Web-spinning or spider mites are common pests on a variety of agricultural and ornamental crops. They are not technically insects, but rather arachnids. Outbreaks can be devastating to strawberries, grapes, cane-berries, fruit trees, hops and a variety of other crops. There are several species of spider mites, including Pacific spider mite and two-spotted spider mites, that are very economically important on several crops. Most spider mite species commonly seen in agricultural setting are in the genus Tetranychus, and these different species have very similar life cycles, crop damage, and management strategies.
Spider Mite Identification
Identifying spider mites in the field is not always easy because they are so small. For most people, a 10x hand lens is needed to positively identify mites to the species level. For quantitative sampling, differentiating between adult, juvenile and mite eggs, a microscope may be needed, with leaf samples taken from the field.
Adult mites have an oval shaped body with eight legs. Juvenile mites are usually much smaller than adults and larvae only have six legs after hatching. Eggs are small, clear, spheres.
Two-Spotted Spider Mites, Adults, Nymphs, Eggs, and Persimilis Predator. Photo: Denise Crawford
Spider mites are identified by their silk webbing which they produce on infested leaves. In most crops susceptible to spider mites, the under surfaces of leaves are affected first. Two-spotted spider mite can be idenitfied by its two pronounced black spots on either side of its body. The Pacific and Williamette spider mite species, both common on grapes, for example, are difficult to distinguish between. Pacific mite is larger than the Williamette spider mite and its forelegs are more reddish in color, compared to the pale yellow forelegs seen in Williamette.
Spider Mites Life Cycle
It is important to know a little about the life cycle of spider mites in order to manage their damage. In area with warmer winter climates, spider mites are able to feed and reproduce uninterrupted through winter, if green plant leaves are present, and frost temperatures do no occur. In areas with colder areas, yellowish-orange adult female mites can overwinter under bark in trees, or under ground cover.
These females will emerge in Spring and being feeding and lay eggs. Spider mite reproduction rates are determined by temperature and available food sources. In warm conditions, particularly in dry, warm regions, reproduction can become very rapid. Generations can be completed in as quickly as a week.
There is a Growing-Degree-Day model that has been developed for two-spotted spider mites, as well as some other species. This model can help growers track mites through various growth stages, based on local temperature data. This model, along with regular plant sampling, can be used to predict populations of different spider mite life cycle stages, which well help guide management decisions regarding miticide sprays (which may affect different life-cycle stages) or biological control. The Pest Prophet App can be used to easily obtain the outputs of these models for a field, without no weather stations needed.
Spider mites spread between pants primarily by wind. Plants under water stress are more susceptible to spider mite infections. Areas of a field that are more exposed to dry, or dustier conditions are typically where spider mite infections are found first.
Spider mites populations usually decrease rapidly during fall or winter. Rain or overhead irrigation can wash spider mites off plants and disrupt their feeding and reproduction and cooler temperatures slow down how fast they can reproduce. Predator populations can also rapidly increase later in the season, causing spider mite population to decrease rapidly. Once all green leaves have been removed from the plant, the spider mites will have no food source and will need to overwinter in order to survive.
Spider Mite Damage
Spider mites are piercing-sucking insects, meaning that they feed on leaves by sucking carbohydrates out of leaves. The damage on infected leaves first appears as small dots, and the leaves may start to appear more bronze or yellow in color. Light infestations may go undetected. As the spider mites continue to grow and feed on leaves tissue, leaves wilt and die. When larger populations of spider mites are present, webbing can cover leaves, flowers and fruit. Damage usually occurs first on lower leaves, and then spider mites spread upwards through the canopy.
The economic damage caused by spider mite epidemics can vary between crops, because they damage leaves. In grapes, fruit trees, and other fruit, usually there is a loss in both yield and fruit quality if there are not enough healthy leaves present to produce carbohydrates needed for fruit production. On other crops that require leaves to shade and protect fruit, sunburing can be an issue after leaves die off. For some crops such as sugar peas and beans, pods may be attacked directly by spider mites, causing a direct yield loss. Ornamental flowers may not be as severely infected, unless the plants are completely overtaken and killed by spider mites.
Spider Mite Webbing on Plum. Photo: UCANR
Spider Mite Management
An integrated pest management is recommended for controlling spider mite outbreaks, using a combination of cultural practices, biological controls and, if necessary, carefully timed sprays of soaps, oils, waters, or insecticides. Spider mite outbreaks can be very difficult to stop without yield loss once populations are high, so preventing infestations is very important. Spider mite control must be considered within the entire ecological context. Because natural predators are an important factor in controlling spider mite outbreaks, broad-spectrum insecticides (as well as some fungicides) may actually increase spider mite populations, if natural predators are harmed.
Field Monitoring
Field monitoring for spider mites with regular sampling is a very important aspect of spider mite control for a variety of reasons. Before any management decision is made, it is important to scout for the presence and quantity of the following: adult spider mites, juvenile spider mites and nymphs, eggs, and natural predators. These counts will determine if sprays or biological controls are necessary and, if so, which type of spray. Different active ingredients affect adult, juveniles, eggs and natural predators differently. Natural predators should be monitored before and after any miticide sprays, but also after insecticides and fungicides targeting other pests. If natural predator populations dramatically decrease, spider mite outbreaks have a higher likelihood of occurring.
