Determining whether an insect is a threat of economic consequence is more important than a pest’s presence in the field.
Will it do more damage than it costs to control it with insecticide? In a year when commodity prices are high, the numbers of insects or the amount of damage they are allowed to inflict on a crop fall in relation to whether or not they should be controlled.
In low price years, they rise. The cost of controlling the insects or the narrow window for control are factors influencing producers’ calls to action.
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Further complicating control decisions is the effect of insecticides on parasites that might attack crop pests. Spraying generally kills them also, limiting nature’s ability to deal with farm pests in the future.
“Whether to control a population can be a complicated decision. But one that can be critical to profitability,” said Scott Hartley of Saskatchewan Agriculture.
Choosing the right method to collect insect population data may be the most important decision once a pest is identified.
Provincial crop protection guides offer lists of pests, their approximate economic thresholds and best methods to count them.
It’s important to get the right tool for the job. Sometimes it’s the eye, sometimes a trap and sometimes it’s a sweep net.
Observation is the method best used for insects that will disappear quickly or be mashed together in a gooey ball during capture.
With the aid of a hand counter, grasshoppers can be counted as they jump. Pace off 50 metres adjacent to the area to be evaluated. Then walk slowly but loudly, counting the jumping insects in about a one metre square area. Once one area is disturbed and counted, move on to the next and repeat. At the end, divide by 50 and an average number per square metre can be estimated.
For tiny and elusive pests, such as flea beetles, damage to the plants sets the threshold for control. Leaf loss or pod predation will govern whether the sprayer needs to be called into action.
Entomologists say using the sweep net properly and knowing how many insects are caught in an established number of sweeps of a set pattern are critical to establishing if there is a problem or just a presence.
Nets should have a 15-inch (37.5 cm) opening. Muslin nets are tougher, while polyester is lighter and easier to use. Sailcloth is the toughest.
Following the standard number of sweeps and pattern outlined for each insect will net the appropriate statistical information.
How to do insect field counts with a sweep net:
Before you start: Contact your local agrologist to determine the appropriate number of sweeps to take, the number of samples to make and the economic threshold for each insect and crop.
1. To make a sweep: Begin at your side and swing the net 180 degrees through the crop canopy so the top of the net is at crop height. If there isn’t a canopy or the crop is short, sweep close to the ground. Take at least one complete step forward, then sweep again.
2. At the end of the last sweep, raise the net above the crop and quickly swing it back and forth to force the insects to the bottom.
3. Grab and hold the net 15 to 20 centimetres from the bottom to trap the insects.
4. Count the insects you have trapped. The counting system you adopt will depend on how well and quickly you can identify the insects in your net, the number of species present and the total number of specimens.
- If you have only a few insects to count: Hold the net closed, invert it through its rim, then slowly open it. Count the insects as they escape, or empty them into a plastic bag or bottle.
- If you have a large number of insects to count: Empty the contents of the net into a plastic bag, seal and place the bag in the freezer for 30 minutes to slow the insects down.
