Twospotted spider mite,
Carmine spider mite,
Description. There are two species of mites that commonly feed on cotton plants in Texas: the twospotted spider mite and the carmine spider mite. These two species are extremely difficult to distinguish from one another. The carmine spider mite females are red, while the twospotted spider mite is greenish in color. However, when nearing conditions suitable for initiating diapause, the female twospotted spider mite may also be red. Thus, color is not always a good indicator of speciation. Although the primary character for distinguish these species is the shape of the striae on the mite’s cuticle, recent findings are that these characters are not reliable. To date there is not a reliable means to distinguish carmine from twospotted spider mites. Additionally, the biology and ecology of these two species is very similar; thus the remainder of this discussion will focus on theses two species but will not necessarily differentiate between them.
Spider mites in cotton feed primarily on the underside of the leaves, but under high populations will infest the fruiting structures as well. These spider mites are well known for producing profuse webbing. Webbing can serve a number of purposes, but the primary benefit of webbing is protection. Webbing can hamper predators and will help in repelling light precipitation.
The twospotted spider mite has an extremely large host range and is found throughout much of the world. It is known to feed on over 900 host species including over 150 economically important ornamental and food crops, including grass and broad-leaved crops. Because of its wide distribution and wide host range, the twospotted spider mite is the most economically important phytophagous mite worldwide.
The twospotted spider mite can reproduce year round as long as warm temperatures persist. Spider mite females can lay over 200 eggs, but generally average about 70. They will generally lay 3 to 14 eggs per day. These eggs appear as small pearl to pink colored spheres.
Twospotted spider mites exhibit a type of reproduction where unfertilized eggs will produce males and fertilized eggs will produce females or males, at an approximate ratio of 3 to 1. Eggs will hatch in a few days into a 6 legged larval stage. The immature stages look like small adults in shape. The larval stage is extremely small and somewhat translucent in color. It will molt to two eight legged nymphal stages, protonymph and deutonymph; and finally to the adult which will measure about 1/32nd inch in length. Growth and development is temperature dependent and is influenced by the host plant. The entire life cycle will require 6 to 14 days to complete in summer months. Although the life cycle speeds up at higher temperatures (upper 80’s and 90’s oF), egg production is maximized in the 70’s oF.
There are a number of factors that can influence spider mite population growth. Population growth will increase with increasing temperatures, but when given a preference, twospotted mites prefer moderate temperatures. Surprisingly, although spider mite outbreaks are often associated with hot, dry conditions, they prefer more humid conditions. However, egg laying and survival tend to be higher at low humidity. Mite infestations are often associated with dusty conditions. Although not well understood, dust may impact the effectiveness of mite predators.
Host plant condition plays a significant role in spider mite population growth. High levels of leaf nitrogen, phosphorus and carbohydrates tend to support mite population growth, while water stressed plants tend to have lower mite reproduction. Although mite outbreaks have been associated with potassium deficiency, this correlation has not been positively proven.
Additionally, some pesticides have been implicated in increasing the reproductive potential of spider mites.
Imidacloprid has been shown to increase the fecundity of twospotted spider mite. Imidacloprid and thiamethoxam are commonly used for managements of thrips, aphids and plant bugs in cotton.
Spider mite infestations usually begin on field edges, often near dusty roads. The populations commonly move into cotton from weedy hosts such as morning glory or palmer amaranth, or other crops such soybeans or corn. Thus, predicting a spider mite outbreak in cotton will often depend on the population in the immediate surrounding habitat. The twospotted spider mite overwinters as an adult female. They like to seek out dark, humid environments in leaf litter or similar habitat to overwinter. When temperatures are warm, diapause may be temporarily broken and reproduction may occur on winter weeds. As warm temperatures are sustained, mite populations will build on spring weeds, and as crowding occurs or with host deterioration, the mites will move into adjacent cotton.
