Here’s a quick review on insect monitoring in alfalfa from Vonny Barlow, entomology farm advisor for University of California Cooperative Extension, Riverside County.
The most common insect sampling method in alfalfa utilizes 15”-wide sweep nets. A standard method for sampling: Swing the sweep net in a 180-degree arc, so the rim of the net strikes the top 6-8” of alfalfa. Hold the net slightly less than vertical so the bottom edge strikes the alfalfa before the top edge. This helps move the insects into the net.
Each 180-degree side-to-side sweep counts as one sweep. A common practice is to sweep from right to left, walk a step, and then sweep from left to right. After taking the desired 10 sweeps at each location, quickly pull the net through the air to force the insects into the bottom of the net bag.
Grab the net bag with a hand at about the mid-point. Count the insects and divide the total by 10 to get the average number of insects per sweep. To gain a good insect count, take sweep net samples in four different areas of the field. Refer to the UC-Davis IPM Web site for specific sweep net sampling guidelines for each pest.
For shorter-regrowth alfalfa, don’t rely on sweep-net sampling to determine population levels. Examine plant stems for insects and recently damaged foliage. Randomly choose five stems from four areas per field. Place each stem sample over a white pan and tap. This will dislodge the insects into the pan for assessment.
Yellow sticky traps can be used for estimating relative densities of flying insects. The traps can capture significantly more insects, including ladybird beetles. Sweeps or stem counts are more useful to determine adult density changes over time.
A problem in implementing an IPM program in alfalfa is the lack of information on the compatibility of insecticides with natural enemies. Evaluate the relative toxicity of selected pesticides to natural enemies, including parasites, aphid predators, Lepidoptera and the Egyptian alfalfa weevil.
Some pesticides are highly toxic to some or all life stages of natural enemies, including broad-spectrum insecticides like carbaryl. Products including Bacillus thuringiensis and cyromazine are relatively non-toxic to all life stages of natural enemies.