Colorado potato beetle on Plants

Symptoms

Colorado potato beetles and their larvae feed on leaves. If left unchecked, they can completely defoliate potato vines in a matter of days. On eggplant, the larvae feed on leaves, flowers, growing fruits, buds and the stem.

The adult beetle has an oval body with narrow black stripes on the wing covers, which are called elytra. Eggs are laid in clusters of 20 to 45 on the undersides of leaves. Check the undersides of leaves as well as the tops, because that is where the eggs are laid.

Early-instar larvae are crimson, with black legs and two rows of black spots on the sides of the body. Larvae also have black heads, black legs and spots along their sides. Large larvae appear orange, bloated and humpbacked. The larvae pass through four stages, called instars, before they pupate in soil.

Fourth-instar larvae are responsible for about 75% of the defoliation that growers see. You may also find black, sticky excrement on stems and leaves where beetles and larvae feed.

Causes

Adults overwinter by burrowing deep in the soil in recently cropped potato fields, hedgerows or forested sites. The overwintered adults emerge from the soil and migrate to potato fields. Beetles find host plants largely by chance rather than by active search. Over large areas, the beetle spreads mainly through wind-borne migration.

Egg hatching time depends strongly on temperature. It runs from about 3-4 days at 84 °F (29 °C) to as long as 16 days at 60 °F (16 °C). Larvae pass through four instars before pupating in soil for about two to three weeks. Two complete generations per year are typical in Pennsylvania and the Mid-Atlantic region, with up to three to four generations in North Carolina.

Insecticide resistance is frequent in Colorado potato beetle populations, and it can arise within a few years under repeated use. Resistance has developed against 54 different compounds since the 1960s. In some southern populations, removal of host plants is a primary trigger that induces diapause.

Diagnosis

Confirm the pest by finding the insects, not only the damage. Begin with the crops this beetle is listed on: potato, tomato and eggplant. Turn leaves over and look for egg clusters of 20 to 45 on the undersides. Eggs hatch in as little as 3-4 days at 84 °F (29 °C), or take up to 16 days at 60 °F (16 °C).

Then look for the larvae and adults. Young larvae are crimson with black legs and two rows of black spots along the sides. Large larvae look orange, bloated and humpbacked. Adults have an oval body with narrow black stripes on the wing covers.

Black, sticky excrement on stems and leaves also marks where beetles and larvae have fed. Fourth-instar larvae account for about 75% of the defoliation growers see. If you find none of these insects, eggs or droppings on the plant, the damage may have another cause.

Treatments

Rotate potato or tomato plantings a quarter to a half mile (0.4 to 0.8 km) from the previous year's fields to delay and reduce infestation. Winter wheat or hay between the rotated fields delays colonization further. No-till production or straw mulch reduces colonization by overwintering beetles. In home gardens, hand-pick beetles and larvae and use row covers. Choosing a sowing date that reduces the population of second-generation larvae is another option. Colorado-beetle-resistant potato varieties, where available, serve as a containment strategy.

Natural egg predators include a ladybird beetle (Coleomegilla maculata) and a predaceous stinkbug, both of which feed on the eggs. On eggplant, the parasitoid wasp Edovum puttleri attacks the eggs. Foliar formulations of Bacillus thuringiensis serovar tenebrionis can be applied for pest control. You can also introduce or conserve natural enemies such as the predators Perillus bioculatus and Podisus maculiventris, the fungus Beauveria bassiana and the nematode Steinernema feltiae. Neem-based products reduced feeding and fertility in laboratory tests. Spinosad-based formulations are used in organic farming, though resistance has been reported in some areas.

For chemical control, scout potato fields weekly from plant emergence, sampling 10 sites along a V- or W-shaped path through the field. Treat if counts exceed 50 adults, 75 large larvae or 200 small larvae per 50 stems. Time foliar insecticide applications for when 15-30% of eggs have hatched, rather than waiting for older larvae or targeting fourth-instar larvae. Chemical control begins from the first appearance of overwintered adults.

Rotate insecticides with different modes of action rather than relying on one chemical class. Avoid relying only on neonicotinoid insecticides (Group 4A), and avoid foliar neonicotinoid sprays in fields already treated with neonicotinoid seed or at-planting treatments. Avoid late-season insecticide applications to prevent resistance building up. The product label is the legal instruction, and you must follow it for the crop and site you are treating.

Prevention

Planting potato or tomato a quarter to a half mile (0.4 to 0.8 km) from last year's fields delays and reduces infestation. Overwintering adults burrow into the soil in recently cropped potato fields, hedgerows or forested sites. Winter wheat or hay between rotated fields delays colonization further. No-till production or straw mulch also reduces colonization by overwintering beetles.

Row covers in home gardens, a sowing date that reduces second-generation larvae, and resistant varieties where available are other cultural options. Conserving natural enemies, including the ladybird beetle Coleomegilla maculata, which feeds on the eggs, is another condition worth keeping. Scouting weekly from plant emergence tells you when counts reach the treatment thresholds. Avoiding late-season insecticide applications prevents the build-up of resistance, which can arise within a few years under repeated use.

Plants most often affected

This beetle is recorded on three garden crops: potato, tomato and eggplant. Potato vines can be completely defoliated in days if the beetles go unchecked. On eggplant, the larvae feed on leaves, flowers, growing fruits, buds and the stem. Rotation advice names tomato plantings as well as potato.

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