Now that your potatoes are happily growing, and you have hilled the earth up around them, it is time to start scouting for Colorado potato beetles. This beetle (Leptinotarsa decemlineata) is a voracious feeder and prevention is by far the best cure. While potato plants are tough and can tolerate a small amount of leaf damage, yields will be reduced if the beetles are allowed to do too much damage.
The adults are distinctive: reddish-orange head and thorax with yellowish wing covers sporting 10, bold, dark stripes. They are about six to 11 millimetres long and about three millimetres wide. Beetles over winter in the soil where you grew potatoes last year or in nearby grassy areas. They generally emerge in late May or early June. The adults feed on plants belonging to the Solanaceae family such as potato, tomato, eggplant and nightshade. After mating, the females lay egg clusters on the undersides of leaves from June to late July. The yellow-orange, oblong eggs are laid in groups of 20 to 30, and individual females can lay 300 to 500 eggs. The eggs are obvious if you look on the underside of potato leaves. Remove leaves that have eggs on them, bag and put in the garbage. Ladybugs have similar eggs, also laid in clusters, but they are smaller than CPB eggs. Avoid destroying ladybug eggs as ladybugs are beneficial beetles that help reduce aphid infestations and eat the CPB eggs.
CPB eggs hatch in four to nine days. Larvae have soft, orange-pink bodies with black spots and a small black head. Initially the larvae are very small and stay in clusters, feeding on the same leaf. [Note: The ladybug larvae look very different to CPB larvae: they have longer, narrower bodies that are predominantly blue to black with some orange patches.] As they grow and molt through their four larval stages, they become larger, more noticeable and move to separate leaves and from plant to plant. Significant larval populations can build up by mid-July, and reach peak numbers by the end of the July. The later (i.e. larger) stages can destroy a lot of leaf tissue, so they should be removed from plants. You can pick them off and drop them into a container of soapy water that will kill them. Or if you wish, you can just squash them on the leaf.
The larval stage lasts 14 to 21 days. During this time they can eat a lot of leaves. The final larval stage eventually burrows a few centimetres into the ground, develops into a pupa, and a new adult beetle one to two weeks later. In most locations these adults feed briefly and as fall approaches they burrow into the soil to find a protected spot to hibernate for the winter. There is usually only one generation per growing season on the Prairies.
Management practices in your garden can help reduce the risk of CPB damage. Avoid growing potatoes in the same location in your garden for at least three years. Remove all over wintering beetles as they move onto your crop. This reduces the number of eggs that are laid and the subsequent number of hatchlings (larvae).
Bacillus thuringiensis var. tenebrionis (Bt) is a control agent available to home gardeners. It can kill the smaller first and second larval stages but not the bigger larvae, so it needs to be applied every few days as the eggs start to hatch. Avoid using broad-spectrum insecticides as these will also kill the beneficial insects.
If you have had a severe infestation, rototill the ground where the beetles are over wintering to expose them to cold temperatures and dehydrating conditions. This reduces the number of over wintering adults. Unfortunately the spring adults can fly and can possibly move into your crop from protected, over wintering locations.
Jill Thomson is a plant disease specialist (retired) who enjoys gardening in Saskatoon with her family, including the dogs.
— This column is provided courtesy of the Saskatchewan Perennial Society (SPS; saskperennial@hotmail.com ). Check www.saskperennial.ca or Facebook page (www.facebook.com/saskperennial) for a list of upcoming gardening events: July 20 - 1 to 9 p.m. and July 21, 9:30 a.m. to 3 p.m. Canadian Prairie Lily Society 52nd Annual Lily Show at Lawson Heights Mall, Saskatoon (www.prairielily.ca) .
