Reducing Insecticide Resistance of Colorado Potato Beetle

Increasing the dose of an insecticide once the number of resistant beetles has begun to increase will not prevent resistance from developing. The steps provided in this factsheet can slow down the development of resistance. Resistance of Colorado Potato Beetle Resistance of Colorado potato beetle to some insecticides has recently been detected in Manitoba, and has been a problem in areas east and south of the province. Potato producers can take steps to prevent the development of widespread insecticide resistance in Manitoba. All pest populations have a very small number of individuals that are naturally resistant to a particular pesticide (or pesticide group). These resistant individuals are normally an extremely small portion of the total pest population. Continuous use of the same insecticide or insecticide group will allow resistant individuals to become a greater portion of the total population.

Rotate Insecticides

To reduce insecticide resistance, it is important to rotate between insecticides from different chemical families, particularly if several insecticide applications are made in a season. Insecticides in different chemical families generally kill insects in different ways, whereas insecticides in the same chemical family often kill insects in the same way. Insects that survive application of a particular insecticide may be killed by an insecticide that kills the insect differently. By selecting products from different chemical families for an insecticide rotation program, the development of insecticide resistance may be reduced.

Use Insecticides Only When Needed

Overuse of insecticides can speed up resistance to the chemical. To avoid overuse of insecticides, fields should be monitored to determine the number of Colorado potato beetle per plant. Insecticide application is recommended when Colorado potato beetle larvae or adults are abundant enough that the value of the crop they consume is greater than the cost of controlling them.  This number is called an economic threshold. Insect densities at or above the economic threshold should be controlled. The economic threshold will depend on the cost of the control and the value of the crop.

 

Time Insecticide Applications for Maximum Effectiveness Larvae of the Colorado potato beetle have four stages of growth. Most of the damage by Colorado potato beetle is due to feeding by the largest (fourth) stage of larvae, when they are about 1/4 inch (8 mm) long. Insecticide applications should be targeted when the oldest larvae are in the third stage of growth 1/8 inch (about 5 mm long). Spraying too early means fewer eggs will have hatched and more applications may be needed. It is important to read the insecticide label carefully for information about when to spray. Some insecticides, such as Novodor, work best when applied to small larvae. Some insecticides do not work well under certain weather conditions. For example, many of the pyrethroid insecticides do no work well when temperatures are above 25oC. Spraying when conditions favour optimum effectiveness of the insecticide will also increase the level of control.

Rotate Crops When Possible

Crop rotation is the most important means of reducing resistance development to insecticides. Colorado potato beetles overwinter as adults within the field of origin, and within uncultivated areas near the field, such as woody borders and drainage ditches. Cool spring weather often forces overwintered beetles to walk to a new food source. The farther a potato field is from overwintering sites, the less the probability that it will be colonized by migrating beetles. This can reduce the need for insecticide applications.

One consequence of crop rotation is that early season populations of Colorado potato beetle may be concentrated at the edge of the fields. Sampling should be performed to determine if beetles are concentrated at the edge of the fields, and if so insecticides may need to be applied only to those areas.

Selecting an Insecticide

Insecticide rotations should be planned so that insecticides from different chemical families are used in consecutive treatments. The following table will help in selecting insecticides from different chemical families for control of Colorado potato beetle. More information on each insecticide can be found in the MAFRI Guide to Crop Protection.

Table 1. Chemical Families of Insecticides Used to Control Colorado Potato Beetle
 

CHEMICAL FAMILY	TRADE NAME	COMMON NAME
CARBAMATES	Sevin XLR Plus	carbaryl
	Furadan	carbofuran
ORGANOPHOSPHATES	Guthion, Sniper	azinphos-methyl
	Lorsban, Pyrinex	chlorpyrifos
	Monitor	methamidophos
	Supracide	methidathion
	Thimet	phorate
	Di-Syston	disulfoton
ORGANOCHLORINES	Endosulfan, Thiodan	endosulfan
PYRETHROIDS	Cymbush, Ripcord	cypermethrin
	Decis	deltamethrin
	Ambush, Pounce	permethrin
	Matador	cyhalothrin-lambda
BIOLOGICALS	Novodor	B.t. tenebrionis

For further information, contact your GO representative.