Tetranychus urticae (soybean)
Authors: Buyung Hadi, Robert Wright, Ken Ostlie, Bruce Potter, Robert J. Whitworth, J. P. Michaud and Phillip E. Sloderbeck
Twospotted spider mite (Tetranychus urticae Koch), the species of mite usually found in soybean, is a tiny arachnid (<0.02 inch). The eight-legged nymph and adult twospotted spider mite body is translucent, yellow to green, with two characteristic darker spots on the sides of the body. The dark spots are caused by waste particles that accumulate in the mite's gut. Twospotted spider mite colonies are accompanied by fine silk webbing. These characteristics will distinguish twospotted spider mite from many other plant feeding mites and predatory mites in the field.
Life Cycle and Seasonal History
In winter, twospotted spider mites primarily overwinter in alfalfa and other plants on field borders. Crop residues may also host overwintering mites. The overwintering forms of two-spotted spider mite do not feed and the overwintering females do not lay eggs. The manifestation of this overwintering form is initiated by environmental cues such as shortened day length, decreased temperature and deteriorating quality of food sources. Overwintering mites can survive freezing temperature and require relatively high humidity for survival. The overwintering mites actively resume feeding and egg laying after time at warmer ambient temperature has elapsed in early spring.
In spring, female spider mites spin a web on the first feeding site and lay eggs within three days. The eggs hatch after 3-4 days and undergo a larval and two nymphal stages in the 5-10 days before the mites are fully developed. In general, the twospotted spider mite development occurs when the temperature ranges between 54 and 104°F (12-40°C) with an optimum temperature range of 86-89.6°F (30-32°C).
In the spring and summer, the mites migrate to adjacent soybean fields assisted by wind or by active crawling. Dispersing mites crawl to upper leaves, spin a silk thread that is caught by the wind and drift until the silk catches on another plant. Twospotted spider mites usually feed and lay their eggs on the undersides of leaves. Younger leaves are preferred but in established infestations older, lower canopy leaves are colonized first and more heavily damaged. The ability of twospotted spider mites to spin silk threads for dispersal and for webbing on leaf surfaces earn it the name 'spider' mite.
Hot and dry summers are optimum for mite populations buildup on soybean, especially if accompanied by drought stress on host plants. The high temperatures coincide with optimum temperature range for mite development. Average daily temperature above 85°F and dry weather also limit the development of Neozygites floridana, a naturally occurring, mite-infecting fungus. Mowing or drought stress of alfalfa and and other vegetation on soybean field borders triggers mass migration of mites into the field. Furthermore, drought stress improves the quality of soybean as mite food source. Thus, the risk of mite infestation increases when hot and dry weather persist for an extended period.
Plant Injury and Damage
Twospotted spider mites attack a wide variety of plants; over 250 host plants have been identified, including soybeans, corn, alfalfa and dry beans. Mites feed by puncturing plant cells with their mouthparts and sucking the cell content. Groups of these punctured cells are visible as yellow or whitish spots (stipples)on the leaves. Stipples may be clustered in areas of colony activity in early infested lower and mid-canopy leaves. As the infestation builds, stippling becomes more widespread on the leaf, and mites move up the plant to colonize younger leaves. Heavily infested leaves take on a bronze appearance and can eventually yellow and die. Premature senescence and leaf drop progresses from the bottom upwards on the plant. Mite symptoms in the soybean canopy, stipples, yellowing, bronzing, accelerated leaf loss and rarely plant death, are often misconstrued as drought stress.
Damage depends on timing and severity of mite infestation and weather. On soybean, mite feeding injury causes uncontrolled water loss and reduces photosynthetic capability of the leaves. Heavy mite feeding in late vegetative and early reproductive stages will stunt plants, retard initiation of new leaves, and limit canopy expansion. Intense mite injury occurring through reproductive stages will reduce pod number, seeds per pod and, if it continues, seed size. Severe infestations may lead to premature senescence, excessive pod shattering during harvest and up to 40-60% yield reductions. Prolonged drought conditions only intensify impacts of spider mites and vice versa.
Scouting and Threshold
Confirming the presence of twospotted spider mite on stippled leaves is important since foliar disease, drought stress, and herbicide injury may cause similar symptom. Mites can be seen with the aid of a good hand lens. Tapping a stippled leaf on white paper sheet may help in seeing the mites as dark specks moving quickly on the white paper sheet. Hand lens with 10x magnification can be used to further ascertain mite infestation in the field. The presence of silk webbing on the leaf surface can also be used to confirm that a mite infestation caused the stippling but not that mites are still present.
