Sweeping Fields Tracks Pest Populations and Protects Crops

More worms are showing up in alfalfa fields. Lygus bugs are turning up more often in cotton fields.

Keeping track of the number of bad bugs and good bugs that are making a home in your cotton and alfalfa fields is quite important as summer heats up. Pest populations can suddenly explode and gobble away at your harvest time profits.

“Pest populations are growing. They can grow pretty fast,” says field scout Carlos Silva.

Carlos uses his sweep net to check on pests in an alfalfa field.

Each week, Carlos heads into the blistering heat to survey pest populations in cotton and alfalfa fields across the San Joaquin Valley. His scouting provides a second set of eyes for growers, supplementing information gathered by their pest control advisors and the farmers themselves.

A close-up of alfalfa weevils in his sweep net.

His tool of the trade is a sweep net – a kind of oversized butterfly net. Carlos will pick three locations in a field and use tennis-like forehand and backhand moves and counting the number of passes as the net snags collect bugs hiding in the fields. “Take time to look at the net,” he says about carefully inspecting the pest haul.

 There’s a certain art and science about using a sweep net. The effectiveness of this sampling practice hinges on how the net is used. Following standard practices ensures results from different people are comparable.

Here’s a refresher course covering alfalfa and cotton:

Alfalfa: Sweep the field when the plants are at least 6- to 10-inches tall. Swing the net in a 180-degree arc so the net’s rim hits the top 6 to 8 inches. Hold the net less than vertical to ensure the bottom edge hits the alfalfa first. Many people will sweep from right to left, and then take a step and sweep again left to right. After 5 sweeps, pull the net through the air to push the bugs into the bottom of the net bag. UC IPM recommends taking a sample in four different spots in the field.

If it’s difficult to count the bugs in the field, place the insects in a bag and cool the contents to slow done pest movement.  UC IPM provides information about treatment thresholds for various pests.

For Cotton: Start to sweep for lygus at first cotton plant square, taking samples twice a week in the field. UC IPM recommends sampling each quarter of a field and taking even more samples in fields larger than 8 acres. Do 50 sweeps across one row of cotton to sample for lygus, making sure the sweeps don’t overlap. Lygus monitoring can end when acala plants have 5 nodes above white flower and Pima has 3.5.

  

You can record your pest information on a form for alfalfa and a form for cotton that can be downloaded online from UC IPM.