During a recent visit to some potato fields by an Extension Educator in LaGrange County, it was evident that growers need to be on the lookout and take action against some of our major pests in this cropping system. These recent observations revealed cutworms present and impacting every 5th plant in the field, as well as an abundance of all life stages of the Colorado potato beetle, evidence of flea beetle damage, and potato leafhoppers bouncing about, leaving minor traces of hopper burn on some plants. Regardless of where you are in the state, if you haven’t done so in a while, go out and scout your potatoes! Below I will review the diagnosis and recommendations of each of these pests.
Cutworms
This is a common name for caterpillars from the Lepidopteran family Noctuidae. The adult moths and larvae are active at night. To diagnose the damage in potatoes, you should dig at the base of plants that display chewing damage on the leaves or stems. Buried in the soil, you may find a drab-looking caterpillar that will curl into a ‘c’ shape when disturbed (Figure 1). In this particular potato field, you could see the cut stem of the plant, sap or wet spots on the soil, and loose disturbed soil at the base of affected plants. During the night these caterpillars will dig out of the soil and feed on the plant material.
Figure 1. Cutworm damage and caterpillar from a potato field (Photo by Jeff Burbrink).
Colorado Potato Beetle (CPB)
This Chrysomelid, or leaf-feeding beetle, overwinters in the soil near the previous year’s crop, emerges in the spring, and migrates to the new potato planting. The adults will then feed on the foliage (Figure 2), deposit yellow egg masses (Figure 3; similar to ladybeetle egg masses) on the leaves, and eventually die. The larva hatch from the egg, feed on foliage and they transition through several larval stages (Figure 4), then drop to the soil to pupate. The best way to manage this pest is to scout for emerging adults in the spring and treat the plants when the eggs are present and beginning to hatch. This will minimize their impact on the crop and reduce the second generation. Despite the name, CPB can feed on a variety of solanaceous crops, including eggplant, pepper, and tomato, however potato is preferred. For potatoes, prior to flowering, the plants can withstand up to 30% defoliation while retaining yields. After flowering, the damage level reduces to 6-8% defoliation, beyond that you will experience reductions in yield.
Figure 2. Colorado potato beetle adult (Photo by John Obermeyer).
Figure 3. Colorado potato beetle eggs (Photo by John Obermeyer).
Figure 4. Colorado potato beetle larvae (Photo by John Obermeyer).
Flea Beetles
There are nine different species of flea beetles in Indiana. This insect was the featured spotlight in the last issue of the hotline. The species that specializes on solanaceous plants is the potato flea beetle, one of the relatively smaller species. They move quickly and jump from the plant when disturbed so you may not see the insect, but their damage to leaf tissue is diagnostic. The adults emerge from the soil in mid-spring and feed on young foliage of potatoes, tomatoes, eggplants, and peppers, along with a variety of common weeds that belong to the solanaceous family. They will not directly damage the fruits or tubers but leave shot holes in the leaf tissue. High infestations can reduce fruit/tuber yield as the loss of the plant materials decreases the capacity of the plant to photosynthesize. Flea beetle populations rarely reach economically damaging levels in potatoes but can be controlled with foliar insecticide applications. To monitor, use a sweep net and walk in an ‘M’ shape across the field. Sweep the net back and forth with each step. After 50 steps (sweeps), carefully examine the contents of the net. If you collect 10 or more beetles per 50 sweeps using a 15-inch net, treatment is warranted.
Figure 5. Flea beetles and their associated damage on potatoes (Photo by John Obermeyer).
Leafhoppers
Leafhoppers are small green insects belonging to the Order Hemiptera, i.e., true bugs. They feed on plants with a piercing-sucking mouthpart, much like aphids, and therefore do not leave holes behind on the plant. However, the saliva that they use to lubricate their mouth parts and facilitate feeding contains toxins that change the appearance of the plant, causing hopper burn (Figure 6). This is a yellowing that occurs around the leaf edges where the insect has fed. These insects are once again small and move quickly, making them difficult to see. However, instead of jumping like flea beetles, they kind of shuffle sideways on the plant. Review images of the larva and adult to aid in scouting recognition (Figure 7). Plants should be treated with a foliar insecticide application when one or more nymphs per 10 leaves are detected. The hopper burn caused by their feeding is diagnostic of their presence in the crop. Waiting for hopper burn to occur may be too late to avoid yield-reducing damage.
Figure 6. Potato leafhopper damage on a potato plant (Photo by John Obermeyer).
Figure 7. Various life stages of the potato leafhopper (Photo by John Obermeyer).
Management
To see the latest pest management recommendations for potato insects, refer to the Midwest Vegetable Production Guide (ID-56) or check out the potato insect management bulletin (E- 96). All of these insect pests can be managed with foliar insecticide sprays. Resistance has been documented among several different populations so be sure to monitor the efficacy of products you apply and always rotate the active ingredients, referencing the IRAC (insecticide resistance action committee) code of each product and moving between codes with each application. For organic growers, Spinosad products are best for CPB control and need to be used strategically to maximize the impact and delay resistance. Critical timing of an application when egg masses have hatched and the larvae are young provides excellent control.