How to Manage Three Key Potato Pests

How to Manage Three Key Potato Pests

Potato-beetle-eggs

Brightly colored egg masses laid by Colorado potato beetle stand out in foliage as does the damage left by what eventually hatches.
Photo by Bonnie C. Wells

The Colorado potato beetle, potato leafhopper, and wireworms have been known to cause significant losses in potato crops across the country. Control for all three pests requires a combination of integrated pest management (IPM) strategies, including regular scouting, sanitation, and chemical and cultural controls.

Russell Groves, Professor and Vegetable Extension Specialist at the University of Madison-Wisconsin, provides information on key characteristics and control strategies to keep these pests off your crop and out of your field.

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How to ID the Pests

Colorado Potato Beetle: The adult stage of this pest has 10 alternating orange and black lines that run transversely along its back. It lays clusters of bright-orange eggs and once hatched, the immatures go through four immature (larval) stages, and then develop into a pupa in the soil. An adult then re-emerges in a week to 10 days to start a new generation. In the potato production regions of the U.S., there are typically one to two generations per year, according to Groves. Potato beetle tends to overwinter in areas where it is abundant, and reoccurs locally. After it re-emerges from overwintering, it walks instead of flying to find its crops.

Potato Leafhopper: This is a migratory pest that flies in from southern latitudes in its adult form, carried on the wind by major weather events. Once they have arrived in potato production areas, they can have as many as three to five generations a year depending on weather conditions.
The lime-green leafhopper lays eggs in the foliage, which then hatch into nymphs. The nymphs resemble the adults and are small, bullet-shaped, lime green in color, but lack wings. They are no longer than a 1/8 of an inch in length during the nymphal stage, and are ¼ inch long as adults.

Wireworms: Wireworms are the larval stages of the click beetle, and are slender, yellowish-brown in color and approximately ¾-inch long when in late instar stages of development.

They overwinter in the soil as larvae, emerging as adults in late spring to mate and lay eggs on the soil surface. Once the eggs hatch in approximately two to four weeks, young larvae make their way through the soil to find food. The adult beetles have a preference for egg laying in gray areas, so they tend to aggregate in old pastures. The larvae that are already present can feed directly on potato tubers.

The Damage these Pests Can Do

Colorado Potato Beetle: Both the larval and adult form are general defoliators. They chew through the plant’s leaves, and while the early larvae don’t cause as much damage as later stage larvae, the late stage larvae can rapidly defoliate a potato canopy if not controlled effectively, Groves explains.
If left uncontrolled, they begin by feeding on the outermost new growth of the plant and work their way into the older growth.

Potato Leafhopper: The potato leafhopper has a sucking mouthpart that it uses to pierce into the vascular tissue of the plant and feed on the plant juice, sapping it of nutrients.

However, according to Groves, damage is most commonly seen on the plant as a result of a condition called hopper-burn, which is the plant’s physiological response to the leafhopper’s saliva. The saliva causes the vascular system to be disrupted, and the leaves that have been fed on curl up and can become necrotic.

Wireworm

Wireworms: This pest attacks germinating seeds, roots, and tubers. The damage is often seen as shallow to deep holes in the potatoes as a result of burrowing while feeding. While they do not tunnel all the way through the tuber, their damage causes reduced quality and unmarketable tubers. The tunnel may also create an entry point for pathogens, which may lead to tuber rot.

Scouting and Cultural Control Tips

Colorado Potato Beetle: Because the pest overwinters locally to previous year’s potatoes, its presence is somewhat predictable. If potatoes are grown in an area where the pest has previously been prevalent, you can use a degree day model to help predict when the beetles will emerge from the ground and colonize emerging potatoes.

Overwintered adult beetles do not tend to feed intensively, according to Groves. After they emerge from overwintering they are typically only feeding to acquire enough energy to mate and lay eggs.

For this reason, Groves suggests to not spend much time controlling adult beetles as they do not cause any economic defoliation. The pest should be scouted for as it’s laying eggs and any controls should be applied to the first generation, and specifically the first and second instar stages of this first generation.

A very successful cultural control for potato beetle is crop rotation, according to Groves. Because the pest walks instead of flying to get to its food in the early season, rotating potatoes at least ¼ mile away from previous year’s potato is an effective cultural control measure.

Potato-leafhopper

Potato leafhopper. Photo courtesy of University of Wisconsin, Vegetable Entomology

Potato Leafhopper: Scouting should be frequent and begin at the end of May or by early June. The current threshold is one adult per sweep using a standard insect sweep net.

In the spring, when there are several, successive weather systems that can transport large numbers of adult leafhoppers it may be necessary to treat for this pest four to six times during a growing season. If there are no weather events that bring the pest in and population levels remain very low, you may need to apply controls only once or twice.

One key way to control potato leafhopper is through varietal selection. According to Groves, there are some varieties in the market that possess greater tolerance to leafhopper feeding, or hopperburn, and other varieties that are not as tolerant.

Varieties that Groves say don’t suffer as much from leafhopper damage include yellow fleshed tubers such as ‘Yukon Gold’, whereas round white market classes are often quite susceptible.

Wireworms: Scouting for wireworms is challenging, Groves says, but past history of the pest can be a good indicator for the likelihood of its presence. If you are planting into new ground, any time you till you should scout to see if the pest is present in the soil.

Groves also suggests cultivating the soil a year in advance of planting in the fall to bring any wireworms to the surface so they can be picked at by natural enemies. A later fall planting before the pest gets too deep into the soil to overwinter also might be effective. This limits the insect’s ability to reach depths in the soil where overwintering success is greater.

Chemical Controls You Can Use on these Pests

Colorado Potato Beetle: Chemicals in the spinosad family work very well for control of the first and second instar stages of the Colorado potato beetle, and Groves has seen success specifically with Blackhawk (Dow AgroSciences), Radiant (Dow AgroSciences), and for organic growers, Entrust (Dow AgroSciencs). These should all be applied during the first generation only at the early larval stages of development.

Another active ingredient Groves says can be used at this time includes abamectin, including products such as Agri-Mek (Syngenta) and Epi-Mek (Syngenta). Another product that can be used is the insect growth regulator novaluron, registered as the product named Rimon (Arysta).

Potato Leafhopper: According to Groves, once you have reached the threshold of one adult/sweep with the leafhopper, they are fairly easy to control with a synthetic pyrethroid insecticide such as Asana (Valent), Warrior (Syngenta), and Mustang (FMC). For organic growers, a natural pyrethrum compound known as PyGanic (MGK) can be used.

Wireworms: Wireworms are difficult to control chemically, says Groves, and the best practices here are cultural ones. Avoiding areas where there has been wireworm damage and tilling one to two years in advance of potatoes are best management practices, as there is very little that can be done chemically once the pest is present