Vegetable Field Scouting Guide: Diamondback Moth

Diamondback-Moth-adult-Stormy-Sparks
An adult diamondback moth. Photo by Alton “Stormy” Sparks

American Vegetable Grower® queried several experts in the field to learn how to scout for and manage the notorious diamondback moth (DBM). This month, our contributors are Alton “Stormy” Sparks, University of Georgia; John Polumbo, University of Arizona; and Bonnie C. Wells of the University of Florida.

Diamondback Moth Basics

Scientific name: Plutella xylostella
Crops affected: Brassica vegetables (preference for cabbage, broccoli, cauliflower, and kale). It also can be a pest of canola.
Geographical range: It’s nearly cosmopolitan (Americas, Europe, Southeast Asia, Australia, and New Zealand).

DBM’s Economic Impact

Wells: DBM is the most destructive pest of cabbage and other cruciferous vegetables worldwide. 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. 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.

Sparks: In Georgia, it is primarily a problem in the southern part of the state simply because that is where host crops are grown. When we have high populations, growers can easily spray twice or more per week throughout the season (thus considerable control costs) and still lose entire crops. I would estimate that a 20% to 30% loss is not at all uncommon. Loss of entire fields does occur.

Polumbo: The pest is normally considered a minor pest here, but was a major problem this year throughout Arizona and some areas of southern California. Surveys of local PCAs and growers indicated that yield losses in 2016-2017 averaged about 16% in cauliflower, broccoli, and cabbage. An average of eight applications were necessary to prevent economic losses (normally growers would spray one to two times for Lepidopteran insects on cole crops).

Photo by Alton “Stormy” Sparks

How to ID a DBM Problem

Sparks: Larvae feed on foliage. Crops that produce a head can be directly damaged by the larvae, pupae, and frass present contamination problems. Other caterpillar pests like young imported cabbage worms cause similar damage, but “window pane” leaf damage is usually associated with DBM. With some experience, the damage can be distinguished, but generally identification of the larvae is used to determine the species present.

Recommended Management

Sparks: In general, resistance develops to whatever DBM are exposed to. So, for an individual field, look at what has been used and try something different.

Traditional

Product

Active Ingredient

Mode of Action

Who’s making the recommendation

Radiant (Dow) spinetoram Spinosyn-nicotinic acetylcholine receptor allosteric agonist John Polumbo, University of Arizona
Proclaim (Syngenta) emamectin benzoate Avermectin-chloride channel activator Polumbo
Avaunt (DuPont) indoxacarb Voltage-dependent sodium channel blockers Alton “Stormy” Sparks, University of Georgia
Rimon (Arysta LifeScience) novaluron Inhibitors of chitin biosynthesis, Type O Sparks
Coragen, Exirel and Verimark (DuPont) chlorantraniliprole Ryanodine receptor modulators Sparks
Lannate (DuPont) methomyl Acetylcholinesterase inhibitors Sparks

Organic

Product

Active Ingredient

Mode of Action

Who’s making the recommendation

Entrust (Valent) spinosad Spinosyn-nicotinic acetyl-choline receptor allosteric agonist Polumbo
Xentari (Valent) Bt subsp. aizawai Microbial disruptors of insect midgut membranes Polumbo

Cultural

Wells: Plants in the Brassicae 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.

Known Resistance

Wells: 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.

Sparks: There are multiple classes of chemistry that “can” be effective against DBM; however, we have also had resistance to all of them.
Polumbo: We documented DBM resistance to chlorantraniliprole (Coragen, DuPont and Voliam Xpress, Syngenta) among several populations in Arizona during 2016-2017.

What Growers Can Do

Wells: 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.

Sparks: Rotation of chemistries and not exposing subsequent generations to the same chemistry is recommended for resistance management. This gets tricky when populations are high and when we have resistance to multiple chemistries. Keep Bt products in the rotation. This also helps in resistance management.

Polumbo: Scout for eggs and larvae soon after emergence or transplanting.

Wells’ 3 Tips to Managing DBM

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. 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.

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