The Colorado potato beetle (Leptinotarsa decemlineata), also called the potato bug, is essentially specific to the potato plant, at least in our gardens. Although it can theoretically also attack the foliage of other plants in the Solanaceae family, such tomatoes, peppers, daturas, petunias and others, it almost never does.
At this time of the year, you’ll soon be seeing the adults popping out of the ground and chowing down on the leaves of your potato plants, if indeed they’re not already present. And although the pest was originally limited to the Rocky Mountains (whence the name “Colorado”), it is now widespread across North America, Europe, and Asia: wherever potatoes are grown.
It is a very easy insect to recognize: it’s a rounded beetle, much like a giant ladybug, but yellow with 10 black stripes rather than being spotted like true ladybugs. The larvae are orange marked with black points. Since each female can lay up to 800 eggs, it is very prolific!
Controlling the Colorado potato beetle is very difficult… unless you stop growing potatoes! Many community gardens have learned this and have banned potatoes from their lots. They have learned from experience that gardeners struggling with potato beetles will stop at nothing in trying to control them and tend to treat their potatoes with particularly lethal pesticides, which in turn provokes the ire of neighboring gardeners less accepting of poisonous insecticides. So they ban potatoes all together, allowing everyone to garden in harmony.
That’s a valuable life lesson the laidback gardener should consider: no potatoes, no beetles. Problem solved! You simply have to go buy your potatoes in the supermarket like everybody else.
But if you really want to grow potatoes in spite of the need to control the Colorado potato beetle, here are some tips to help you.
Prevention is Better than Cure
First, if the beetles are not yet present on your plants, you can keep them off by covering them with a floating row cover securely fixed to the ground. Water, air and sunlight can penetrate this barrier, but not the beetles.
For this technique to work, though, it is essential that you carry out crop rotation. In other words, you have to have planted your potatoes in a section of garden where there were no potatoes the previous year. If not, when the beetles emerge (they spend the winter in the soil at the foot of last year’s potato plants), they won’t be excluded by the barrier, but instead trapped inside it… where they can enjoy a scrumptious potato leaf meal safe from your efforts to control them!
The Horse is Out of the Barn?
If the beetles have already arrived, try one of the following treatments:
• Pick hand adults and larvae and crush them or drop them into soapy water. Or, in the case of larvae, just knock them to the ground, as at that stage they don’t have the ability to climb back onto the plant;
• Run a hand vacuum over the foliage to pick up both adults and larvae;
• Search and crush the orange eggs found beneath the leaves
• Liberate Asian ladybugs (predators on potato beetles) on your potato plants and hope they don’t all fly away;
• Spray regularly with appropriate insecticides (pyrethrin, neem, etc.);
• Treat with BTT (Bacillus thuringiensis tenebrionis)… if you can find it. This is a natural bacteria specific to Colorado potato beetles that will kill them if applied correctly. Note that this is not the usual BT found in garden centers. That is BTK (Bacillus thuringiensis kurstaki), specific to caterpillars, not beetles. Nor is it BTI (Bacillus thuringiensis israelensis), sold to treat mosquitoes. Unfortunately, BTT (“potato beetle BT”) is not widely available. In Canada, it’s not available at all; you’d have to import it from the USA.
• Finally, the second most laidback trick for controlling potato beetles (the first being not to plant potatoes at all) is to plant your potatoes not in rows, which simply concentrates the scent that attracts potato beetles, but here and there among your other vegetables. That way potato beetles often can’t even find your plants!