Flea beetles: row covers, trap crops and timing

How to recognize flea beetles

Flea beetles are small, shiny beetles with enlarged hind legs that jump when disturbed (UC IPM). Minnesota puts most adults at 1/16 to 1/8 inch long, colored black, bronze, bluish, brown or metallic gray, some with stripes (UMN). Most species are host-specific; the pale-striped flea beetle is a generalist (UMN).

Damage. Adults make shallow pits and small, rounded, irregular holes, usually under 1/8 inch, a pattern Minnesota calls unique to flea beetles (UMN). On waxy broccoli leaves they leave pits; on mustard and potato, shot holes about 1/16 to 1/8 inch wide (WSU PNW640). Heavy feeding can kill seedlings and young transplants and stunt older plants (WSU PNW640). Larvae feed on roots, usually without much harm (UMN, WSU PNW640), but tuber flea beetle larvae groove and tunnel into potatoes (WSU PNW640), and on carrots larvae can cause sunken spots (UC IPM). Some species spread plant diseases, such as bacterial wilt of corn (ATTRA, UMD).

When they feed

• Winter. Adults overwinter in leaf litter, hedgerows, windbreaks and wooded areas (UMN), or in plant debris, grassy field borders and ditch banks (WSU PNW640). Vermont says brushy or woody areas near fields, not grassy areas (UVM).
• Spring emergence. In early spring, at about 50 degrees (UVM); in Maryland, late April to early May, which makes them among the earliest insects to appear (UMD). They feed on weeds until crops are available, so seedbeds and new transplants are hit first (ATTRA).
• Eggs and larvae. Eggs are laid in the soil at the base of host plants and hatch in one to two weeks; larvae feed on roots and then pupate in the soil for 11 to 13 days (UVM).
• Generations. Usually one or two a year (UMN, UC IPM, UMD). In Washington the crucifer flea beetle has one and the potato-feeding species two to three; Oregon and Idaho see more (WSU PNW640). UMass describes two flushes of solanaceous flea beetle adults, late May to early June and mid-July to mid-August (UMass).
• Peak damage. Spring is the most damaging time; by summer, crops at the 4- or 5-leaf stage usually tolerate feeding and adult numbers drop (UMN). Populations tend to be high after mild winters and in hot, dry spring weather (UMD).

Crops at risk

Minnesota lists radishes, broccoli, cabbage, turnips, eggplant, peppers, tomatoes, potatoes, spinach and melons (UMN). Maryland names eggplant, corn and the cabbage family as most susceptible (UMD). In the cabbage family, Vermont separates two groups: European brassicas such as broccoli, cabbage, collards and kale grow waxier leaves and are less at risk past the two- or three-leaf stage, while Chinese cabbage, tatsoi, mizuna, komatsuna, turnip, mustards, red Russian kale and rutabaga stay vulnerable at every stage. Radish, daikon and arugula are highly attractive (UVM). Eggplant seedlings establish slowly and can be turned into lace, while tomatoes and potatoes often outgrow the damage (UMass). See the radish and kale pages for those crops.

Step 1: time the planting

1. Plant susceptible crops as late as practical; warmer weather speeds growth, so plants tolerate more feeding (UMN). Vermont delays planting until overwintered beetles have emerged (UVM).
2. For a fall crop, Washington State suggests planting brassicas in mid to late July (WSU PNW640). The frost dates page explains how to find your own season.
3. For eggplant, Maryland sets transplants out two weeks after all danger of frost and lays black plastic mulch about a month before planting (UMD).
4. Avoid planting highly susceptible crops where potatoes grew before, where tuber flea beetles build up (WSU PNW640). Rotation alone does little, because adults move easily (WSU PNW640, ATTRA).

