With the hot dry weather we have been having, spider mites are beginning to become an issue. The two-spotted spider mites are usually more prevalent in areas of heavier drought stress. We have begun to get more rain which will deter the mites slightly, but they may have already laid eggs on the plants. Spider mites cause foliar damage or discoloration on the top of the soybean leaves. They leave yellow patches or stippling on the leaves. When scouting for spider mites, you’ll want to look at the underside of the leaves. They will be along the field margins initially and make their way farther into the field in half circle pattern. Mites are extremely small and hard to detect with the naked eye. A hand lens may be required to properly identify them. Eggs will appear as small clear or yellow marbles and newly hatched nymphs will have 6 legs. The eggs will hatch within 3-5
days of being laid. Older more mature nymphs will have 8 legs. Their life cycle is short, lasting anywhere between 5-30 days which means they can produce up to 10 generations per year during our growing season. However, unlike many pests, spider mites do not have an economic threshold in which they need to be kept under. If the area that is infested begins to grow larger, that is an indication that action should be taken to control their spread. If the weather forecast continues to call for hot, dry, weather, that is another indication that a spray application may need to take place. However, with the heavy rains some places have been experiencing, the storms may be enough to knock the mites off of the plants and reduce their impact.
At this point in the season, many aphid populations have already been sprayed for control. Like the spider mites, aphids are found on the underside of the upper soybean leaves. They are small and yellow and are more visible to the naked eye. Aphids are born as pregnant females and they lay live young rather than eggs. When you are scouting for aphids, you want to sample from around 30 locations in the field to get an average number per plant. Unlike the spider mites, they will not be concentrated on the field edges; they will be spread throughout the field. The economic threshold when scouting for aphids is 250 aphids per plant. This threshold signals that an insecticide should be applied to control damage. If the levels reach 1,000 aphids per plant or higher, that is when economic damage may ensue, so be sure to spray before the populations reach that level.
A typical rule of thumb when scouting fields is if you see ladybugs throughout the field, you have aphids. Lady beetles are natural predators to aphids and will control the population to a certain extent. If they are not controlled, they can stunt the soybeans, curl the leaves and make them appear a yellow shade around the edges, as well as promote the growth of mold and other diseases.
– Cassidy Fletcher, Northeast Sales Agronomist and Technical Lead