Colorado Potato Beetle

Leptinotarsa decemlineata

Colorado potato beetle (CPB) is a key pest of potato and eggplant throughout the Northeast and requires a combination of cultural, biological and chemical strategies for effective control. 

Increasing temperatures result in faster development and feeding rates. Cold, rainy weather slows both crop and insect growth, so eggs that are laid can pile up and then all hatch at once when it warms up. Knowing what to look for and getting out into the field to scout is key in determining when to use appropriate controls. CPB is also an important pest of eggplant, so these fields should be monitored as well. Good control of CPB in June will not only protect vulnerable crops throughout the growing season; it will also reduce the number of beetles in the next generation that will survive to feed on next year’s crops.

Life Cycle & Identification:

In the Northeast, CPB feeds on solanaceous crops and weeds including potato, eggplant, tomato (primarily seedlings), horsenettle, and Eastern black nightshade. CPB overwinters in the adult stage, generally in soil (up to 12 inches deep) in the woods and brushy borders next to host crops, though some burrow into soil right in the field. In late spring, the beetles emerge to search for host plants. Adult CPB are 1/2" long by 3/8" wide, oval with a rounded back, and each forewing is yellow with five black stripes. They are fairly slow and clumsy; they can fly when temperatures are warm enough, but more often they walk into fields from overwintering sites. Heavy feeding may occur on edges of non-rotated fields. If beetles do not find host plants via walking, they may fly in search of food. 

Once they reach a host plant, adults feed, mate, and lay clusters of 20-35 elongated, yellow-orange eggs on the undersides of leaves. One female can lay up to 300 eggs in her lifetime. Eggs grow darker as they develop and hatch in 7 to 10 days, depending on temperature. The larvae are reddish-orange and grub-like with rows of black spots along the sides of their abdomen. They, along with the feeding damage they cause, are easily seen on leaves. Larvae go through four molts before they pupate—each stage of the larva is called an “instar”. In the first instar, the larvae are about the same size as the eggs and in the second instar they are about ⅛” long. Mature, fourth instar larvae are hump-backed and plump, and reach ⅝” before they drop to the soil and pupate. Adults emerge from pupae after 10 to 14 days, leaving round exit holes at the soil surface. In southern New England, there is a second generation of eggs, larvae, and adults in late July, while there is only one generation in northern New England. Beetles fly or walk out of fields in August, seeking overwintering sites at field edges.

Feeding damage and larvae are easily seen on leaves. Larvae go through four molts (instars) before they pupate. In the first instar, the larvae are about the same size as the eggs and in the second instar they are about 1/8" long. Mature, fourth instar larvae are hump-backed and plump, and reach 5/8" long before they drop to the soil and pupate. Adults emerge from pupae after 10-14 days, leaving round exit holes at the soil surface. In southern New England, there is second generation of eggs, larvae, and adults, while there is only one generation in northern New England. Beetles walk or fly out of fields in August, seeking overwintering sites at field edges.

Crop Injury:

Both adults and larvae cause feeding damage, but larval damage is the most severe. Because the fourth larval stage (instar) does 85% of the feeding damage, it is critical to control larvae while they are small. Potatoes can tolerate 20% defoliation (or even more, depending on time of the season and cultivar) without reduction in yield. Eggplant is more sensitive to damage and can only tolerate up to 15% defoliation before yield is affected.

Monitoring & Thresholds:

Scout weekly while CPB are active. If you are getting near the spray threshold, pay extra attention and scout again in 3-4 days. The New England Vegetable Management Guide recommends this tighter scouting schedule if numbers are above 15 adults, 75 small larvae, or 30 large larvae per 50 plants/stalks. 

The scouting procedure below was established for potato, but a similar process can be used in eggplant. Spray thresholds for both crops are listed in Table 1. There are no established thresholds for CPB in tomato, as it is not a preferred crop.

