Food security

How do farmers and agronomists determine which pests need to be managed?

Have you ever looked for insects? Insects are ubiquitous in our environment. For farmers, many types of insects live in their fields. Some are beneficial like pollinators and predators. Others, though, can be called pests because they cause serious crop loss.

When does an insect population go from being tolerated by the farmer to a must kill-decision? Agronomists and farmers use integrated pest management to decide if pesticide application is warranted.

two photos - one shows green and yellow catepillar rolled into circle it is very small compared to a finger, other photo shows green catepillar which is about twice as long as the finger is wide
Cutworm (L) and cabbage looper (R) caterpillars before pupa stage. Integrated pest management guidelines are to not spray at this size. Photo credit: Lesley Lubenow

Integrated pest management gives clear guidelines to when it is appropriate to spray pesticides. Each guideline details the growth period(s) during which crops are most susceptible to damage. This helps growers know when to keep a keen eye out for problems. The guidelines also help with identification of the pest. Finally, they help to determine what size pest population will damage yields. This helps farmers determine when to treat the pest problem.

Here is an example with a crop of canola. Canola is susceptible to damage by insects named flea beetles from the time the seedlings emerge from the ground until the plant has four leaves.

Using the integrated pest management guidelines, farmers and agronomists must scout the fields every 24 hours during this important growth stage to protect their crops. They are looking to see if flea beetles are causing 15-20% defoliation damage to the plants. When this threshold is met, farmers are advised to use a pesticide to kill the flea beetles. Flea beetle damage below 15% defoliation is tolerated. And, once the canola plants have grown past having four leaves, no more scouting is needed because the plant is strong enough to tolerate more flea beetle feeding.

small green leaves with about 20 holes in them and a shiny small beetle on the leaf
Flea beetles (Phyllotreta spp.) feeding on young canola plants. Since the leaf defoliation is below 15 to 20% of the total area the correct decision is do not spray and scout again in 24 hours. Credit: Lesley Lubenow

Other integrated pest management guidelines take into consideration the behaviors of the pest. Some pests like the flea beetles (above) and grasshoppers invade fields from edges of the field – the field margins. Farmers may choose to spray the field edges for the pest instead of the whole field. This action reduces environmental impacts of insecticides and saves the farm operation money.

In other cases, it’s not the size of the plant – or the damage – that can influence a decision. Some insects behave predictably and are known to visit farm fields for only short periods. They are considered “occasional pests.” The damage they cause is worth much less than the cost of treatment.

Blister beetles are an example of this type of pest. They migrate to faba beans fields only for two to three days before they move on. The integrated pest management guideline for this pest is to spray only when crop injury is widespread across the field. The plant injury might look terrible on individual plants. But if only a few beetles are feeding in a small 10 x 10 ft area, the damage of a few plants will not decrease total crop yield across a large 160-acre field. From a net income standpoint, the farmer costs of the insecticide and water, tractor and sprayer operation, and use of work time are rarely economical. It’s best to rely on the habits of these beetles and know they will leave the field without impacting yield.

Nuttall’s blister beetle (Lytta nattalli) overlooks its injury on faba bean. Since this beetle doesn’t stay in one place long, most farmers do not treat this insect pest. Credit: Lesley Lubenow

Other integrated pest management guidelines take into consideration the life stage of the insect. As hungry caterpillars grow into pupas, they can eat a lot of leaves. You may be familiar with cutworm pests who lop off tomato transplants in the spring. They also attack sunflowers, soybeans and other crops. Integrated pest management guidelines for cutworms state that once the caterpillar has reached 1.25 inches long, farmers do not use a pesticide to kill them. At this point, the cutworm has grown to their maximum size, ate most the leaf material it needs to grow, and will be changing into a pupa soon.  If the farmer finds large amounts of smaller caterpillars, then the decision to use a pesticide is made.

Integrated pest management guidelines also suggest non-chemical solutions to problems. Farmers can choose to plant earlier or later to miss the window when an insect appears. They can use a cover crop to hide plant scents from searching insects. For gardeners, non-chemical solutions include using a water hose to wash aphids off a plant, picking off Colorado potato bugs into a bottle, and other solutions.

To spray or not spray? That is what integrated pest management guidelines answer. Crop science is continuing to research and answer these questions. IPM guidelines are published and available to the public by state cooperative extension systems. Additionally, extension educators teach crop scout schools and publish IPM information for farmers and agronomists to stay up-to-date during the cropping season.

Answered by Lesley Lubenow, North Dakota State University Extension

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

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