Horticulture News: Potato beetles and larvae can wreak havoc on your garden

By Adam Leonberger, Published:

The common black and yellow-striped “potato bug,” a familiar insect to home gardeners, is the most serious insect pest of potatoes. Also it can be found on tomato, eggplant and peppers. Both the striped beetle and the black-spotted, red larva feed on plant leaves. Their damage can greatly reduce yield and even kill plants.

The Colorado potato beetle is notorious for its ability to rapidly develop resistance to insecticides. This has been a serious problem on the east coast for some time, and is becoming more of a problem in Kentucky. With a limited number of insecticides available, some homeowners feel they have exhausted their control options.

Colorado potato beetles overwinter in the soil as adults. They become active in the spring as temperatures rise and begin to feed on weeds and volunteer or early planted potatoes, even entering the soil to attack emerging foliage.

Females lay orange-yellow eggs in batches of about 2 dozen on the underside of the leaves. Each female can lay 500 or more eggs over a four to five week period. Eggs hatch in four to nine days and the larvae begin to feed on potato foliage.

The larvae are humpbacked with two rows of black spots on each side. They usually feed in groups and damage can be severe. The larval stage lasts two to three weeks.

Full-grown larvae burrow in the ground to pupate. In five to 10 days, the adult beetle emerges. This insect can go from egg to adult in as little as 21 days. The newly emerged adult female feeds for a few days before egg laying begins.

There are two full and occasionally a partial third generation each year. If foliar sprays are used, an effort should be made to treat just after most eggs have hatched but before serious plant damage occurs.

Control

Insecticides should only be used when needed. Potato plants can withstand considerable defoliation without yield loss. Additionally, some beneficial insects, such as green lacewings, predatory stink bugs and parasitic flies, will help to reduce Colorado potato beetle numbers somewhat. Generally, insecticides do not need to be applied unless there is more than an average of one beetle or larva per plant.

Non-chemical control measures such as hand picking of adult beetles and immature stages is encouraged. Hand picking can be particularly effective in reducing the numbers of overwintering beetles.

Drop adults and larvae in a pail filled with soapy water to kill the insects. Also remove or crush the yellowish orange eggs on the underside of leaves. 

Bacillius thuringiensis var tenebrionis (Bt) is effective against small larvae (less than 1/4 inch) and should be applied at egg hatch or when larvae are first seen. Larger larvae are more difficult to control with Bt. Azatin, an extract of the neem seed, prevents the larvae from developing normally. Esfenvalerate (Ortho Bug-B-Gone Multipurpose Spray) and spinosad (Bonide Captain Jack’s) are also effective against Colorado potato beetle.

Resistance management

Insecticides in the same chemical class usually have the same mode of action, the same method of killing the insect. Resistance develops more rapidly to an insecticide when that insecticide is used repeatedly as the only control measure. Repeated use of one class of chemical kills susceptible beetles, leaving those that are resistant.

Rotation needs to include various chemicals, each coming from among different classes of insecticides. Homeowners have several chemicals that can be used in a rotational spray program for these insects because they are in different classes.

The chemical Sevin is in the carbamate insecticide class; Bonide Garden Dust is a pyrethroid; and neem oil is an insect growth regulator. 

For more information, contact the Franklin County Extension Office, 502-695-9035, or Adam.Leonberger@uky.edu.

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