Colorado Potato Beetle

Colorado potato beetle, Leptinotarsa decemlineata (Say), is a major pest of potato, Solanum tuberosum L., in commercial production and home gardens.

Adult Colorado potato beetles 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 . 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.

The Colorado potato beetle feeds on the leaves of several solanaceous plants and is known as a serious pest of potato and eggplant, as well as an occasional pest of tomatoes. Many beetle populations have developed resistance to the pesticides that have been widely used against them.

Life Cycle : Colorado beetles spend the winter months several inches below the soil surface. In the spring these adults emerge, feed, and mate. Females lay twenty to sixty bright orange eggs on the underside of leaves on early planted potatoes. Over a lifetime, a female may lay as many as 3500 eggs. When the larvae hatch, they begin feeding on potato plant leaves, stopping only to molt (shed their skins in order to grow).

They undergo four larval stages or “instars” in ten. to twenty days, depending on the temperature. Mature larvae drop off the leaves and enter the soil for pupation, a stage in the beetle’s development lasting five to ten days. During this inactive stage in the ground, the larvae transform into adults.

The adults from this first generation emerge during the potato growing season, mate, and lay eggs, thus beginning a second generation of larvae with voracious appetites. The adults that result from this second generation of larvae are the ones that overwinter. The following spring, the cycle repeats.

Damage : Adults and larvae both feed on host plants. Adult damage is generally considered less severe, except in cases where they “stem” young host plants early in the season. These severed plants often die. Moderate plant defoliation, less than about 15%, has no impact on yield, particularly if it is early in the season. However, potato beetles are frequently sufficiently abundant to totally defoliate plants, often killing them prematurely and severely reducing yield.

Larvae chew holes and tunnel into the roots. Damage by the larvae, except under dry conditions, is usually considered minor. The first generation of adults emerges in late June and early July to feed on the foliage and flowers. Feeding damage by cucumber beetles to foliage is usually very minor, but severe feeding on flowers can result in poor fruit set. The second generation emerges in September and October.

Management : The Colorado potato beetle is one of the few “super” pests in agriculture. This species has developed resistance to most insecticides. Care must be used when targeting Colorado potato beetles to select an effective insecticide. You should also recognize that once you have to increase the rate to get adequate control, resistance to that insecticide is likely present in the population. Commercial growers have a number of insecticides that perform well. Fortunately, many of the newer insecticides have a unique mode of action giving growers the opportunity to delay the onset of resistance by rotating chemical classes.

If growers carefully select insecticides for Colorado potato beetle control, these products should remain effective for years to come. Repeated use of the same insecticide or other insecticides from the same chemical class will speed development of insecticide resistance. Slowing the onset or even preventing insecticide resistance is called resistance management and all growers should practice some form of resistance management.

For the home gardener and certified organic producer, the choice of effective chemicals is limited and none will give perfect control. There are more insecticides available to commercial growers but nearly all of these insecticides are classified as restricted use materials so that to purchase them you must be a certified pesticide applicator. Also, many insecticides are sold only in large quantities making them generally impractical for the home gardener.

Resistance Management :There are several key components of any resistance management program. First and foremost is to not repeatedly use the same insecticide or other insecticides having a similar mode of action to control Colorado potato beetles on the same farm in successive years. In fact, growers should not use the same insecticide to control the overwintering generation and the summer adults and any larvae they produce (2nd generation) in a given growing season. Historically, it has taken from 4 to 10 generations of repeated exposure to the same or similar insecticide for resistance to occur in Colorado potato beetles.

Colorado potato beetles have a single generation most years. A good “rule of thumb” is if an insecticide performed well for you against the overwintering adults and young larvae don’t use it on the summer adults or again the following spring.

Cultural Control: Crop rotation is an effective method of reducing your risk to Colorado potato beetles. The greater distance a potato field is located from a last year’s potato field, the later in the season the field will become colonized with beetles, greatly reducing the potential for damage. Colorado potato beetles cannot fly unless temperatures reach 70 F. In early spring in Minnesota, daytime temperatures can be substantially below 70 F effectively making beetles walk from the overwintering site to locate a potato field.

Chemical Control: Commercial production of potatoes is nearly impossible without using insecticides to control Colorado potato beetles. Commercial growers have the option of using systemic insecticides applied to the soil or seed at planting or apply insecticides to foliage after crop emergence. Using a systemic insecticide at planting is justified if there is a history of beetle treatment each growing season. If beetle pressure is not always severe, foliar applied insecticides can be the better choice because that permits assessment of beetle pressure before deciding on treatment. Also, invasion of a field may occur from one side of the field and treating field edges or “hot spots” is an option available to those who use a foliar applied insecticide.

All insecticides are most effective on very young larvae. Eggs and pupae are not susceptible to chemical control and adults can be difficult to control. Targeting application toward young larvae helps prevent damaging populations and allows resistant adult beetles to mate with susceptible beetles keeping selection pressure for insecticides resistance low. If sprays are targeted against overwintering adults, the only survivors will be resistant individuals. When resistant individuals mate, their offspring are resistant thus fixing the resistance gene in the population. Once resistance has become fixed in a population, reversion to susceptibility occurs only after many generations of non-exposure and may never revert to pre-exposure levels.

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