Causal agent(s) and transmission
The Colorado potato beetle (CPB) is a small insect from the order Coleoptera, originating from central America but which is now present worldwide. It may cause severe problems in areas with a hot growing period, where several generations of the insect may exist during the season.
The adult has a length of about 10 to 12 mm and a rather rounded aspect. Its anterior part (head and thorax) has an orange-yellow-colour with several black spots and the two hardened forewings (elytra) are pale yellow with five black stripes on each elytra (photos 1 and 2).
The adult lays orange-yellow eggs (photos 3 and 4) grouped in clusters on the underside of the leaves.
The larva, which is slightly curved and orange-red in colour, has a double row of black spots on the abdomen (photo 5). Larvae pass through four larval stages and those of the third stage (L3) are especially voracious. Numerous larvae and/or adults can quickly defoliate a crop.
At the end of the last field generation, the larvae enter the soil of potato fields or adjacent borders and transform into pupae and later on into adults which can overwinter in the soil.
Colorado potato beetle develops mainly on potatoes but it can also attack other cultivated or wild Solanaceae (eggplants, tomatoes…).
In the event of high infestation, larvae of the Colorado potato beetle can severely defoliate the potato crop and seriously impair the enlargement of the tubers. Early attacks of the Colorado potato beetle can thus result in heavy yield losses.
The introduction of this pest is forbidden in some protected areas which impose strict controls on imported plant materials.
However, a warm climate is an essential factor for its development.
Symptoms on foliage
The adults overwinter in the soil at a depth of about 20 to 30 cm and they move upwards to the soil surface in spring when humidity and temperatures are favourable for flying or for food search. Sometimes, the adults can even damage the stems.
After egg laying by the mated females, the young emerging larvae can be highly destructive and as a result of their voracity they can consume all parts of the foliage (photo 1).
The larvae start to devour the leaves which carry them, then move onto the neighbouring leaves up to the top of the plant. They are voracious and they can rapidly defoliate a complete plant.
As well as the impact of high insect populations in the environment in the previous year and the presence of host plants, the climate is the most important regulating factor in the development of the Colorado potato beetle.
In fact, the duration of the life cycle from the egg to the formation of summer adults (beetle) is highly variable and depends on the summer
temperatures: it lasts for about one month in hot summers but may last for 2 to 3 months, in cool summers.
Consequently, the temperature has a direct effect on the number of generations formed annually and one to four generations may be formed each year depending on the climate.
The management of the Colorado potato beetle consists of a combination of several methods, which may include cultural, biological, physical and chemical practices:
- control of groundkeepers and weeds as well as crop rotations can reduce or delay the pest pressure. Because overwintering adults need to feed before walking or flying into new fields, controlling groundkeepers and weeds, such as nightshades, reduces the early food source for the emerging adults and this may reduce or delay population growth. As these beetles are weak flyers, crop rotation and planting crops such as cereals after potatoes help to reduce their migrations from overwintering sites to new fields;
- avoid cultural operations when the larvae are trying to penetrate the soil (summer), as this helps them to bury themselves deep down;
- other control methods have been developed in some countries to trap or kill the insects (with the use of plastic-lined trenches, propane flamers, or vacuum traps) to limit the use of insecticides (although the cost of such techniques is usually higher than that of chemical control methods);
- several insecticides are effective for controlling the Colorado potato beetle, but it is recommended to change the type of the applied product at each application in order to avoid the development of resistance. Many of the insecticides authorised against aphids are also effective against the Colorado beetle, especially on the young larvae which are the most susceptible to insecticides, whereas the adults, eggs and pupae are very resistant. Early chemical treatment of young larvae can more efficiently prevent the feeding damage made by the older larvae which are difficult to kill and can be devastating;
- biological insecticides such as Bacillus thurigiensis are authorized in some areas for the management of Colorado potato beetles in potato crops.