STATE COLLEGE — If left unchecked, leaf-feeding Colorado potato beetles can completely defoliate potato vines in a matter of days.

Potato plants can tolerate up to 20-30% defoliation prior to flowering, 5-10% defoliation after flowering, and 30% defoliation at tuber formation before a reduction in yield is observed. Growers should begin scouting their potato fields at plant emergence and weekly thereafter. For commercial growers scouting potato fields, selecting 10 sites per field is recommended, and using a V or W-shaped path through the field to scout. At each site, choose one potato stem from each of five plant adjacent plants and count and record all adults, large larvae, and small larvae. If more than 50 adults, 75 large larvae, or 200 small larvae are counted per 50 stems, insecticide treatment is recommended.

Home gardeners with small potato patches can hand-pick larvae and adults. Insecticides are available to help suppress populations. However, Colorado potato beetle (CPB) may be resistant to some common products such as carbaryl and malathion because of repeated use over the last few decades. Biological insecticides such as neem, Beauvaria bassiana, and spinosad may help suppress populations, but they should be used early in the season before larvae are large or develop into adults.

Commercial growers have more products available but are cautioned not to rely on the neonicotinoid class (Group 4A) of insecticides. Further, do not use foliar neonicotinoids in fields previously treated with seed treatment or at-planting neonicotinoids. Using all available management strategies such as crop rotation, scouting, threshold treatments, and alternate insecticide classes is important. Refer to the Mid-Atlantic Commercial Vegetable Production Guide for a list of labeled insecticides, or contact Extension educator Leah Fronk at lxf339@psu.edu or (717) 363-9062.

The adult CPB has an oval body with black, narrow stripes on the wing covers. To overwinter, CPB adults burrow deep in the soil in recently cropped potato fields, hedgerows, or forested sites. In late April to early May, adult CPB will emerge from the soil and migrate to potato fields to feed and lay clusters of bright yellow eggs on the lower leaf surfaces. A single adult female CPB can lay up to 350 eggs in her lifetime.

Colorado potato beetle egg hatch can vary according to temperature. Entomologists note that egg hatch can occur in as little as three to four days under warm temperatures (84°F) or as long as 16 days under cooler temperatures (60°F). After hatching, CPB larvae will go through four larval stages. The first instar of the CPB appears hump-backed and brick red in color, but subsequent instars will appear light orange to tan in coloration. Colorado potato beetle larvae also have blackheads and black legs with two rows of black spots along both sides of the larva’s body.

When larvae mature, they drop to the ground and will burrow three to six inches into the soil to pupate for two to three weeks before emerging as adults. Upon emerging from the soil, adult CPB will begin feeding aggressively on the potato foliage before initiating a new round of egg-laying. Potato growers may observe two complete generations a year in the Mid-Atlantic region. In the south (NC), growers may observe as many as three to four generations of Colorado potato beetles per year.

Vegetable entomologists recommend that foliar insecticides be applied to potato foliage when 15-30% of the eggs hatch for the best control. Treatment of 4th-instar larvae and adults are particularly ill-advised since they are more difficult to kill, and 4th-instar larvae are responsible for 75% of the defoliation observed by growers.