Scout beetles on 30-50 plants (or later in the season, stalks). One recommended procedure is to walk the field in a V-shaped pattern and stop at 10 sites across the field. Randomize your selection of sites using a set number of paces (e.g. stop every 10 paces) depending on field size. At each location, select 3-5 plants if plants are less than 12-18” tall; if plants are larger, select 3-5 stalks. Alternatively, select 30-50 plants or stalks individually at random across the field. Count adults, large larvae (greater than half-grown) and small larvae (less than half-grown) separately. Take note of the presence of orange-yellow egg masses on undersides of leaves - eggs will hatch into larvae 7-10 days after being laid. Record percent defoliation of each plant/stalk. 

A spray is warranted if any of the following thresholds are met:

Potatoes can tolerate 20% defoliation without reduction in yield (or even more, depending on time of the season and cultivar). Damage by adults in rotated fields may not be significant, so you can wait for egg hatch to kill both adults and larvae. Damage to eggplant seedlings from adult feeding is often severe enough to warrant control of the adults.

Use the following scouting sheets to help keep track of the CPB populations: Potato, Eggplant, Tomato. These can be used for several different insects and diseases in each crop. There are no established thresholds for CPB in tomato, as it is not a preferred crop. 

Cultural Controls & Prevention:

• Crop rotation: The single most important tactic for CPB management is to rotate potatoes, eggplants, and tomatoes to a field that is at least 200 yards from the previous year’s fields. Barriers such as roads, rivers, woodlands, and fields with other crops are helpful, because CPB adults are such slow and clumsy movers. This delays and reduces colonization by adults, reducing the number of eggs and larvae in the field later.
• Crop health: Starting with healthy seed and maintaining good crop nutrition helps plants grow well and withstand feeding injury.
• Early planting: Green sprouting, also know as chitting, prepares whole seed potatoes to emerge rapidly, gaining about 7-10 days to harvest. This early start makes it easier for the crop to put on growth and size before CPB adults and larvae arrive. While it won’t avoid damage altogether, it may reduce the need for insecticides.
• Late planting: Planting after mid-June and using a short season variety often avoids CPB damage, as adults that do not find food will leave the field in search of greener pastures. This practice may eliminate the need for controls.
• Physical barriers such as straw mulch, trap crops, and trench traps may delay and reduce infestation.
• Mulching: It has been well documented that when potatoes or eggplants are mulched with straw, fewer CPB adults will settle on the plants and fewer eggs will be laid. This can be accomplished on larger plantings by planting into a rye cover crop, mowing down the rye, then pushing the rye straw over the plants after they emerge. For smaller plots, straw may be carried in.
• Perimeter trap cropping: Potato trap crops may be planted earlier than the main crop to attract beetles before the main crop emerges or planted between overwintering sites and this season’s crop. Flame, vacuum, or spray the border crop before beetles move into the main crop. Another approach is to plant 3 to 5 rows of potatoes treated with a systemic insecticide in a perimeter around the field; this treated border will kill up to 80% of the colonizing beetles. Planting main potato crops later than normal may also cause beetles to leave the field before potatoes emerge, resulting in lower beetle numbers.
• Trenches: One common recommendation is to install plastic-lined trench traps next to overwintering sites at least 1 week before adults emerge, with trenches 1 - 2’ deep and 6 - 24” wide at the top. They can be U- or V-shaped with side walls sloping at angles between 65° and 90°. Beetles walking from field borders fall into the trench and cannot fly out.
• Hand removal: For smaller plantings and early infestations, walking through the crop to squish egg masses or drop adults into buckets of soapy water can be very effective to delay the build-up of damaging populations.
• Flaming: Flame weeders can be used to kill colonizing adult beetles in potatoes when the crop is less than 5” tall. Move rapidly using a tractor-mounted or hand-held flamer. The goal is to scorch beetles, not burn them to death— injury to antennae and legs renders them unable to orient and climb plants, while burning them to death will result in excessive plant damage. At this early stage, healthy emerging potatoes have sufficient reserves to regrow foliage and establish well.

Biological Controls:

Naturally occurring predators and parasites of CPB help suppress populations and prevent crop injury. Natural enemies that attack CPB eggs or larvae include twelve-spotted lady beetle (Coleomegilla maculata), spined soldier bug (Podisus maculiventris), a ground beetle (Lebia grandis), and a parasitic tachinid fly (Myiopharus doryphorae). The fungus Beauvaria bassiana (e.g. Mycotrol) has been shown to suppress beetle populations, though it does not provide immediate control. Using selective rather than broad-spectrum insecticides can help to conserve these natural enemies. Be aware that lady beetle egg masses look very similar to CPB egg masses, but lady beetle eggs are more yellow and slightly smaller (~1mm) than CPB eggs (~1.7 to 1.8mm), which are more orange.

