Two-Spotted Spider Mite Management in Soybean and Corn

A3890 wo-spotted spider mite T management in soybean and corn eileen cullen & sarah schramm Common Name–Two-spotted Spider Mite Scientific Name–Tetranychus urticae Koch (Arachnida: Acari: Tetranychidae) 0.006 inch Two-spotted spider mite (Tetranychus Appearance urticae) is distributed worldwide with Eggs. Adult females deposit an extensive host range. Because it can round, translucent eggs singly on the affect orchard, row, and greenhouse undersides of leaves (figure 1). A hand crops, this mite is one of the most lens (at least 10x magnification) is 0.016 inch important agricultural pests throughout needed to see eggs, 0.006 inch (0.14 the world. A closer relative of spiders mm), on crop foliage in the field. Eggs than insects, mites are tiny webspin- become opaque to pearly white, with ning arthropods with eight legs. In the red eyespots of the immature larva Wisconsin and the Upper Midwest, visible just before hatch. two-spotted spider mite is an occasion- Figure 1. Two-spotted spider mite eggs, Immatures. Development in- adult and nymph al pest of soybean and corn. However, cludes one translucent six-legged larval Photo by Peter Sonnentag and Tom Klubertanz this mite only becomes a problem in stage followed by two eight-legged dry weather conditions. In most years, nymphal stages. Immatures feed on Life cycle with adequate rainfall and a fungal plant foliage just as adults do. pathogen as a primary natural control, In northern states, two-spotted spi- Adults. Adults are tiny, about outbreaks do not occur. In the absence 0.016 inch (0.42 mm), with four pairs der mites overwinter as adult females of these checks, spider mites reproduce of legs. They range in color from in field margins and similarly sheltered quickly with overlapping generations greenish yellow to dull orange with two areas. The time it takes for eggs to de- during which eggs, nymphs, and adults irregularly shaped black spots, one on velop into reproductive adults depends can be found together on infested either side of the abdomen (figure 1). upon summer temperature, humidity, plants. and host plant. Generations are typically completed in 4–14 days with faster developmental rates above 91°F. Spider mites have a high reproductive potential, with 7–10 generations or more in one growing season if hot, dry weather conditions persist. Outbreaks are most likely with extended dry weather conditions, even at relatively moderate temperatures. Over a 30-day life span females lay an average of 90–110 eggs, though up to 300 have been reported, resulting in exponential population growth. Spider mites do not have wings. Mites crawl to locate new leaves within the soybean or corn canopy and move to adjacent plants. To disperse greater distances, mites climb to the top of a plant and spin silken web strands that, Figure 2. Two-spotted spider mite damage symptoms at the field level in soybean when caught on breezes, carry the Photo by Mike Rankin mites to new host plants and fields. two-spotted spider mite management in soybean and corn agement decision-making due to the small size of this pest. Scouting and action thresholds rely upon confirmation of spider mite colonies in the field and rating of plant injury symptoms. Crop injury from drought stress or foliar dis- eases can be confused with spider mite damage. Be sure to base miticide treatment decisions on confirmed presence of mite colonies in the field rather than relying only on apparent plant injury symptoms. Soybean scouting. Begin at field edges where infestations are likely Figure 3. Foliar bronzing symptoms in Figure 4. Two-spotted spider mite feeding to start. Green plants within the field injury on a corn leaf soybean caused by two-spotted spider mites may also be affected. The entire field Photo by Mike Rankin Photo by University of Wisconsin-Extension should be inspected. Damage spider mites during late vegetative Examine lower, middle, and upper and early reproductive growth have leaves for stippling. Turn leaves over to Two-spotted spider mites feed by confirm presence of spider mite adults, extracting contents of individual leaf recorded yield reductions of 40–60%. Additionally, pods on mite-stressed nymphs, and/or eggs with a hand lens cells with needle-like mouthparts. At (10x magnification). Adults can be de- the plant level, spider mite injury ap- plants are more likely to shatter, which compounds yield loss. Differences tected by tapping soybean plants onto a pears as white or yellow specks called white sheet of paper. Dislodged spider stipples. Stippling can occur on both between soybean cultivars in two- spotted spider mite reproductive rates mites will be noticeable by their dark sides of the leaf, and is often noticed abdominal spots and can be seen as tiny first on the underside. This cell-dam- have been noted, but no mechanisms of mite host plant resistance have been dark specks moving on the white paper. aging feeding style lowers chlorophyll Look for webbing on the underside of content, reduces plant photosynthetic