Populations of the Colorado potato beetle have an amazing ability to develop resistance to insecticides, including many of the carbamate, organophosphate, pyrethroid, spinosyn, and neonicotinoid insecticides that are widely used today.
The beetle's ability to develop resistance is not surprising given that this pest had to adapt to the high concentrations of toxic glycoalkaloids in its Solanaceae hosts. In fact, these beetles are pre-prepared to fight toxins of various nature.
What's more, the Colorado potato beetle produces a huge number of offspring, which greatly increases the chance of random insecticide-resistant mutants. 50 eggs per plant multiplied by 20 potato plants per acre can produce a million beetles in just one generation. And often there are two or three generations of beetle per season.
When an insecticide is first applied to a crop, a small number of target pests (perhaps as little as one in 10 million) may survive due to random mutation. If nothing else kills them, a few resistant mutants will sprout. And many of their offspring will inherit insecticide resistance.
Tips for controlling the Colorado potato beetle and preventing insecticide resistance
Potato crop rotation and spatial isolation
Adult beetles overwinter in the soil in last year's potato fields and emerge in the spring to feed, mate and lay eggs. The larvae emerge from the eggs, feed for a while, and then fall to the ground to pupate and start the cycle over again. Placing this year's potatoes far from last year's makes it difficult for the beetles to find food when they emerge.
The beetles feed only on nightshade crops and weeds and do not travel long distances. For this strategy to work, separate fields by at least 0,25 miles.
Some insecticides deteriorate if the spray solution is too acidic (eg spinosad, spinetoram). Others begin to deteriorate if the solution is alkaline neutral (eg phosmet). Some insecticides with adjuvants (eg, abamectin and thiamethoxam) should not be used.
Stages of development and growth rate
You will usually achieve a more effective insecticidal treatment if you wait for the larvae to appear. Most insecticides work best against newly hatched small larvae. It is generally recommended to foliar feed when around 50% hatch. Use the full amount (dosage) of the insecticide listed on the label for best results.
Alternate insecticides with different mechanisms of action
The use of a single insecticide (or multiple insecticides with the same mechanism of action) can quickly lead to resistance or cross-resistance in Colorado potato beetle populations. It is necessary to alternate insecticides from different groups with each application. If you don't kill the resistant mutants with one, you can probably control them with a subsequent application of a completely different insecticide.
Many potato growers report excellent Colorado potato beetle control when they apply neonicotinoids (IRAC group 4) at planting or hilling. It is known to provide 80 to 100 days of residual control. But exposure to long-lasting insecticides in the foliage can create strong selection pressures for resistance. There are many neonicotinoid-resistant populations in the East and Midwest.
Some populations of the Colorado potato beetle avoid exposure to insecticides by staying underground longer and delaying their development. This resistance strategy is called extended diapause. This is another reason to keep a close eye on beetle populations in potato fields. You can only spray when pests are present and not spray according to a calendar.
The author of this article is Carrie Huffman Wohleb, an associate professor of potato, vegetable, and seed crops at the University of Washington.