Chemical Controls & Pesticides:

Scout to determine if a damaging population is present. Labeled conventional products include pyrethroids, neonicotinoids, novaluron (e.g. Rimon), cyromazine (e.g. Trigard), and diamides (e.g. Verimark, Exirel). In recent years, we have observed resistance to both neonicotinoids (e.g. Assail, Belay, Venom, Actara, Cruiser, Platinum) and synthetic spinosads (e.g. Radiant) in New England. When using products that control only larvae or only small larvae, scout for eggs, note egg hatch, and apply controls before larvae reach third instar to avoid the worst feeding injury. For materials that control all stages, you may wait and scout for adults and larvae to determine the need to apply insecticides.

For organically managed fields, the selection of insecticides is limited to fewer active ingredients including spinosad (Entrust), azadirachtin (Azatin), pyrethrin (Pyganic), and Beauvaria bassiana (Mycotrol O, Botanigard), which can be tank-mixed and/or rotated. The Bt product Trident, which was developed a few years ago for CPB control, is still off the market due to formulation issues. With few products to choose from, it’s more important to time pesticide applications so that they are as effective as possible and therefore reduce the need for subsequent applications. 

Ledprona is a new insecticide that was registered in 2024 by the EPA for controlling CPB and is manufactured under the brand name Calantha. It is the first sprayable insecticide utilizing a novel mode of action based on RNA interference (RNAi) with the goal of providing effective and specific control of CPB. RNAi is a natural biological process which exists in most organisms and regulates the translation of messenger RNA, a step leading to the synthesis of proteins. Ingestion of certain manufactured double-stranded RNA (dsRNA) can activate this interference process in some organisms such as CPB, disrupting the expression of a vital enzyme in the insect’s metabolism and resulting in mortality within a few days of consumption. While the potential for dsRNA technology to cause non-target effects has been shown in other organisms, field studies of Ledprona have not found any significant negative effects on non-target arthropods. This, along with dsRNA’s rapid degradation in the environment, may allow for a decrease in environmental impacts stemming from CPB management. However, like with any other insecticide, resistance to RNAi-based biopesticides is likely to emerge if resistance management strategies are not used. Cautious use of this insecticide in combination with other IPM principles shows promise as a method of providing effective control of CPB while mitigating environmental risks.

Do not try to kill every beetle in the field. Potato crops can withstand 15% defoliation without affecting yields. Avoid treating potatoes for CPB late in the season, as defoliation less than two weeks before senescence will have little effect on final tuber bulking.

Resistance management must be part of every potato grower’s plan. CPB has a remarkable capacity to develop resistance to insecticides. Based on a fifty-year track record, we can expect that any insecticide that is used repeatedly on the same population of CPB (that is, those in the same field or farm) will lose its efficacy in less than five years. Where potato production is concentrated and rotation has been limited, resistance may develop on a region-wide basis. It’s up to you to manage resistance in the population of beetles on your farm, and keeping insecticides effective with careful rotations is a worthwhile investment.

In the New England Vegetable Management Guide, as well as on pesticide labels, each insecticide has an IRAC (Insecticide Resistance Action Committee) Group Number, which identifies chemistries with the same mode of action. Growers should note the resistance group number of each insecticide, rotate classes of insecticides, and avoid using the same chemistry more than once per year, or even better, once every other year. Do not use the same chemical class on successive generations in the same year. Use newer chemistries first. For conventionally managed fields, there are enough different products to do a two-year rotation that will effectively control CPB while effectively delaying resistance to any one product. For organic growers, there are fewer products available, and cultural control practices should be followed to avoid dependency on a product.

See the potato and eggplant insect control sections of the New England Vegetable Management Guide for full lists of labeled materials.

Crops that are affected by this insect:

• Eggplant
• Potato
• Tomato, Field
• Tomato, Greenhouse