Population dispersal from weeds into cotton may occur from mites crawl from one host to another. This type of dispersal may be most evident in situations where weedy areas are directly adjacent to cotton fields. Mites will also move via the wind. Mites may be carried short distances on the breezes directly, or by ballooning, where the mite spins a strand of silk to catch the wind. This type of dispersal may carry the mite for miles. Mites can also disperse by catching rides on animals or machinery.
Spider mite outbreaks tend to be more common in irrigated or well watered cotton than in dryland cotton. The reasons for this are not certain but may relate to a lack of nearby weeds around most dryland cotton, tougher leaves, decreased reproduction on water stressed plants, lower nitrogen inputs or reduced insecticide use in dryland cotton.
Damage. Spider mites feed by piercing plant cells with their mouthparts and sucking the contents. This damage usually occurs in clusters and damaged clusters appear as a white or yellowish specs on the leaf, termed stipules. Specking damage is termed Phase I damage. As feeding increases and the mites persist, the damage spreads and the leaf takes on a reddened appearance (Phase II damage) and eventually necrosis. Cotton photosynthesis has been correlated with this damage and appears to sharply decline when Phase II is reached; about 20 mites per leaf.
Management and decision making.
Cultural management. Spider mite outbreaks tend to develop on field borders adjacent to spider mite infested weeds or other crops. Managing weeds along field margins will often prevent migration of mites into cotton. Additionally, excessive nitrogen fertilizer should also be avoided. Spider mites have been shown to occur more frequently in fields grown under higher than necessary nitrogen.
Biological control. Predators are often the most effective means of managing spider mites. There are a number of predaceous arthropods that prey on mites including among others, several species of thrips, lacewing larvae and minute pirate bugs.
In Texas, one of the key predators of mites in cotton is western flower thrips. Although a pest of early season cotton, western flower thrips can effectively prevent mites from reaching damaging levels. The population of thrips, particularly immature thrips, should be monitored when mite are encountered. Because of the importance of predators in mitigating mite outbreaks, it is important that care be taken to avoid killing them when treating for other pests such as cotton aphids, fleahoppers, bollworms or Lygus.
Scouting. Spider mite infestations are usually first detected on weedy field margins, and often along dusty roads. Thus it is a good idea to pay particular attention to this type of habitat. Sample for mites in the weeds and in adjacent crops to determine if a mite infestation exist. In cotton, mites should be scouted by sampling 40 to 60 leaves from 4 or more regions of the field. Pull leaves equally from the top, middle and lower portions of the plant canopy. A 10X hand lenses will be necessary to see immature mites and eggs. Additionally, scout for mites by looking for areas with cotton that has speckled or reddened leaves. Mite hot spots can often be easily detected from a distance, and while mites may be numerous in these spots, they may occur in very low numbers in the remainder of the field.
Chemical control and Action thresholds.
The current action threshold for spider mites has not been fully evaluated, but the recommended spider mite action threshold can be used as a general guide. Prior to bloom, cotton should be protected from spider mite induced defoliation. After bloom, it is essential that the leaves responsible for boll filling be protected from spider mites. Research has demonstrated that cotton should be protected from severe spider mite damage for 650-750 DD60s beyond cutout or NAWF + 5.
Since spider mite outbreaks usually occur along field margins or in hot spots, treating only those infested areas may be a means for reducing costs.
Consider alternatives to the pyrethroids for managing pests, such as bollworms and lygus, or neonicotinoids for aphids when mites are present. Maximize insecticide coverage when treating for mites. Drop nozzles and high spray volumes can greatly enhance mite control.
|Suggested Insecticides and Rates for Managing Spider Mites in Cotton
ingredient per acre
|Amount of formulated
|Acres treated per gal or lb of
|Mode of Action Group (IRAC)|
(Epi-Mek 0.15EC, generics)
|0.0047-0.0094||4-8 fl oz||32-16||6|
|0.625||16 fl oz||8||21A|
(Comite II 6)
|0.94-1.69||20-36 fl oz||6.4-3.6||12C|
|0.125-0.25||4-8 fl oz||32-16||23|
1rates below 8 fl oz are recommended only for early-season cotton less than 10 inches high.