identified. Corn. leaves. capacity, and causes water loss from No numeric economic thresholds leaf tissue, resulting in yield loss Two-spotted spider mite have been developed for two-spotted through reduction in dry matter and colonies are uniformly distributed spider mite in soybean. Begin by grain production. on the corn plant. Feeding injury estimating the percentage of leaf Soybean. appears on corn(figure leaves 4) as chlorotic, surface with stippling and chlorotic discoloration. Current action thresh- Because outbreaks pale yellow or yellow-white areas olds recommend treatment at 10–15% happen irregularly, and spider mites along the leaf . Most spider leaf discoloration from R1 (beginning are so small, this pest can be easy miteEffects feeding on andcorn injuries yield are occur more be - bloom) through R5 (beginning seed). to overlook. On a field scale, spi- severetween whenpollen mites shed damage and denting. leaves at or Spider mites can cause yield reductions der mite damage appears similar above ear level. Infestations have been (figure 2) as long as green pods are present. Soy- to soybean plant moisture stress associated with accelerated plant dry bean plants recover from mite injury . Mite infestations usu- down in the fall. In addition to grain after treatment, although less com- ally start in areas prone to drought yield reduction, quality and yield of pensation is possible in later develop- stress, such as along field edges and silage corn may decline due to mite ment stages. Check the pesticide label on higher ground within fields. As feeding. preharvest interval when considering damage increases, leaves become late season treatment. Do not spray yellow, bronzed, and/or brown, and Scouting and treatment (figure 3) after R7; treatment at this point will not may prematurely drop off the plant guidelines impact yield. Damage can occur anytime Management of two-spotted spider The spider mite treatment decision during the growing season when hot mites in soybean and corn depends rating scale (table 1) is based on data and dry weather conditions persist. upon scouting, action thresholds, and obtained during previous two-spotted In outbreak years in Wisconsin, miticides. Although the density of mites spider mite outbreaks on soybean in the two-spotted spider mite injury and per plant is the ideal sampling unit, this Midwest. treatmentFields infested have typically with two-spotted occurred in is impractical for integrated pest man- mid- to late-July through August. Corn scouting. Two-spotted cover extensive leaf area, discoloring fungal pathogen to disperse throughout spider mites do not usually cause leaves near the ear and spreading to the two-spotted spider mite population economic damage in field corn in Wis- other leaves over a significant area of in a field. Mites die within 1–3 days of consin. When this pest occurs in dry the field. Control is suggested if you infection, and populations can decline years, yield losses are more consistent find active mite colonies on one-third quickly once the pathogen has spread. in soybean. of the leaves of 50% of the plants, or if Several species of insects, mites, Begin by checking the field for 15–20% of the leaf area is covered with and spiders prey on spider mites in presence or absence of spider mites on mites and their damage. corn and soybean. Predatory mites in individual green leaves on corn plants Further yield loss and economic the family Phytoseiidae are the primary along field edges. Repeat the procedure benefit to chemical control are unlikely arthropod natural enemy of two-spotted on at least 10 plants at several locations once corn reaches dent stage. spider mites. Predaceous mites have within the field. Moderate infestations been utilized through conservation and will result in leaf stippling and chlorot- Control mass release for spider mite manage- ic spotting on the leaf surface. Look for ment in orchard, garden, and green- Biological and natural evidence of spider mite adults, nymphs, house crops. Although predaceous controls and/or eggs, as well as webbing on the mites occur in corn and soybean, The most effective natural control underside of leaves. Severe infestations augmentative releases of commercially of two-spotted spider mite in soy- can cause entire leaves to turn yellow reared predaceous mites are currently bean and corn is the fungal pathogen, and brown. cost prohibitive in field crop systems. Neozygites floridana. It attacks all mite As with soybean, determining the stages, and is host-specific to spider density of tiny spider mite adults and Spider mite infestations and mites. Infected mites have a waxy or nymphs per plant is not practical. Treat- weather cloudy appearance. This mite-killing ment guidelines are based on indicators Miticide treatment may be delayed fungus depends on environmental con- such as live mite colonies, leaf damage if rain with cooler temperatures and ditions cooler than 85°F, coupled with symptoms, and continued low rain- high humidity are expected.

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