The Unique Challenge of Colorado Potato Beetles
It wasn’t until about 150 years ago that Colorado potato beetles began to play such a dramatic role in potato production in the US. Before then, this harmless insect fed on a handful of weed species in the foothills of the Rocky Mountains. As the first pioneers brought potato production to the western territories, the potato beetle found a new source of food. This pernicious pest migrated eastward along the routes of the pioneers, reaching as far as the East coast by 1874.
Most of us are bitterly familiar with the Colorado potato beetle and the rampant destruction which it can cause. This dome-shaped insect is easily identified by the five black stripes on each yellow wing cover. Adults overwinter deep in the soil and emerge in late spring with a fierce appetite. Upon emergence, potato beetles travel on foot for several days and sometimes over a mile to seek out a food source. While they predominantly feed on the tender leaves of young potato plants, all members of the nightshade family (potatoes, tomatoes, eggplants, peppers, tomatillos) are at risk.
The female Colorado potato beetle can lay up to five hundred bright yellow-orange eggs in clumps of 15-25. From these eggs hatch the larvae, which do the most damage to foliage. These larvae resemble plump orange grubs and feed for 2-3 weeks through four stages of molting, finally crawling into the soil to pupate. Here in the Northeast, a second generation emerges after a brief resting period. Regions south of here experience yet a third generation before adults go into hibernation in late fall.
Organic Methods of Controlling Populations
As damage to potato crops has grown ever greater, farmers have employed a multitude of synthetic pesticides. Given its rapid rate of reproduction, the potato beetle developed resistance to many of these pesticides, resulting in more regular spraying of the most persistent chemicals. Most organic growers take a multi-faceted approach to control of the Colorado potato beetle.
Prevention. Organic control of the Colorado potato beetle depends greatly on cultural practices, the most important of which is crop rotation. Moving host plants to a new location can cut populations significantly as they struggle to find a viable food source in the springtime. As I mentioned above, post-emergence adults are known to travel great distances, but every little bit helps. Barriers such as rivers or roads between fields can help to amplify the effect of rotation.
Physical exclusion is best achieved by lightweight row cover set over potato plants before the beetles arrive in the spring. Early placement and well-sealed edges ensure that any hungry beetles cannot reach young plants. Using straw mulch around the base of plants after an early hilling has been shown to be an effective measure of exclusion as well. The mulch makes it harder for beetles to find potato plants, as well as creates an environment that favors natural predators.
Trap Crops. Many growers use traps as borders for their potato crops. The most common of these is a wide ditch dug around the perimeter of a potato field. This ditch is steep on both sides and is lined with black plastic. Since the beetles can only walk after emerging from hibernation, they must cross this ditch, where they slip on the steep sides and reach their timely demise under the hot sun.
Physical Removal. Many potato growers resort to picking adult beetles off by hand, killing one at a time by drowning or crushing them. Squishing the orange egg deposits on the undersides of potato leaves is a great way to efficiently extinguish the next generation. For obvious reasons, this tactic is only practical for small scale producers and home gardeners.
Flaming has become more popular in recent years as the equipment has become a bit more polished. This must be done as the adults first inhabit the plants and on a sunny day while the beetles are active on top of the foliage. The young potato plants will be able to recover from a brief exposure to intense heat, but the beetles will not.
Organic Pesticides. In recent years, a strain of the naturally occurring Bacillus thuringiensis bacterium has been touted as an organic alternative to these dangerous synthetic pesticides. While this is certainly an effective organic treatment, it should be cautioned that the development of resistance remains a concern whenever we rely heavily upon a single class of pesticide for control. There are a multitude of bio-insecticide brand names in which B.t. serves as the active ingredient.
Here at High Mowing, we know the frustrations that these guys can cause—the past two years we’ve worked hard to protect our small potato crops so those tricky potato beetles they have instead found and decimated our eggplants all the way back to the stems. It can feel dire sometimes—and I assure you that picking potato beetle larvae and squishing eggs is not among our favorite summer activities—but it’s a good reminder of how it pays off to maintain diligent scouting, take immediate action, and always start out with healthy plants that will be more likely to be able to bounce back after significant damage.