Scouting and systematic crop sampling are also important for using targeted spider mites control methods. Spider mite outbreaks usually first occur in “hot-spots”, or smaller highly infected areas within the field. These areas can be mapped and used for targeted sprays or release of biological control predators.
Usually the damage associated with spider mites are the first symptoms noticed. If there are damaged or yellow leaves, the undersides should be examined using a hand lens for presence of mites or webbing. Shaking leaves onto pieces of blank white paper can be helpful to find and count mites, or leaves can be sampled and examined later with a microscope or hand lens.
Once the presence of spider mites is determined and clearly correlated with observed leaf damage, scouting and rating plants for spider mite damage may be a faster way to create maps or determine where outbreaks are occurring. Drones or overhead imaging can be used for this process, but the accuracy is not always high, and there are variety of other factors which can affect the outcome. These results should always be verified by in field scouting by qualified individuals.
Cultural Control Practices for Spider Mites
There are many cultural practices which can greatly affect spider mite population and risk of infestation. Outbreaks usually occur in dusty parts of the field. Applying water to roads and paths can greatly reduce the amount of dust present in the area. Encouraging drivers to reduce speed on dirt roads is also very helpful. In some crops, overhead irrigation using sprinklers or micro-sprinklers can be used to clean dust off crops, and can also be helpful in controlling temperature and humidity in the canopy. In some cases of lighter infestation, sprays of only water may be used to control spider mites. Make sure to reach the undersides of the leaves to achieve thorough coverage with water.
Irrigation related stress should also be avoided throughout the season. Plants that are injured by phyto-toxical chemical affects (from herbicides or over-spraying pesticides) will also be more susceptible to spider mite infestation. Plants with damaged root systems, as a result of soil-borne disease or pests, may also be more susceptible to spider mite damage.
Biological Control of Spider Mites
If spider mites are found in a field, action should be taken to prevent further outbreaks. However, this does not always mean that spraying is the best option. Biological control, through releasing commercially available predators, or conserving existing predators, can play an important role in spider mite outbreaks. In many cases, biological and cultural crops may be sufficient to prevent economic damage from spider mites.
Spider mites have a wide variety of natural predators, including the western predatory mite and Phytoseiulus persimilis. Predatory mites can be distinguished from spider mites by typically being more teardrop shaped as well has having longer legs and being more active. Phytoseilus persimilis is reddish-orange in color. Other types of insects are also predators on spider mites: sixspotted thrips, spider mite destroyer lady beetle larvae and adults, and generalist predators such as minute pirate bugs, big-eyed bugs, and lacewing larvae. Western flower thrips can also feed on spider mite eggs and larvae, (but can also damage plants). Care should be given to avoid damaging populations of these predators by insecticide sprays.
A number of these natural predators are commercially available for field release. Western predatory mite and Phytoseiulus persimilis are the most commonly used commercial biological controls. Western predatory mites are more effective under hot and dry conditions, while persimilis mites prefer more humidity. These predatory mites require spider mites to feed on, otherwise they will starve, cannibalize themselves or migrate away from the field. They should only be released where there are high populations of spider mites.
It is recommended to distribute these predators in a field based on where spider populations or damage are most present, rather than a completely even distribution. This will allow for the predator populations to establish quicker. Using sprinklers with persimilis to increase humidity within the canopy can also help establish their population and optimize their chances for successfully controlling spider mites.
As a general rule, 1 predatory mite is needed for every 10 spider mites present. Multiple, successive releases may also be necessary to establish populations of natural predators. Monitoring and sampling before and after releases is important to determine if more predators are necessary. On perennial crops, it is possible to establish predator populations that are able to successfully control spider mites for multiple years without having to spray insecticides.
Neoseiulus (=Amblyseius) californicus is another commercially available option which is preferred for conditions where there are less spider mites present. N. Californicus is able to feed on a wider variety of mites and are able to travel further than persimilis and western predatory mites in order to find its prey.
Chemical Control of Spider Mites
Spider mite outbreaks often occur after broad insecticides are used and natural predators are harmed. There are studies that have showed insecticides having other effects that can increase spider mites populations. Studies have shown that Spider mites exposed to Carbaryl have had faster reproduction rates. Some insecticides also can increase the level of nitrogen in leaves which indirectly makes them more susceptible to spider mite infestation.
If biological and cultural controls are not sufficient to prevent spider mite outbreaks or in cases of severe infestation, spraying may be necessary to reduce the spider mite population. Using sprays before releasing predators can also be an effective strategy. If insecticides are used, selective materials are recommended to avoid damaging natural predator populations.
Insecticidal soap or oil as well as other horticultural oils can be used. For organic or home gardens, there are some plant extracts that have shown to have some control over spider mites. Oils and soaps kill mites by directly contacting them, so complete spray coverage is important, especially because spider mites live primarily on the undersides of leaves. Repeat applications may be necessary, to achieve full control.
Chemical miticides may also be used, but labels need to be carefully reviewed. Different active ingredients will affect natural predators differently, as well different stages in spider mites’ life-cycle. Some products will kill eggs, for example, and some will not. A sampling program, along with Growing Degree Days (calculated through Pest Prophet app, or by other means) can be used to determine which miticide is most appropriate for a given situation. Miticides on different crops can also have additional regulatory or safety precautions. Sampling before and after spray applications, (once it is safe to re-enter field) is recommended to assess level of control and effect on natural predators.
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