How to Control Two-Spotted Spider Mites Without Harmful Pesticides

Two-spotted spider mites or red spider mites (Tetranychus urticae) are tiny sap-sucking pests belonging to the Tetranychidae family that are barely visible to the naked eye, being only 0.3-0.5mm long, that live on the underside of leaves. Their body color varies, it can be pale green, greenish-amber or yellowish-green, and they have two characteristic dark spots on each side of their body, hence their name.

Mature spider mite males are much smaller in size than the females, usually around a fifth of the weight, and tend to remain straw-coloured.

Mites are not insects, but arachnids, as they have eight legs (insects have six) and belong to the class Arachnida which also includes spiders, scorpions and ticks.

These pests occur worldwide and attack a wide range of plants, and they’ve been found on over 300 species of plants!

Two-spotted spider mites are serious pests on a range of vegetable crops such as tomatoes, cucumbers, peppers, and beans; berries such as strawberries, raspberries, and blueberries; farm crops such as corn, soybeans, and cotton; trees such as apples, peaches, and almonds; and ornamental plants, such roses, chrysanthemums, and petunias, just to name a few.

When two‐spotted spider mites reach high population densities, they can cause severe crop damage and seriously weaken plants, causing defoliation and reductions in crop quality and yields.

These pests thrive in hot, dry weather with less than 50% humidity, especially when nights are also warm. When such weather persists for prolonged periods, spider mite populations multiply rapidly.

Spider mite damage to plants is common in summer and more prevalent during drought conditions. Water-stressed plants tend to be more susceptible to attack and are more easily damaged by spider mites. In crops that encounter both drought stress and spider mite attack, crop losses of 50% or more are possible.

As spider mites dislike damp conditions, rainy weather helps to keep spider mite population levels in check.

Spider Mite Life Cycle

Spider mites can breed very quickly, and under ideal conditions during the active breeding season, a new generation of spider mites can develop every 5–7 days.

Female spider mites lay their eggs on the undersides of leaves, which hatch to develop through four distinct stages. The first is six-legged larval stage, followed by two nymphal stages, the eight-legged protonymph, deutonymph and final adult stage. Molting takes place between each stage for the mite to reach maturity.

Adult female spider mites are capable of overwintering, surviving right through the cold winter season, to start the breeding cycle once again when the weather warms up in springtime.

Identifying Spider Mite Plant Damage

Two-spotted spider mites have long probe-like sucking/piercing mouthparts known as chelicerae, and feed mainly on the lower surface of leaves by puncturing plant cells, namely the parenchyma cells (simple, thin walled, undifferentiated cells which form a large majority of many plant tissues) and sucking out the contents. The resulting plant injury causes a reduction in photosynthesis and an increase in water loss from the plant.

Leaf Damage Symptoms – Spider mite feeding damage produces a characteristic ‘stippling’ on plant leaves, where they become unhealthy looking, with the upper surface of the leaves covered with a mottled or stippled pattern of tiny whitish speckled spots, short streaks or short lines. This eventually leads to chlorotic yellowing and bronzing of the leaves as they become dry and shriveled.

Not all plants respond to spider mite damage in the same way and may vary depending on the plant and leaf type. Leaves may turn grayish-green to yellow, show stippling and then drop off. Some leaves may display bronzing, where they develop a bronze, coppery or brownish cast to them, which may appear after, or instead of stippling.

Feeding damage caused by spider mites is first observed on the lower part of a plant and gradually progresses upward as the mite population increases in numbers.

How to Identify the Presence of Spider Mites on Plants

Apart from the plant damage described previously, the presence of spider mites can be also identified in the following ways:

Presence of Webbing – Spider mites are able to spin fine silky webs similar to spider webs, and the presence of these webs on a plant is an indicator of a spider mite infestation.

The spider mites use their fine silky webbing in two ways:

1. Protection – The webbing is used to line their feeding areas in order to protect their colonies and eggs from predators. It’s usually found on new growth and between leaves, but the webbing can cover whole leaves, and in cases of heavy infestations, a large portion of the affected plant. The mites use the webbing as a ‘highway system’ which permits them to safely travel from one feeding area of a plant to another. Two-spotted spider mites are easiest to observe when they’re moving around inside their webbing, they appear like tiny moving darker spots against the white webbing background.
2. Migration – Additionally, webbing is used as a ‘launching point’ to assist spider mites to disperse by the wind so they can migrate to new plants. When mite populations grow too large, two-spotted spider mites may relocate by moving onto webbing on the outer edges of leaves where they can catch the breeze and allow the wind to carry them elsewhere onto new plants to feed on.

