Organic pest management of cabbage flea beetles

Adult flea beetle with chewing leaf damage
Adult flea beetle with chewing leaf damage

Description

The following tools and techniques provide examples of organic tactics for managing cabbage flea beetles (scientific name Phyllotreta cruciferae).

Target

Our target is the adult of the cabbage flea beetle adults AND soil-dwelling larval forms. Adults: tiny, black, and shiny beetles that have a flea-like jumping action. Larval forms: Rarely seen, the immature stages live in the soil until spring warmth causes them to emerge. Note: Cabbage flea beetles are not related to fleas at all.

Timing

General calendar time: adults emerge between late May through early June (in Minnesota)

Degree days: adults emerge between 150-250 degree days (DD)

The time to control these beetles adults is when they emerge from the ground in the spring. In Minnesota, and around the Twin Cities metropolitan area, a common time for cabbage flea beetle adults to emerge is late May (around May 20) through early June (around June 1).

We can improve the precision of our timing by using a simple degree day (DD) calculation formula. This method take the average daily temperature (the daily high temperature and the daily low temperature divided by 2) and subtract it by the lowest average temperature a typical insect begins to move (50 degrees F). If the number at the end of it is over 50, we count that as insect “degree days”.

By using the simple degree day formula, growers can more precisely zero in on a more reliable window of time that an insect will emerge.

Simple Degree Day Online Calculator (University of Wisconsin Extension)

Tools

Tool #1: Trapping with yellow sticky cards (Type: Mechanical/Monitoring)

For all-sized plots: Place yellow sticky cards at regular intervals. around perimeter and within plot area at least 1-3 days before earliest expected emergence. Suggested placements:

  • Small plots. 12×12 sq. ft. or less. 8 traps total. Place at least one recipe card-sized trap at each corner, and additional four within a 4×4 square within the plot.

12x12 (1000 × 563 px) (750 × 422 px)

 

  • Medium plot. 24×24 sq. ft. 16 traps total. Place one trap at each corner, plus one in the middle of each side (~8 traps total around the perimeter). Do the same at least once within a square that is 8×8 within the plot (~8 traps within the growing area). 

24x24 (1000 × 563 px) (750 × 422 px)

 

  • Large plot. 36×36 sq. ft. 28 traps total. Place one trap about every six feet around the perimeter of the plot; place at least 8 additional sticky traps within a 12×12 square within the plot about every 4 feet.

36x36 (1000 × 563 px) (750 × 422 px)

Tool #2: Plant a “trap crop” (Type: Cultural)

For small to very large plots: Plant a cool-hardy “trap crop” among your primary crop that attracts adult cabbage flea beetles. Their attraction will distract them from your primary crop, diminish their feeding time, and lure them to lay eggs away from your primary crop.

  • Late April/Early May: Plant trap crops in mid-to-late April
  • Late May/Early June: Let adult flea beetles emerge on trap crop
  • Early to mid June: Treat trap crop with insecticidal soap with neem oil, or burn or use organic-approved botanical-based pesticide, such as Pyganic

 

Tool #3: Spray insecticidal soap with neem oil (Type: Chemical)

Insecticidal soap with neem oil basically coats the insects with a soap-like substance that slows them down. In addition to slowing them down physically, the neem oil’s natural antifeedant properties interferes with their digestion and/or willingness to feed. Without feeding, the cabbage flea beetles don’t get as much energy. Females who feed less will lay fewer eggs.


Tool #4: Use row cover material to protect crops (Type: Cultural)

Row covers are effective. Though they may be time intensive to setup, take down, and store, their effectiveness is guaranteed to protect against plant-hopping cabbage flea beetles — and other adult plant-feeding insects. In addition to protection from insects, row covers may also provide some protection from sun/wind exposure, heavy rains, hail, and other weather-related pressures. Row covers may be even more effective when used in conjunction with nematodes, who can feed on soil-dwelling stages of flea beetles.


Tool #5: Apply pesticides with Beauvaria bassiana to attack soil-dwelling stages

The Beauveraia bassiana is a soil-dwelling fungus that attacks the soft-bodied stages of certain insects like flea beetles. The B. bassiana strains GHA and ATCC 74040 are effective in reducing flea beetles, but these formulations might need to be purchased from vendors who specialize in agricultural pest management tools.

Insects killed by fungus Beauvaria bassiana

Technique(s)

Technique for tool #1: Aggressive trap approach
  • Increase # of traps during heavy infestations.

Technique for tool #2: Plant a “trap crop” (Type: Cultural)
  • Use radishes as a trap crop
  • Remove/compost trap crops after 1st emergence

Technique for tool #3: Spray insecticidal soap with neem oil
  • Apply more than one application before plants flower and to increase effectiveness and likelihood of contact.
  • Apply 1x early in the morning and 1x late afternoon during heavy infestations

Technique for tool #4: Use row cover material to protect crops
  • Apply neem insecticides for plants under the row cover to ensure that flea beetles that might emerge under the row cover slo come into contact with the neem oil

Control measures outside of the growing season

Tool: Biological control with beneficial nematodes

Flea beetles lay eggs close to the base of plants. Larvae hatch from these eggs and feed on plant roots during the growing season. If you receive regular and high pressure from cabbage flea beetles, consider applying beneficial nematodes after the adult flea beetles reach their peak in mid-to-late July. The beneficial nematodes, if they establish, should provide pressure to future populations of cabbage flea beetles within the soil areas where they overwinter. Apply nematodes early in the A.M. or late in the evening when soil is moist to wet. Cool, cloudy weather is ideal for releasing nematodes during the growing season. Water to help nematodes disperse throughout the growing area.

Steinernema spp. nematodes emerging from a larva

For more information

Contact Green Noise LLC at neil.d.cunningham@gmail.com

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