July is here, and so are the bugs! Two insect species are notorious pests on growing potatoes in mid-summer: potato leafhoppers and Colorado potato beetles.

Potato leafhoppers

Potato leafhoppers (Empoasca fabae) are small green or brown insects of no more than ¼ inch that move and jump very quickly, so it can be hard to get a good look at them. These insects suck the sap from growing plants. To aid in feeding, they release chemicals from their saliva into plants. These chemicals cause potato leaves to curl, and eventually to become brown and crisp on the edges. The symptoms caused by leafhopper feeding are usually called hopperburn. Even mild hopperburn will reduce plant growth. Most potato varieties are susceptible to hopperburn. As far as we know, leafhoppers cause damage mainly due to the toxin in their saliva and not because they are transmitting any significant potato diseases.

A useful presentation on leafhoppers in potato is here: http://www.nysipm.cornell.edu/vegetables/nofa/NOFA_08_plh.pdf

The most effective method we have found to manage leafhoppers is to use tolerant potato varieties. For example, in our weed-control trials, we use Freedom Russet, which is quite tolerant to leafhoppers. We also spray plots with the OMRI-approved pyrethrum-containing insecticide PyGanic, once we reach the threshold of one adult for every two sweeps with a net. It’s important to recognize that pyrethrum is a broad-spectrum insecticide and can kill beneficial insects including bees. Since pyrethrum breaks down in UV light, it is recommended to apply it at dusk, and this may reduce the effect on bee populations. Providing unsprayed habitat is an important aspect of  protecting beneficial insects – the predators, pollinators and parasites. A planting guide is here: http://www.motherearthnews.com/organic-gardening/plants-to-attract-beneficial-insects-zl0z1005zvau.aspx

As with all pesticides, it is important to follow the label directions and treat pyrethrum-containing insecticides with care.

Colorado potato beetles

Colorado potato beetles (Leptinotarsa decemlineata) are common throughout the Midwest. The adults overwinter underground and emerge in late spring. Adult females lay clusters of bright orange eggs on potato leaves, which hatch into hungry larvae. The larvae eat potato leaves and, if enough are present, they can strip all of the leaves from a growing potato plant. Eventually, the larvae will turn into prepupae and burrow into the soil, where the insect will turn into an adult and the cycle will begin again. In Wisconsin, these beetles can go through more than one cycle per summer.

Colorado potato beetles only cause damage from eating potato leaves. They do not produce a toxin, so do not stunt potato growth. Also, Colorado potato beetles do not spread any important potato pathogens. Useful photos and information about Colorado potato beetles are available here: http://en.wikipedia.org/wiki/Colorado_potato_beetle

Organic farmers can use weaknesses in the beetle life cycle to manage them. For example, adult potato beetles do not usually move far from where they emerge from the soil, so planting potatoes a few hundred yards from where they were the previous year will reduce losses. Farmers can also wait to plant until July and avoid the first round of emergence of Colorado potato beetle adults.

If you have a large number of potato varieties growing in a small space, you may notice that the beetles show a strong preference for some varieties and will leave others alone. Some farmers try to take advantage of this preference by planting eggplant near potatoes since Colorado potato beetles have been reported to prefer to feed on eggplants. We have had poor outcomes with this strategy. If you have a large amount of a single potato variety growing in a field, the beetles will eat the leaves, even if they ignored them in a mixed-variety plot. This is because when the beetles have no choice, they will voraciously eat any potato variety that is present.

Some options that may work for beetle management in larger fields include plastic lined trenches that trap adult beetles walking into the field and straw mulches that harbor beetle predators. Row covers and straw mulches are also recommended for beetle control. We have had poor outcomes with row covers, with low yield being the largest problem. In contrast, outcomes with straw mulch for yield, and weed and soil moisture management have been positive. We are still collecting data on insect management in these plots. These management methods are detailed here: https://attra.ncat.org/attra-pub/summaries/summary.php?pub=132

This year, we have had low numbers of beetles in our research plots, and have been able to manage them by crushing the egg clusters, and knocking the larvae off the leaves into a bucket of soapy water. Recently-emerged larvae cluster together at the top of the plant, and can be easily collected. This is an ideal management method for small potato fields (and I have to admit it’s kind of fun). We have previously managed Colorado potato beetles using Entrust, a spinosad product. This is a bacterial toxin that stops the larvae from feeding. After a few days, the larvae will starve to death. Spinosad can affect some beneficial insects, so it is best to spray it only in badly infested areas, and to spray at dusk. Entrust can be tank mixed with PyGanic. Spinosad history and use are described here: http://www.motherearthnews.com/organic-gardening/spinosad-zmgz11zrog.aspx#axzz36VmGwNI6

What are your methods for managing and avoiding pests, and for protecting the beneficial insects in your potato fields?

This article was posted in Blog Posts and tagged Colorado potato beetle, organic potato, potato leafhopper, variety trials.