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Insect economic thresholds: what do they mean?

Q & A with CPS

Q: What do insect economic thresholds mean?

A: Insects can eat into your profits. They can appear at any time during the growing season and can cause damage that is patchy, scattered and difficult to gauge. Accurate estimation of both pest population and potential crop damage levels can only be obtained by thorough field scouting. Proper pest identification is key as damage is unique to each insect species.

Targeted pest control measures should be engaged when pest populations reach levels of economic significance — this is called the economic threshold. An economic threshold is the insect population level, or the extent of crop damage, where the value of the crop loss exceeds the cost of pest control. Typically, thresholds focus on potential yield loss and costs associated with control measures.

Economic thresholds can be based on pest population or crop damage. The threshold for lygus bugs is based on pest population. A sweep net is often used to determine the number of lygus bugs present. In canola, research has shown lygus bugs can cause a yield loss of approximately 0.09 to 0.13 bushels per acre per insect per sweep. Since damage per insect has been determined we can calculate how many insects per sweep need to be present to cause crop loss that is greater than the cost of control measures.

Sweeping for lygus bugs can reveal how much of an impact their presence will have on your crop.
photo: Jordan Peterson

Crop damage can sometimes be deceiving. This can lead to an overestimation of damage and early activation of control. The economic threshold for flea beetles is based on crop damage. The economic threshold is the 25 per cent defoliation level for canola at the cotyledon growth stage. This level of defoliation can cause yield loss due to seedling death and reduced plant growth.

Economic thresholds give growers a valuable piece of information to help make difficult pest management decisions easier.

Jordan Peterson is a manager of agronomic services with Crop Production Services in northern Alberta.

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