Mite infestation typically starts near field edges. Thus, symptoms of mite feeding (stippling, leaf discoloration, accelerated lower leaf loss) are likely to be noted first on plants near the edges. Start scouting at field edge, especially near permanent vegetation (roadside ditches, grass waterways, drainage ditches, pastures, fence lines) and alfalfa fields. Pull plants and examine the leaves from the bottom upwards. Look at the underside of leaves. Note stippling, webbing, and examine for live mites with a hand lens or tapping onto a sheet of paper. Examine how far up the plant mites and symptoms have progressed. If mite presence is verified, it’s time to progress into the field. If mite presence is verified, move into the field at least 100 feet (30.5 m) before making the first stop. Walk the field in a "U" pattern, stop randomly at about 20 locations along the pattern and check at least 2 plants at every stop. Note the stippling, leaf yellowing, leaf dropping, and mite presence. Assess the infestation based on the following scale from University of Minnesota:
|0||-||No spider mites or injury observed|
|1||-||Stippling on lower leaves, no premature yellowing observed|
|2||-||Stippling common on lower leaves, no premature yellowing observed|
|3||-||Heavy stippling on lower leaves with some stippling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common. Small areas with lower leaf loss.|
|4||-||Lower leaf yellowing readily apparent. Leaf drop common. Stippling, webbing and mites common in middle canopy. Mites and minor stippling present in upper canopy.|
|5||-||Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stippling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.|
Note: Symptoms of spider mite damage are reduced when temperatures are cool. Yellowing or leaf drop will be much less pronounced than when temperatures are high. The stippling and physiological damage is still present however.
If live mites are present and abundant, their damage symptoms are found through the soybean field and infestation status has reached a 3 or higher on this scale, apply a miticide. Only a 10-15% reduction in effective leaf area will justify a miticide spray. Damage is not reversible, so it’s important to protect the middle and upper canopy leaves. Edge treatment is not recommended since the mites are likely to have spread already into the field by the time severe symptoms (score=4 or 5) are noted at the field edge.
If mites are present but the infestation hasn't reached a score of 3, scout the field every 4-5 days as long as drought conditions persist since infestations may buildup quickly during drought conditions.
In the field, spider mite populations are naturally controlled by the combination of weather, low host quality of well-watered soybean, predators and a fungal pathogen. Application of a broad spectrum insecticide may disrupt the natural control of spider mite population by killing predators while leaving spider mite eggs unaffected. Thus an insecticide application can trigger a rapid resurgence of the spider mite infestation. Similarly, fungicide use in soybean could adversely affect the fungal pathogen that attacks spider mites.
The predatory mite, Neoseiulus fallacis, is an important predator of spider mites. The predatory mite is slightly larger in size, and the body lacks dark pigmentation of herbivorous mites.
The mite destroyer beetle, Stethorus sp., is a small black lady beetle. The adults feed on mites and the larvae feed on mite eggs. The female beetle lays its eggs in active mite colonies.
The sixspotted thrips is a small (1/16 inch long), tan and cigar shaped. Both adults and immatures feed on mites. Minute pirate bugs, Orius spp., are another small insect (about 1/16 inch long) whose adults and immatures prey on mites.
In addition to the predators, the fungal pathogen, Neozygites floridana can infect and kill twospotted spider mites. The efficacy of this fungal pathogen to regulate spider mite populations is dependent upon the weather. Temperatures below 85°F (29.4°C) and relative humidity above 90% is optimum for fungal growth. Several cool, damp days in a row may increase the likelihood of fungal infection on mite population. Prolonged drought may decrease fungal inoculum in the field.
Few insecticides used in soybeans will control twospotted spider mites. Only products containing chlorpyrifos and bifenthrin are effective. Because the efficacy of these products depends on coating the leaves, ensure thorough leaf coverage. Don’t skimp on water. Neither of these miticides will kill eggs so don’t count on a single application to eliminate the infestation, especially if drought conditions persist. Re-check fields within 10-14 days.
Applications of other pyrethroid insecticides widely used to control soybean aphids are ineffective against mites and may even aggravate the mite situation. These insecticides, however, are highly toxic for naturally occurring predators. Some pyrethroids may even temporarily increase mite reproductive rates. As a result, mite populations may ‘flare up’ after pyrethroid insecticide application. Be sure to re-check fields for spider mite flare-ups within 10-14 days of a soybean aphid spray, particularly when the weather is favorable for spider mites population buildup.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com