Colorado potato beetles
Quick facts about Colorado potato beetles
The Colorado potato beetle (Leptinotarsa decemlineata) is a major potato pest throughout North America.
- It also feeds on eggplant, tomato, pepper and other plants in the nightshade family (Solanaceae).
- They become active in spring (May) when the potatoes emerge.
- Larvae and adults feed on foliage and can completely defoliate plants.
- Most 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
Adult Colorado potato beetles
Adults are active from June through September.
- 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.
Life cycle of Colorado potato beetles
Colorado potato beetle adults live through the winter 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 (i.e. the last or 4th larval instar) are responsible for as much as 75% of feeding damage.
- Potatoes can usually tolerate up to 30% 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% 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.
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 can control beetles
Natural enemies will have little impact on Colorado potato beetle numbers, but may help with some control.
- Stink bugs and lady beetles will prey upon Colorado potato beetle eggs
- The fungus Beauveria bassiana will kill both larvae and adults
Using microbial pesticide
Bacillus thuringiensis var. tenebrionis, a naturally occurring bacterial disease, can control young Colorado potato beetle larvae (1st and 2nd instars).
It is not as effective on larger larvae and adults.
- Apply the product every few days, beginning when egg masses begin to hatch.
- If you wait to apply this material when larvae are in the 4th instar, this method will not be effective.
The use of synthetic pesticides like carbaryl and permethrin is generally not suggested. Colorado potato beetles have become resistant to these pesticides.
Anytime you use a pesticide and it does not seem to kill Colorado potato beetles, switch to a different active ingredient.
- Esfenvalerate - relatively new synthetic pesticide that is still effective on Colorado potato beetles.
- Pyrethrin - can be effective but it has a short residual. Smaller larvae should be targeted to achieve the best results. Some Colorado potato beetles may have developed resistance to pyrethrins.
- Azadirachtin (Neem) - a pesticide derived from the Neem tree of Asia and Africa, may provide some control.
- Spinosad - is also an effective product, made from the soil bacterium, Saccharopolyspora spinosa.
Colorado potato beetles are not resistant to azadirachtin or spinosad. These products are also “soft” on natural enemies.
Azadirachtin needs to be reapplied frequently and provides poorer control of large larvae and adults.
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 2018