropping systems and can decrease the presence of natural enemies and favor insecticide resistance.
A truly integrated strategy will incorporate several complementary tactics over large areas that work to suppress multiple pests. When used together, these tactics can act synergistically to enhance pest suppression, reduce insecticide resistance, and improve the sustainability of the overall pest management program in crucifer cropping systems.
1. Crop Rotation
Because DBM has a narrow host range, feeding exclusively on cruciferous plants, crop rotation away from a host plant can significantly reduce the number of pests and subsequent damage in cropping systems. However, crop demand and price may dictate if crucifer-free periods are economically feasible for commercial vegetable producers.
2. Trap Cropping
Trap cropping, or using highly pest attractive plants on field borders, can be used to intercept and retain pests such as the DBM and other crucifer pests. Collards and Indian mustard are recommended trap crops for suppressing DBM. Because collards are highly attractive to DBM, they can be planted on the periphery of cabbage or broccoli fields for pest management.
3. Insecticide Knowledge and Resistance Awareness
Insecticides have been widely used on cruciferous crops for the control of numerous different pests. Preventative applications have been historically used for the control of early season pests and has led to the use of protective applications for later season pests such as DBM. However, use patterns significantly vary between regions and change depending on resistance development and new products becoming available for use.
Because the control of the DBM has relied heavily on insecticide applications, the pest has developed resistance to nearly every class of insecticides. According to the arthropod pesticide resistance database, DBM has developed resistance to more than 80 active ingredients in more than 20 countries. DBM was one of the first agricultural pests to develop resistance to DDT in the 1950s and later to the microbial insecticide Bacillus thuringiensis (Bt).
In North America, DBM has developed resistance to all pyrethroids, most carbamates and to Bt; however, the resistance varies greatly by region. DBM is highly adaptable and has proven to develop high levels of resistance to insecticides after only a few years of use. When new insecticides hit the market, it is absolutely imperative they are used judiciously and combined with other management tactics for the control of DBM and other cruciferous pest species, so that insecticide resistance can be prevented.
For more information on the management of DBM and other insect pests of crucifers, the following links might be of help: