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Chewing insects can cause serious yield and quality losses in potatoes by feeding on the leaves, stems or tubers. Sucking insects such as aphids and leafhoppers cause indirect losses by transmitting viral diseases.
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Effective insect control depends on a combination of cultural and chemical practices. Good control of weeds and volunteer potatoes in and around potato fields removes alternative food sources for many of the pest insects, particularly early in the spring before the crops have emerged from the ground. Field rotation will reduce the numbers of many pest insects, particularly Colorado potato beetles. Insecticides can reduce wireworm populations, however, regardless of an insecticide application, growers can expect reduced potato quality if they plant potatoes immediately following pasture or perennial grasses. Potato plants are more severely affected by insect pests when they are also suffering other stresses, so adequate moisture and fertility, as well as effective disease and weed management will minimize losses due to insects. Despite the effectiveness of cultural practices in reducing insect populations, insecticides are still required in most potato insect control programs.Adult Colorado potato beetles are orange and black striped insects about 1/4" (0.6 cm) long (Plate 3). Adult beetles over-winter in the soil and in response to warmer temperatures in late May or early June they emerge and migrate by walking or flying to potato plants. Beetles cannot fly except when the temperature exceeds approximately 77°F (25°C) for several hours, so the majority of beetles walk to potato fields.
Colorado potato beetle
Colorado potato beetle adult
Courtesy of C. Schaupmeyer
Once in the potato field, beetles feed; mate and the females lay orange eggs in groups of approximately 30 on the underside of potato leaflets.
Eggs are laid over a period of several weeks in June, and usually hatch within 5 - 10 days. Because of the length of the egg laying period, some larvae may have completed development before others have hatched. Unless controlled, larvae are present in the field in late June and throughout July.
Colorado potato beetle eggs
Courtesy of C. Schaupmeyer
The young larvae are brick red with black spots; older larvae are orange with black spots. Larvae are slow moving, soft-bodied, hump-backed and feed on the leaves of the potato plant. Larvae are generally found near the top of the plant and they seldom move far from the plant on which they hatch, unless all the leaves are eaten. There are four larval stages, which differ mainly in size; newly hatched, first-stage larvae are about 1/10" (2.5 mm) long; fully-grown larvae of the fourth stage are about 2/5" (10-12 mm) long. From hatching to completion of the larval stage takes 2-3 weeks, and larvae feed almost continuously, except when moulting from one stage to the next. The third and fourth stage larvae (over 1/5" or 5-6 mm long) cause the most damage to the potato plant as they consume about seven times as much food as the previous two stages. In addition to the damage caused by feeding, the Colorado potato beetle is capable of transmitting spindle tuber virus and bacterial ring rot.
When the larvae have completed their development, they enter an inactive pupal stage in the soil. After 1 - 2 weeks, adults emerge from the pupae and begin to feed on the potato plants. Some years, the new adults may lay eggs that hatch into larvae in August or September. These larvae frequently die of starvation or cold, and do not contribute to future populations of Colorado potato beetles. Defoliation by larvae just before harvesting has negligible effect on yield. In late summer, the adults respond to senescence of potato foliage, shorter days and cooler temperatures by burrowing deep in, or close to, the potato fields in which they were feeding.
Using cultural control methods and augmenting them with insecticides only when necessary to prevent loss can reduce the development of insecticide resistance. The single most important cultural control method for Colorado potato beetle is field rotation where there is a 650 ft. (200 m) separation between last season’s and this season’s potato fields. Colorado potato beetles usually over-winter close to last season’s potato fields and are very vulnerable when they emerge in the spring if no food plants are close at hand. A 650 ft. (200 m) separation between consecutive years’ crops will reduce the survival rate of migrating adult Colorado potato beetles.
Migrating Colorado potato beetles will often be concentrated at the edge of the potato crop. This phenomenon is called the edge effect. The edge effect occurs because under normal spring climactic conditions the adult beetles walk into new potato fields in the spring, and tend to stop after encountering the first plants. In this case, the density of females laying eggs, and of the resulting larvae, is much higher at the edge than in the middle of the field. There may be no Colorado potato beetles in the middle of extremely large fields. The pattern of higher densities at the edge is less distinct when spring temperatures are abnormally warm, allowing adult beetles to fly into the field. The pattern may also change if the plants emerge unevenly. The first plants to emerge may attract more beetles.
The edge effect provides an opportunity to reduce the area where Colorado potato beetle densities are high and where insecticide application is required. The proportion of edge effect is reduced in larger relative to smaller fields. Increasing the size of field increases the proportion of the field where Colorado potato beetle densities are lower, and where insecticide treatments may not be required.
Insecticides should be applied only when they are justified economically; that is, when the cost of control is exceeded by the benefits of increased marketable yield. Applying insecticides when they are not economically justified reduces net return and promotes development of resistance. The level of insect population above which control is economically justified is called the economic threshold. Economic thresholds have been determined for Russet Burbank and Norland for the normal control situation in which spray applications are applied against larval populations in early summer.
The economic thresholds for Russet Burbank and Norland are quite low, because if the larvae are not controlled they will mature into adults that will also feed on the potato plants. Continuous defoliation throughout the season has a marked effect on yield. However, plants are quite tolerant of defoliation early in the season, so large numbers of larvae can be allowed to feed for a short time.
The decision to make an insecticide application is not based entirely on the number of feeding larvae exceeding the economic threshold. The degree to which hatching is complete and the extent of defoliation must also be considered. Hatching takes place over a 5 - 10 day period, depending upon environmental conditions. The larvae are protected from an insecticide application while inside the egg. If a spray is applied during the early stages of hatching, the unhatched larvae will not be controlled and a second application may be required later in the season. Knowing that the plant is tolerant to defoliation early in the season allows for the insecticide application to be delayed until a greater portion of the larvae have hatched.
Larvae that have matured and are pupating in the soil will not be controlled by an insecticide application. If a spray is applied too late in the season while some larvae are pupating, a second application may be required when the pupating larvae emerge as adults.
The information contained in the box below shows how to calculate the economic threshold for Russet Burbank and Norland potato varieties.
Scouting for Colorado potato beetle larvae should begin 2 weeks after crop emerges. Twice weekly sample along field edges as this is where beetles will first appear, providing proper crop rotations are employed. Edge sampling consists of walking around the edge of the field and at corners and the midpoint of each side, counting the total number of the specified stages on 20 plants. Do not sample plants less than 1 yard (1 metre) from the field edge. Do not sample immediately neighboring plants, skip a plant in between or move to the next row over. Record the number of beetles and perform the economic threshold calculation. Note also the percent defoliation. If the result of sampling indicates that the economic threshold has not been reached, repeat the sampling at the next scheduled interval. If the economic threshold has been exceeded and 10% or more of the foliage has been consumed, sample the field intensively to determine which parts of the field require treatment and apply insecticide immediately to areas where defoliation exceeds 10%. Sample again about 3 days after insecticide application (observe re-entry precautions on insecticide label) to determine whether application gave effective kill. If the insecticide was effective, sample intensively twice weekly until 6 weeks after plant emergence. If the total numbers of all larval stages and adults again exceed the economic threshold, apply the insecticide as soon as the largest larvae reach third stage more than 1/5" (5 mm).
Written by B. Elliott |
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