Flea Beetle, Brassica

Phyllotreta cruciferae

Identification:

The crucifer flea beetle is uniformly black and shiny, about 2 mm in length, while the striped flea beetle has two yellow stripes on its back. These flea beetles only feed on brassica crops; those found on corn or solanaceous crops are different species. Though they prefer the tender leaves of Brassica rapa and B. juncea crops such as arugula, tatsoi, mizuna, bak choi, and mustard, they will also feed on the more waxy Brassica oleracea crops such as broccoli, cabbage, kale, and collard.

Life Cycle:

Adults spend the winter outside the field, in shrubby or woody borders, and move into fields in May and begin feeding and mating. Eggs are laid in soil near the plant. Larvae will hatch in 11-13 days. Larvae feed on root hairs and pupate underground. . After about 2 weeks, the larvae will pupate, and emerge after another week as the next generation of adults in mid-June. This cycle repeats itself and a second summer generation emerges in late July to feed on fall brassica crops before moving outside of the field to forested areas for the winter. Feeding declines in September.

Crop Injury:

Adults feed on leaves and stems, leaving small round holes.  Heavy feeding can kill seedlings, and moderate damage can delay maturity, reduce yield, or make crops unmarketable. Crops with more waxy leaves (cabbage, broccoli,kale) are less attractive and feeding is more restricted to leaf margins, especially as crop matures. Crops with glossy leaves (bok choy, Nappa cabbage, mustard) are highly attractive, and the whole leaf is damaged. These crops are susceptible from planting until harvest.

Monitoring & Thresholds:

Maturing plants should be scouted frequently. When plants are young, an average of 1 beetle per plant or 10% average leaf damage is a reasonable threshold for chemical intervention. Because brassica crops differ greatly in susceptibility and attractiveness there is no fixed economic threshold that applies to all crops. Beetles can be difficult to count, as they hop when disturbed. Yellow sticky cards placed in the canopy or feeding damage can be used to determine beetle populations. Control if damage to cotyledons or seedlings is stunting growth, or if damage to greens will reduce marketability.

Cultural Controls & Prevention:

• Rotate spring crops as far as possible from last season's fall brassica crops, and where flea beetles will overwinter.
• Separate early and late-season brassica crops.
• Avoid early brassicas (before July) to break the reproductive cycle. (This may not be economically feasible for many growers.)
• Cover crops with spun-bonded row covers. Row cover provides excellent protection if well secured with soil or bags around all edges immediately after seeding or transplanting. Remove and replace the same day for cultivation, as needed.
• Provide adequate water and nutrients for crop growth; avoid soil compaction.
• Incorporate and till crops immediately after harvest to kill larvae feeding on roots. 
• Control brassica weeds. Brassica weeds also harbor flea beetles (both adults and larvae) and reduce the efficacy of your crop rotation schemes that aim to break the pest cycle by changing crop families. 
• Trap cropping can take advantage of the flea beetles' preference for particular brassicas by using the preferred species or varieties as a draw. Their numbers will build up in the more attractive plants, and can be killed there with an insecticide application, protecting the main crop and reducing spray area and time.
  • Make sure the trap crop is established before the main crop (the one you are trying to protect) or is at least as mature (e.g. transplanted same day). Direct-seeded crops can be used around transplants if seeded 7-14 days earlier.
  • Use a fast-growing, vigorous cultivar for the trap crop.
  • Use a border crop to prevent beetles from moving farther into the field. Traps at ends of rows help make a complete perimeter, which stops beetles coming from all directions. Interior trap crops also can act as a ‘sink’ within the field.
  • Spray only the trap crop to kill the accumulated beetles, and to avoid having to spray the main crop. You also want to keep the trap crop healthy enough to do its work, and potentially be harvestable as well—you may need to fertilize, re-seed, or otherwise maintain this trap crop because if it gets too ragged, the beetles will not enjoy feeding on it and will move back into your main crop. Use a longer-residual product, if possible.
  • Combine with a repellent on the main crop, e.g. coat the main crop with Surround WP and use a trap crop as part of a “push-pull” system. 

Chemical Control

Several synthetic pyrethroids (Group 3A), carbamates (Group 1A), neonicotinoids (Group 4A, either as foliar or soil drench), and diamides (Group 28) are labeled for flea beetle in brassicas. Avoid repeated use of one type of chemistry over multiple generations or using both soil and foliar applications of the same group. Systemic insecticides can provide longer-term control against damage, although beetles may still be seen when scouting. Diamide products (Exirel and Harvanta for foliar applications, Verimark for soil), are systemic and provide control against flea beetles as well as other brassica pests like caterpillars, cabbage aphid, and, if applied to the soil pre-plant, cabbage root maggot. Be aware that systemic insecticides may have longer days-to-harvest intervals.

For organic farmers, the choice of effective chemistries is limited to spinosad (Entrust), Surround WP (kaolin clay), and pyrethrin (Pyganic). In UMass trials, Entrust showed the greatest efficacy in suppressing flea beetles and reducing damage. Pyrethrin (Pyganic EC 5) showed poor to moderate efficacy in our trials but is reported by growers to cause a significant short-term knockdown. Abby Seaman, NYS IPM, found in 2013 that Entrust, as well as both Venerate and Grandevo, two OMRI-approved bioinsecticides, were all found to significantly reduce damage from flea beetle on cabbage but pest pressure was very low in that study. Growers often hesitate to use Surround because of difficulty mixing and spraying— some growers have found that using a masonry or sheet-rock drill to mix up the material in a 5-gallon bucket before adding to a backpack sprayer works to get the clay into suspension. If you want to apply Surround using a tractor-mounted sprayer you must have mechanical agitation or the material will not go into suspension and it will clog up your nozzles. It is probably worth figuring out how to do this if you struggle with getting your early season transplants to survive the onslaught of flea beetles, and it can also be useful in protecting cucurbit transplants from striped cucumber beetles, which vector bacterial wilt.

For current information on production methods (including varieties, spacing, seeding, and fertility), weed, disease, and insect management, please visit the New England Vegetable Management Guide website.

Crops that are affected by this insect:

• Cabbage, Broccoli, Cauliflower, and Other Brassica Crops
• Lettuce, Endive, Escarole
• Mesclun
• Radish
• Spinach