This year, I’ve had a handful of people ask me about managing Colorado potato beetle (CPB) in Iowa. Read more about CPB identification and biology here. These conversations were with producers growing over 5 acres of potatoes and having a difficult time reducing CPB pressure. With the help of Dr. Ian MacRae at the University of Minnesota, I put together an IPM plan for this devastating pest. Colorado potato beetle (CPB) is a major pest of potato that is native to America and Mexico. It's been in Iowa for over 150 years and has a long history of devastating outbreaks. This article provides some basic information on CPB identification life cycle and damage to potato.
Adult CPBs are oval in shape and 3/8 inch long. They 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. Eggs turn dark red just before hatching. When larvae first hatch from eggs, they have brick red bodies 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.
Adult CPBs overwinter in potato fields, field margins and windbreaks. They become active in the spring at about the same time potatoes emerge (sometime in May). 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 over 3-5 weeks.
Eggs begin to hatch within two weeks, depending upon temperatures. Larvae remain aggregated near the egg mass when young but begin to move throughout the plant as they eat the leaves. Larvae can complete development in as little as 10 days if average temperatures are in the mid 80’s. It will take over a month if temperatures average near 60°F. Larvae mature through four instars before they drop from the plant, burrow into the soil and pupate. There can be two generations in Iowa. Because eggs are laid over time, all life stages of CPB can be present at the same time in a potato field by July.
Both larvae and adults feed on the foliage of potatoes and, if left untreated, can completely defoliate plants. In addition to potato, they may also feed on eggplant, tomato, pepper, and other plants in the nightshade family (Solanaceae). Old larvae (i.e., 4th instars) are responsible for as much as 75% of feeding damage. Potatoes can usually tolerate substantial defoliation, up to 30%, when they are in the vegetative stage, but they are much more sensitive to the effects of defoliation when tubers are beginning to bulk and they can only tolerate about 10% defoliation. Tuber bulking begins soon after flowering, making this time critical for CPB management.
In general, CPB are very difficult to suppress because of their biology. Successful suppression will take an integrated approach with a focus on being proactive. I often recommend using IPM (integrated pest management) to help with field crop pests. It should include some or all of the following tactics:
1. Cultural. Much of what farmers/gardeners do to grow plants favors insect development. If we can mix up our growing conditions to interfere with the food and habitat pests need to survive, that will greatly discourage them from devastating our crops. For example:
- Sanitation, or removing potential food and habitat, is an effective starvation technique. If potatoes or solanaceous plants (e.g., eggplant, peppers, tomatoes), are not available when CPB adults first emerge in the spring, they will seek out alternate hosts, such as nightshade and ground cherry. Remove old plants and weeds in and around potato plots before, during and after the season to eliminate food sources.
- For small plantings, hand removal can be effective. Drop CPB adults and larvae in a pail filled with soapy water. Also remove or crush the eggs on the underside of leaves. Adults can fly into plots so be sure to check your potatoes regularly. Hand removal may be less practical in larger plots.
- Crop rotation, or only growing potatoes only every other year, may help reduce beetle populations if no potatoes are being grown within a radius of ¼-½ mile away and temperatures are not excessively warm. Moving the area that is planted with potato, is largely ineffective because the CPB can fly long distances when temperatures exceed 70°F.
- Date of planting will probably not be that effective tactic like for other pests. Most potatoes will not germinate until the soil temp is 40°F and the adult tend to emerge 2-4 weeks later. Planting seed pieces too early can actually cause germination issues, like diseases and rotting.
- Tillage is not effective unless deep tillage is used (and even then they can survive). The adults overwinter in soil and leaf litter and are quite capable of digging into and out of the soil after being buried.
- Exclusion with cages or row covers is not recommended for CPB because the supplies are expensive and time consuming to maintain.
2. Genetic. Planting an early-maturing variety will allow you 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.
3. Biological. There are few natural enemies of CPB, like predatory stink bugs and beetles. There is also a naturally-occurring fungus, Beauveria bassiana, that will kill larvae and adults. Unfortunately, biological control has minimal impact on CPB populations compared to other crop pests. I wouldn’t recommend planting flowering crops around or within potato plots to attract natural enemies, or releasing predators to suppress CPB.
