Colorado potato beetles
The Colorado potato beetle (Leptinotarsa decemlineata) is a major potato pest throughout North America.
- Adults become active in spring, about the same time potato plants emerge from the ground.
- Larvae and adults feed on leaves and can completely defoliate plants.
- Many pesticides are ineffective because of pesticide resistance of the Colorado potato beetle.
- A combination of pest management tactics can reduce Colorado potato beetle numbers.
How to identify Colorado potato beetles
Oval in shape and 3/8 inch long.
- Have a yellow-orange prothorax (the area behind the head) and yellowish white wing covers with 10 narrow black stripes.
- Females lay clusters of bright yellowish-orange oval eggs on the underside of leaves.
- When young larvae first hatch, they are brick red with black heads.
- Older larvae are pink to salmon colored with black heads.
- All larvae have two rows of dark spots on each side of their bodies.
Biology of Colorado potato beetles
Plants they attack
Colorado potato beetles feed primarily on potatoes. They can also attack other plants in the night shade family (Solanaceae), including:
- Ground cherry
Colorado potato beetle adults spend the winter 5-10 inches underground in potato fields, field margins, windbreaks and gardens.
- Adults feed for a short time in the spring, and then begin to mate and lay clusters of 10-30 eggs on the undersides of leaves.
- Each female can lay up to 350 eggs during her adult life which can last several weeks.
- Eggs begin to hatch within 2 weeks, depending upon temperatures.
- Larvae cluster near the egg mass when young but begin to move throughout the plant as they eat the leaves.
- Larvae can complete development within 10 days if average temperatures are in the mid 80s F while it can take over a month if temperatures average near 60 F.
- The fourth instar larvae drop from the plant, burrow into the soil and transform into pupae.
In southern and central Minnesota there is generally a second generation. By midsummer, all stages of Colorado potato beetles, eggs, larvae and adults can be present in a potato field.
Damage caused by Colorado potato beetles
- Old larvae (the last or 4th larval instar) are responsible for as much as 75 percent of feeding damage.
- Potatoes can usually tolerate up to 30 percent defoliation when they are in the vegetative stage.
- They are much more sensitive when tubers are beginning to bulk and can only tolerate about 10 percent defoliation.
- Tuber bulking begins soon after flowering, making this time critical for beetle management.
How to protect your plants from Colorado potato beetles
Treatment of Colorado potato beetles in home gardens can be challenging. Use a combination of different pest management tactics to reduce Colorado potato beetle numbers.
Keep your garden clean
When Colorado potato beetles first emerge in the spring, they will look for other hosts in the absence of potato plants.
Clean up weeds like nightshade and ground cherry near your garden, as these weeds can act as a possible food source.
Plant early maturing varieties
Plant an early maturing variety to escape much of the damage caused by adults emerging in midsummer.
- Check seed catalogs for varieties that mature in less than 80 days.
- Yields on early maturing varieties are not as large, and often these varieties do not store as well as the popular Russet Burbank potato.
Growing potatoes only every other year may help reduce beetle populations if:
- No potatoes are being grown within a radius of ¼ to ½ mile away, and
- Temperatures are not excessively warm.
Pick beetles off plants
Handpicking in small gardens can be effective.
- Drop adults and larvae in a pail filled with soapy water.
- Remove or crush the yellowish orange eggs on the underside of leaves.
- New adult beetles can fly into gardens so be sure to check your potatoes regularly.
- Handpicking may be less practical in larger gardens.
Natural enemies of Colorado potato beetles
There are a few natural enemies of Colorado potato beetles
- Stink bugs and lady beetles will prey upon Colorado potato beetle eggs.
- The fungus Beauveria bassiana will kill both larvae and adults.
- Unfortunately, natural enemies have little impact on overall Colorado potato beetle numbers.
Colorado potato beetles are resistant to essentially all synthetic pesticides like carbaryl, cypermethrin, deltamethrin, lambda cyhalothrin, imidacloprid, permethrin, and pyrethrins. These products are unlikely to be effective and their use is not suggested.
Anytime you use a pesticide and it does not seem to kill Colorado potato beetles, switch to a different active ingredient.
Colorado potato beetles are not resistant to azadirachtin or spinosad. These products are also “soft” on natural enemies.
- Azadirachtin (Neem) – is derived from the Neem tree of Asia and Africa. It is effective for a couple of days and repeat applications are probably necessary. Azadirachtin provides poorer management of large larvae and adults
- Spinosad – is made from the soil bacterium, Saccharopolyspora spinosa. It is effective for about 10 - 14 days.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
Be sure that the vegetable you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.
Reviewed in 2019