Insect Management

Colorado Potato Beetle

Colorado Potato Beetle, Leptinotarsa decemlineata, egg mass

      •Managing Colorado Potato Beetle

MANAGING COLORADO POTATO BEETLE

Colorado potato beetles (CPB) move into potato and eggplant crops in early June to feed and lay eggs. Some adult beetles spend the winter in last year’s potato fields, but most move into the woods and brushy borders next to these fields, where they burrow into the soil up to a depth of 12 inches. In spring the beetles have to regrow their flight muscles before they are able to fly. At first they search for food plants by walking from the field edges. This is why the edge of non-rotated crops are attacked first. If beetles do not find host plants via walking they will fly some distance in search of food. Once host plants are found adults begin to feed and lay eggs. The beetles will have mated the previous fall or late summer; hence they have no need to mate in the spring to produce viable eggs. However, they do continue to mate in spring. The bright yellow eggs are laid in clumps that average 30-35 eggs, generally on the undersides of leaves.  

Crop Rotation. The single most important tactic for CPB management is to rotate potatoes or eggplant 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. Rotated fields tend to be colonized 1-4 weeks later in the season. Also, the total population of adult beetles is lower, producing fewer larvae to control. Early June is the time to scout for adults, eggs and egg hatch. Walk your fields and look for CPB adults and eggs. The economic threshold for adult beetles in potato is 1 beetle per 2 plants (or per 2 stalks, in midseason). Eggplant seedlings have a low tolerance to damage. Look on the undersides of leaves for the orange-yellow egg masses. The fresher the eggs, the brighter orange the eggs will appear. Eggs hatch in 7-10 days, depending on temperature. If you want to know when the earliest eggs are hatching, you can flag the earliest egg masses you find with bright tape or flags, and then keep an eye on the hatch. Larvae go through four stages before they drop to the soil and pupate. In the first stage, the larvae are about the same size as the eggs and in the second stage they are about an eighth of an inch long. As the larvae get bigger, they do more feeding. The fourth, or largest, stage does 85% of the feeding damage. It’s a good idea to prevent beetles from ever reaching the fourth instar! After larvae complete their growth, they drop to the ground and burrow into the ground to pupate. Ten days later the next generation of adults emerge and feed. If they emerge before August 1, they will lay more eggs. After August 1, they feed and head to overwintering sites.

Systemic treatments. Neo-nicotinoid insecticides (Resistance Group 4) that are labeled for CPB include Assail 70 WP, Platinum, and Admire (2F or Pro). These are systemic insecticides that are applied as an in-furrow, banded drench or, in eggplant, as a drip irrigation application to the root zone. Admire may also be used as a transplant drench prior to placing in the field. Foliar products with the same active ingredients (eg Provado) are also labeled. Colorado potato beetles have a phenomenal ability to develop resistance to insecticides. In the Connecticut Valley, there are fields where CPB resistance to imidacloprid (Admire) is 300 times that of susceptible populations. Control requires higher rates, does not last as long, or does not happen at all. There is cross-resistance among products in the neo-nicotinoid group. For resistance management, do not use a product in this group on more than one generation per year.

Spray timing and thresholds. If you are using Bt (Novodor), you want to make the first application when 20-30% of the eggs have hatched. If you are using spinosad (Spintor 2SC or Entrust, the organic formulation) or a broad-spectrum insecticide, you can wait until more larvae have hatched, when the oldest larvae reach the third instar, when they are about 1/3 inch long. Applications made at this time with Spintor, Provado, AgriMek, or synthetic pyrethroids will kill all the larvae that have hatched up to this point. The threshold for small larvae is 4 per plant; for large larvae, 1.5 per plant (or per stalk in midseason), based on a count of 50 plants or stalks. Thresholds established in the Northeast for eggplants from seedling to fruiting stage include: 15 CPB per 10 plants (Rutgers) or 2 small/1 large larvae per plant (<6 inches) or 4 small larvae /2 large per plant (>6 inches) (Cornell). In eggplant, in addition to defoliation, beetles sometimes clip the stems of flowers or flower buds. This directly reduces fruit formation and marketable yield. On the other hand, potatoes can tolerate 20% defoliation without reduction in yield (or even more, depending on time of the season and cultivar). To prevent resistance, alternate among classes of insecticides in each generation, and throughout the season. If you used Admire at planting, do not use foliar applications of Provado. Classes (different modes of action) include synthetic pyrethroid (Asana, Pounce, Baythroid or Ambush) but many populations are already resistant to these; carbamates (Thionex, Vydate); neonicotinoids (Admire, Platinum,

Actara, Provado); spinosad, (Spintor, Entrust); Bacillus thuringiensis (Novodor); and abamectin (Agri-Mek). Use the resistance group numbers (listed in the 2006-2007 New England Management Vegetable Management Guide and on each pesticide label) to help know which pesticide is in which group. One low-risk strategy would be to use a material such as spinosad, which control adults and larvae for the first spray, followed by a Bt to kill emerging young larvae or Agrimek to control all stages of larvae. The following three insecticides each have a different mode of action. They have not shown field resistance in this area (yet) and provide good options for alternate insecticides that provide effective control:

·Spinosad (SpinTor 2SC, a liquid formulation or Entrust, an organic formulation, dry powder) gives excellent control of all stages of CPB at a 3.5 to 4.5 fl oz rate. For a heavy population, two applications about 10 days apart, with the initial application when third instars occur, will control the first generation. Spintor has the advantage that it will control adult CPB and also European corn borer if a grower has that pest on early potatoes. It is currently the only effective CPB insecticide approved for organic growers.

·Abamectin (AgriMek 0.15EC) is mainly a contact material, which controls larvae. It may be best used early in the season, when good coverage is easier to obtain. Rates of 5-6 fl oz per acre gave effective control in commercial fields in trials on Long Island. The lowest labeled rate is 8 fl oz. Bacillus thuringiensis subspecies tenbrionis (Novodor FC) controls small larvae, through the third instar. Time applications to begin when 30 percent of the eggs have hatched. Where fields are densely populated and eggs are hatching continuously, reapply every 5 to 7 days. Currently there are no formulations of Bt tenebrionis products that are approved for organic use under the National Organic Program. You can check with the MASS organic certification program (MICI, 978-297-3644) for more details.

Perimeter treatments or perimeter trap cropping can be applied to potato. One approach is to plant a barrier crop between overwintering sites and this year’s crop and get it in earlier than the main crop; then control early-arriving beetles with a systemic or foliar insecticide. Another approach is to plant three to five rows of potatoes treated with a systemic insecticide (for example, Admire or Platinum– assuming resistance is not a problem) in a perimeter around the field to be planted to potato, tomato, or eggplant; this treated crop will kill up to 80% of the colonizing beetles. In eggplant or tomato, the perimeter border can be an Italian eggplant type, which is more attractive to both CPB and flea beetles. Treat only the border, as soon as beetles arrive.

Late planting: Another strategy for beating the beetle is to plant late. CPB adults that do not find food leave the field in search of greener pastures. Planting after mid- June, using a short season variety, often avoids CPB damage and eliminates the need for controls.

Updated June 2006

- R Hazzard; thanks to sources including: D Ferro (UMass Amherst), J. Mishanec (CornellUniversity), J Boucher (Univ. of Connecticut).

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