Step 2: cover and seal

• When. Put floating row cover on immediately after seeding or transplanting (UVM, UMass), at or before crop emergence (ATTRA), and before beetles arrive (WSU PNW640).
• Seal it. Seal the edges continuously, ideally with soil; bags of sand or rock must be close enough to hold the seal in wind. Avoid torn covers, re-cover promptly after weeding and check underneath every few days (UVM).
• Start clean. Covers work best where the bed follows a crop flea beetles do not attack; otherwise beetles can emerge from the soil underneath (UVM, ATTRA), where they feed out of reach of natural enemies (WSU PNW640).
• Hoops. Drape the cover over hoops for eggplant so it does not rest on the plants (UMD, UMN, UMass).
• How long. Until the sixth-leaf stage (UC IPM), or until eggplant flowers (UMD). Remove covers before flowers open on crops that need pollinators (UMN).

Step 3: add a trap crop

A trap crop is a more attractive plant sown to pull beetles off the main crop. Flea beetles are drawn to the tallest, earliest plants (UMN).

Once beetles gather on the trap crop, Washington State lists three options: an approved insecticide, hand removal or an insect vacuum followed by freezing, or tilling the trap crop under. Beetles can move back to the crop if the trap plants flower, set seed or suffer drought or heat, and trap crops may not fully protect during heavy infestations (WSU PNW640). The companion planting chart lists the other trap-crop pairings.

Step 4: scout and decide

Scout new plantings every day or two while plants are small, counting beetles without disturbing the leaves (UVM), or on cool mornings, when beetles move more slowly (UMN). Yellow sticky traps help detect beetles (UMN); UC IPM uses white sticky traps, and ATTRA cites traps every 15 to 30 feet of row. Sticky traps also catch beneficial insects, so they are best for monitoring (WSU PNW640). On eggplant, damage is a better measure than counts, because the beetles are hard to count (UMass).

Other measures

• Clean up. Remove crop debris in fall (UC IPM, UMN, UMD), control weeds in and around beds (UMN) and watch for volunteer mustards, a particular problem (WSU PNW640).
• Mulch. UC IPM suggests reflective mulch soon after planting. Straw or large-leaf yard-waste mulch gave moderate improvement in Oregon State potato trials, and living mulches such as clover and vetch may help but compete with the crop (WSU PNW640).
• Kaolin clay. A particle film that may block feeding, with mixed results; reapply after rain (UMN, UMD, UVM).
• Natural enemies. The native wasp Microctonus vittatae kills adults but is not sold commercially (UMN), and parasitism rates are low (WSU PNW640). Lacewing larvae, big-eyed bugs and damsel bugs also feed on them (WSU PNW640).
• Nematodes. Commercial insect-parasitic nematodes applied to the soil attack the larvae; apply when larvae are present and keep the soil moist (UVM, ATTRA).

Insecticides

UC IPM says insecticides are generally not necessary for flea beetles, are not very effective when populations are high, and that often only a few outside rows need treatment. Minnesota suggests spot-treating patchy infestations, including edges (UMN). Active ingredients the sources name include pyrethrins, spinosad, carbaryl, malathion, permethrin, lambda-cyhalothrin and cyfluthrin (UMN); Maryland names pyrethrum, neem and spinosad for severe infestations (UMD). Vermont reports neem and insecticidal soap as not very effective in research, pyrethrins and kaolin as mixed (UVM). ATTRA calls botanical and soap sprays a last resort because they are broad-spectrum, and Washington State warns that natural insecticides can also harm beneficial insects (WSU PNW640). The fungus Beauveria bassiana has some organically approved formulations and is applied in the evening, because sunlight kills the spores (WSU PNW640).

This page gives no rates. Follow the product label, which is the law; check that the vegetable is listed on it and observe the waiting period between application and harvest (UMN). Organic growers should check products with their certifier (UVM).

Where sources differ

Related: cabbage worms, the other main pests of the cabbage family, and the spinach page.

Flea beetle checklist

• Tiny round holes and pits on seedlings, and jumping beetles: flea beetles.
• Susceptible crops planted after the spring flush where possible.
• Row cover on at seeding or transplanting, edges sealed with soil, on a bed that did not hold a host crop.
• Trap crop of radish or mustard sown first and kept young, beetles dealt with promptly.
• Seedlings scouted every day or two; act only above the action levels.
• Any insecticide labeled for the crop, with the pre-harvest interval observed.