4. Scouting. To focus your suppression efforts, I highly recommend the use of tracking air temperature. Insects develop based on accumulating heat units, or degree days (DD) just like most plants and other invertebrates. The lower developmental threshold, a point at which no development will occur, for CPB is 52ºF. The very first adult you see in the spring sets the “biofix” where accumulating DD begins:
- 120 dd post biofix - eggs are maturing, look for the beginning of 1st instars
- 185 dd post biofix - 1st instars completing, look for the beginning of 2nd instars
- 240 dd post biofix - 2nd instars completing, look for the first 3rd instars
- 300 dd post biofix - 3rd instars completing, look for the first 4th instars
- 400 dd post biofix - 4th instars completing, larvae will start dropping to ground to pupate
*You would want to target your scouting and suppression efforts on the first generation, 120-200 DD post biofix. Every summer is a bit different, so it is important to monitor each season rather than depend on calendar dates.
5. Thresholds. Even when implementing proactive IPM tactics, like cultural tools, there will be seasons when CPB still have outbreaks. In general, I recommend applying insecticides to protect yield based on CPB leaf defoliation: 20%-30% before flowering, 5%-10% at flowering, and 15%-20% post flowering. Estimating defoliation by eye takes practice because people tend to overestimate leaf area removed. There could be some seasons where these thresholds are not met and insecticides are not required.
6. Chemical. Depending on the size of the potato plots and the historical outbreaks on the farm, insecticides may be required to protect the crop. This method can be used with other IPM tactics and in moderation (e.g., a last resort when other tactics are not working). I highly recommend scouting and using thresholds to determine if applications are needed (see previous sections). There is a wide spectrum of chemical control products that target CPB, ranging from organic to restricted-use pesticides. It is important to read the label and follow all the directions, particularly how soon you can harvest potatoes after applications. Keep the following considerations in mind:
- It is easier to kill young larvae compared to older larvae and adults.
- Products are more effective when making direct contact with the body. Getting small droplets to contact CPB can be challenging given they feed and aggregate on the undersides of leaves.
- Targeting suppression efforts of the first generation can greatly reduce the impact of the second generation when plants are more sensitive to defoliation.
- Most chemicals are broad spectrum, meaning they will kill most insects that make contact with the application (e.g., pollinators, predators, etc.). This is true of organic and synthetic insecticides.
Microbial insecticides (the following products are approved for organic production and considered low risk to humans):
- Bacillus thuringiensis var. tenebrionis (Btt), a naturally-occurring bacteria, is an effective product against CPB and a great option for potato plots of your size (called Trident). This bacteria can kill young CPB (1st and 2nd instars); it does a progressively poorer job on larger larvae and adults. This form of Bt only targets beetles, so it is not broad spectrum. So far, CPB have not developed resistance to this bacteria like they have for commonly used synthetic insecticides. To be successful, apply the spray every few days when you see egg masses begin to hatch. Do not wait for older instars to develop. Btt has a short residual (<1 day).
- Spinosyn, sometimes called spinosad/spinetoram (called SpinTor, Radiant, Entrust), is derived from a bacteria, Saccharopolyspora spinosa. Spinosyn is another microbial product with a greater level of efficacy (killing power) than Btt. So far, CPB have not developed resistance to this bacteria like that have for commonly used synthetic insecticides. This product can be used on larvae and adults. Spinosyn has a medium residual (10-14 days).
Plant-derived and synthetic insecticides:
- Neem oils (azadirachtin) can be effective in reducing CPB larvae. Neem is derived from the seeds of neem trees. To be successful, apply the spray every few days when you see egg masses begin to hatch. Do not wait for older instars to develop. Neem oils have a short residual (<1 day).
- Pyrethrins are insecticides derived from chrysanthemum flowers. This product can be effective but it has a short residual (<1 day). Smaller larvae should be targeted to achieve the best results. Some CPB may have developed resistance to pyrethrins. To be successful, apply the spray every few days when you see egg masses begin to hatch. Do not wait for older instars to develop. Pyrethrins have a short residual (<1 day).
- Synthetic insecticides (e.g., carbaryl, permethrin, pyrethroids) pose a higher risk to humans; many of these products are overused in commercial potato production and no longer work. They are unlikely to provide adequate CPB management due to problems of insecticide resistance. An exception to this would be pyrethroids, like esfenvalerate. This is a relatively new synthetic insecticide and insecticide resistance is not as well established in CPB. Esfenvalerate would not be considered organic and would have moderate risk to humans; it has a long residual (up to 4 weeks).