Overview
- What are red spider mites?
- What are the different types of red spider mite?
- What is the impact of red spider mites?
- How do I know if I have a red spider mite problem?
- How do I get rid of red spider mites?
- Conclusion and future direction
Red spider mites are significant pests in many crops and other plants. They typically measure less than a millimeter in length, which makes them difficult to spot. They get their nutrients by attacking leaves, eventually leading to leaf death and reduced yield from the affected crop. These tiny mites are found globally, with many species, such as the southern, spruce, and two-spotted spider mite, prevalent in the continental US. Luckily, there are several ways to control red spider mite numbers, including biological methods.
In this blog, we will focus on identifying and getting rid of these common pests. Let’s begin by learning exactly what these mites are.
What are red spider mites?
Red spider mites are members of the arachnid group, which also includes spiders and ticks. They feed on the leaves of a wide range of plants, including house plants and crops like tomatoes and berries. There are over one thousand species of spider mites, but the most common pests belong to the spider mite genus Tetranychus. They are typically small and often red with eight legs, though different species have different characteristics. Red spider mites can be broadly distinguished from small insects by their ability to spin webs, which are particularly visible with large infestations.
Despite the name, these pests can also be yellow or green. Their small size makes it challenging to find red spider mites with the human eye, but they are easily visualized with a 10x lens. Without magnification, they can appear as tiny dots on the upper or underside of leaves, which you may notice while examining crops or gardening. In low numbers, they are not particularly harmful, but large infestations can cause significant damage.
The life cycle of a red spider mite varies depending on environmental conditions. In warm temperatures and with access to green leaves, these bugs can feed all year round. Juveniles can hatch from eggs and reach adulthood within a week. In colder conditions, female red spider mites can remain dormant in fallen leaves and other debris before returning to feeding and laying eggs when conditions become favourable. The rapid life cycle of red spider mites means that large infestations can build up quickly and more readily develop resistance to chemical control measures.
What are the different types of red spider mites?
Species of red spider mite are difficult to distinguish from each other because of their small size and similar damaging effects on crops. Here, we provide an overview of common problem species of red spider mite and how to identify them.
European spider mite (Panonychus ulmi):
These tiny mites can be found on various plant types, including common crops and trees. Though originating in Europe, they spread throughout the North American continent in the twentieth century and are considered particularly damaging to apple and stone fruit trees. Adult females are red and around 0.35mm long, while the males are smaller and have more yellow colouration. Eggs are red and about half the size of the adult male.
Southern spider mite (Oligonychus ilicis):
This species has a broad global distribution, which, despite its name, also includes the northern hemisphere. It affects a wide range of crops and other plants. In the USA, it is known to colonize broad-leaved trees and may be of less concern to crops. Female southern spider mites are around 0.38mm long, and males are smaller, at 0.3mm. Both sexes are red, and the eggs are red and around one-third the size of adults.
Spruce spider mite (Oligonychus ununguis):
This species is more prevalent in colder climates and is native to the USA, where it is considered a particularly harmful pest. In addition to Spruces, this species feeds on other trees, such as firs and pines. Adults are typically 0.5mm long and can have a dark green or reddish-brown color.
Two-spotted spider mite (Tetranychus urticae):
This species is widely distributed in temperate and subtropical climates and is prominent across the continental US. It is observed in many colors, including reddish-brown, green, and yellow. The characteristic spots are waste deposits visible through the semi-translucent body. It feeds on a vast variety of crops and is particularly problematic in greenhouses that maintain optimal mite growth conditions. Adults usually measure around 0.5mm.
What is the impact of red spider mites?
Red spider mites cause damage to crops by sucking food (sap) directly from plant leaves. They have needle-like mouthparts, which help them damage plant cell walls and access the phloem, the vascular tissue that carries sap around plants. The mites preferentially target new leaves, which can significantly reduce yield if attacked early in the season. Host plants may not show symptoms until a large infestation is present, and effects on yield may not be observed until the following season. Damage caused by red spider mites can lead to the loss of leaves, reducing the canopy cover and making crops more susceptible to sunburn, which can also reduce yield.
Are red spider mites dangerous to humans?
No. There is no evidence to suggest that red spider mites can harm humans, either by biting or by transmitting disease. These bugs do not live on humans and typically do not colonize indoor spaces, though significant colonies can emerge in gardens.
How do I know if I have a red spider mite problem?
Early signs of red spider mite damage can be difficult to detect. Their size and preference for colonizing the undersides of leaves make spotting low numbers a significant challenge. However, once a larger colony is established, you may notice a dusty appearance on the leaves. Damage to leaves will initially present as spots of discoloration, which are generally brighter than the surrounding area. Affected leaves will lack the rich green color of unaffected neighbors and will eventually begin to turn brown or yellow.
A large infestation will trigger complete browning and yellowing of leaves, followed by death and leaf drop-off. Red spider mites spin silk webs, which become more apparent the larger the infestation becomes. Due to the rapid proliferation of these tiny mites under warm and dry conditions, infestations can progress rapidly during the summer months. Monitoring leaves is particularly important to catch infestations before they damage plants.
Fortunately, if you discover a red spider mite infestation, there are several methods you can use to get rid of it. These include biological control methods, which offer a natural and safer alternative to chemical solutions.
How do I get rid of red spider mites?
Cultural control:
Removing dust and debris from crop growing areas can help remove red spider mites from the environment. As mentioned above, female red spider mites can overwinter in debris, so keeping the growing area clear can help prevent infestations in future seasons, even if they are not an issue yet. Mites prefer dry conditions, so ensure crops are properly irrigated. If you notice that a crop has developed an infestation, remove infected leaves carefully without bringing them into contact with healthy plants.
Biological control:
Fortunately, many natural solutions offer a potent control method against red spider mites.
Releasing red spider mite predators and pathogens into the environment is known as augmentative biological control. Biological control methods include using natural substances like neem oil or microbials like fungal strains of Beauveria bassiana. Predators include ladybugs and green lacewing larvae. Another option is to release predatory mites like Amblyseius andersoni.
Conservation biological control is different from augmentative biological control in that it aims to create an environment that favours the growth of natural predators. This can be achieved by introducing plants that these predators use for food or shelter. Conservation biocontrol helps predators that are naturally present, but it can also maximize the impact of released predators, leading to more efficient pest management.
Chemical control:
Several chemical control options are available. However, they should only be used as a last resort as they can damage the environment and harm the population of natural predators. Furthermore, red spider mites can become resistant to chemical controls over time. In fact, a study from the University of California found that two-spotted spider mites developed resistance to the chemical hexythiazox on strawberry crops.
Conclusion and future direction
Red spider mites are significant pests affecting crops and other plants both globally and within the United States. Their small size makes identification a challenge, however, significant damage to crops usually occurs with larger infestations, which are more visible. There are over one thousand species of these mites, with the two-spotted spider mite posing a particular challenge to crops in greenhouses and temperate regions. Integrated pest management through cultural control and introducing natural predators provides a robust solution to red spider mite infestations. In contrast, chemical solutions have limited value due to resistance and the damage they cause to the environment.
For more information on different pests, such as bean flies, you can consult the CABI BioProtection Portal resources.
