Colorado Potato Beetle Control on a Small Farm

One week your potato rows look perfect, and ten days later half the leaves are lace and the stems are crawling with fat orange larvae. Colorado potato beetle control on a small farm comes down to catching the first adults before they lay, then keeping each new generation from finishing its cycle.

This is the routine I use on about a quarter acre of potatoes, plus eggplant, which the beetles love even more. None of it needs a sprayer until the very end, and most seasons I never fill one.

Know the Colorado potato beetle life cycle

Every control step works because it hits one stage of the cycle. Learn the stages and timing becomes obvious.

• Overwintering adults spend winter 4 to 10 inches deep in the soil of last year's potato field and the field edges. They emerge in spring as the soil warms past about 50 degrees F.
• Eggs are bright yellow-orange clusters of 20 to 40, laid on the underside of leaves. One female can lay 300 to 500 eggs over a few weeks.
• Larvae hatch in 4 to 9 days. They are red to pink with two rows of black dots and do about 75 percent of the total leaf damage, most of it in the last two of their four growth stages.
• Pupae drop into the soil after 2 to 3 weeks of feeding and emerge as summer adults about 10 days later.

In most of the northern US you get one or two generations a year. Farther south, three is common.

Rotate far, not just next door

Beetles that wake up in spring are hungry and do not fly far at first. Most walk. If your new potato patch sits 20 feet from last year's, they will find it within days.

Moving potatoes and eggplant at least 200 yards from the previous year's plot delays colonization by two to three weeks, and research trials have shown big drops in early season numbers when rotation distance goes up to a quarter mile. On a small farm, 200 yards may not be possible. Even 100 yards, with a hedge or a tall cover crop between plots, helps more than nothing. If your rotation already feels shaky, read why crop rotation plans fail in the second year before you redraw the map.

Cover early and scout twice a week

Lightweight row cover laid over the hills at planting keeps emerging adults off the leaves during the window when they would lay the first and biggest batch of eggs. Seal the edges with soil or sandbags. Potatoes do not need pollination, so you can leave the cover on until plants push hard against it, usually 4 to 6 weeks. Our guide to floating row cover for pest control covers weights and hoop spacing.

Once the cover comes off, walk the rows on a fixed schedule. I go Monday and Thursday mornings, before the sun is high, because adults sit still in the cool. Flip a leaf on every tenth plant and look for egg clusters. If you want a system for recording what you find, the article on reading the early signs of a pest problem lays out a simple tally sheet.

A rough action threshold

Potato plants can lose up to 30 percent of their leaf area before flowering with almost no yield loss, and somewhat more after flowering. A useful rule for small plots: act when you find more than one adult or 4 larvae per plant on average, or when defoliation passes about 20 percent before bloom.

Hand picking that actually keeps up

Hand picking has a reputation as a hobby gardener method. On anything under half an acre, it is the most reliable tool you have, if you do it right.

1. Carry a bucket with 2 inches of water and a few drops of dish soap.
2. Hold the bucket under the leaf and tap the stem. Adults drop and play dead, so let them fall straight in.
3. Crush or pinch off every egg cluster you see. Each one removed is up to 40 larvae you never have to deal with.
4. Knock larvae into the bucket from the top leaves, where they cluster.
5. Repeat every 3 to 4 days through June. Skipping a week lets a new batch of eggs hatch and grow past the easy stage.

A quarter acre takes me about 40 minutes per pass early in the season. By the third week, it drops to 15 minutes because there is far less to find.

The counter-take: do not reach for spinosad first

Most organic advice says to spray spinosad as soon as larvae appear. It does work, and it is approved for organic production. The problem is that the Colorado potato beetle has a long history of developing resistance to nearly every insecticide used against it, and some populations are already showing reduced response to spinosad.

If you spray every generation, you are breeding that resistance in your own field. Save it for the moment hand picking clearly loses, and use it no more than twice per season, on different generations. A better first spray option, when you need one, is Bt tenebrionis, sold as a Bt strain for beetles. It only kills young larvae in their first two stages, so it must go on within a few days of hatching. Timing is the whole game, which is another reason those twice weekly walks matter.

Neem oil gets mentioned a lot too. In my experience it slows feeding a bit but does not drop populations enough to rely on.

Mulch and habitat for the predators you already have

A thick layer of straw mulch, 3 to 4 inches deep, laid after hilling seems to slow beetles walking in from the field edge. It also cools the soil, which potatoes like. Ground beetles, lady beetles, spined soldier bugs and the parasitic fly Myiopharus all feed on Colorado potato beetle eggs and larvae. They do not show up on bare, freshly tilled ground.

Leave a 3 foot strip of unmowed grass or flowering plants such as dill and buckwheat along one side of the potato block. It costs almost nothing and gives predators somewhere to live between meals.

After harvest

Dig promptly, pull every volunteer potato you find next spring, and do not leave cull piles at the field edge. Volunteers and culls are where the late season generation eats and builds up before going into the soil for winter.

Frequently asked questions

What is the fastest way to get rid of Colorado potato beetles?

On a small plot, hand picking adults and crushing egg clusters every 3 to 4 days gives the quickest drop in numbers. If larvae are already widespread, a Bt tenebrionis spray on newly hatched larvae works within a couple of days. Keep spinosad as a backup to slow resistance.

Do Colorado potato beetles kill potato plants?

Heavy infestations can strip plants completely, which stops tuber growth and can kill the plant. Light feeding is usually tolerated, since potatoes can lose up to 30 percent of their leaves before bloom without much yield loss.

Where do Colorado potato beetles come from each spring?

Adults spend the winter in the soil of last year's potato field and nearby edges. They walk or fly to the nearest host plants once the soil warms, which is why rotating far away is so effective.