Two-spotted spider mites will usually feed exclusively on one plant until it starts to decline, and then move on to a new plant.

Dusty, gritty feeling leaves – Spider mite populations leave behind a range of debris on leaves, such as fecal matter, fine webbing, dead mites and the cast-off exoskeletons (the skins they shed when molting), which builds up on the underside of some leaves, making the leaf feel gritty to the touch.

Some of this debris and even mite activity may be observable by using a magnifying glass or a smartphone camera’s closeup (macro) function.

Spider mite pinch test – Since spider mites are barely visible, one quick tell-tale test that gardeners use to determine their presence is the ‘spider mite pinch test’, which is quick and easy to do, and is described as follows:

1. Find a leaf that looks affected, usually off colour or displaying the characteristic stippled pattern.
2. Pinch the leaf between the thumb and forefinger, with the thumb sitting on the top surface of the leaf.
3. While keeping your thumb in place on top of the leaf, pull or drag the forefinger firmly along the underside of the leaf.
4. Examine your index finger, if there are red or brown streaks or lines on your fingertip, it means there are spider mites present!

It’s important to remember when diagnosing spider mite problems, if the plant damage symptoms are clearly visible but not getting worse, and no pests are detectable, one possibility is that the pest population has come and gone. They may have been wiped out by the colder weather of the changing season or by natural predators, but even in such cases, it’s worth treating the affected plants to eliminate any leftover pests or overwintering females to prevent a new pest outbreak.

The Best Ways to Control Two-Spotted Spider Mites

When controlling pest problems, an integrated pest management (IPM) approach, where a combination of two or more techniques are used together to limit pest populations and the damage they cause, is the most successful.

Any of the methods described below can be combined to increase the effectiveness of the overall pest control strategy.

Spider Mite Prevention Methods

The easiest way to deal with pest problems is by preventing them in the first place. Prevention is a far more efficient approach as it saves a lot of the time, energy and cost that goes into controlling pests after they appear.

Cold or rainy weather and higher humidity are the main two natural factors that reduce two-spotted mite populations, and cold winters kill off most of them in outdoor environments (as opposed to those in greenhouses which are protected from the cold).

We can complement nature’s controls to prevent two-spotted spider mite outbreaks using the following prevention methods:

• Keeping plants healthy by feeding, mulching, and watering them. Plants have their own natural defenses and the ability to resists pests if they’re strong, healthy, and vigorous. Weak, unhealthy and drought-stressed plants are more susceptible to spider mite attack.
• Maintaining a diverse garden habitat filled with companion plants whose flowers serve as a nectar source for beneficial insects and predatory mites, such those from the Asteraceae (daisy) family, Apiaceae (parsley) family and other plants with small, shallow, nectar-rich flowers like Sweet Alyssum. Encouraging predatory mites and beneficial insects helps control two-spotted spider mites.
• Mulching the soil, making compost and adding it to the garden, to create a habitat which encourages predatory mites that feed on two-spotted mites. Some predatory mites live in soil, or amongst the leaf litter and other mulch materials on the soil surface.
• Avoiding the use of chemical insecticides which can kill beneficial predatory mites, and cause explosions of pest spider mite populations.
• Misting the underside of plant leaves regularly during hot, dry weather to increase moisture levels and make the less hospitable to spider mites. Best done earlier in the day so leaves dry by nightfall, as leaves that remain wet overnight can encourage fungal diseases.

Physical Controls for Spider Mites

If plants have been attacked by two-spotted spider mites, some simple controls can reduce their numbers before resorting to spraying miticides.

• Prune off affected parts of the plant and destroy them, don’t put them in the compost! After pruning, increase humidity in the area around the plant, especially around new growth, by watering overhead or misting the plants. Don’t wet leaves late in the evening to avoid fungal plant diseases.
• Blast the affected plants by hosing with a jet of water under pressure to dislodge the mites from the undersides of the leaves. This can significantly reduce buildup of spider mite populations. Use only moderate pressure so as not to damage the plants. This method is not suitable for young plants, seedlings, or delicate plants.
• In cases of heavy infestation, where plants can’t be saved, remove and destroy the whole affected plants when practical to prevent spread to other healthy plants.

It’s important to check plants for signs of two-spotted spider mites often during persistent hot, dry conditions. Pest problems are easier to deal with when they’re detected early, and it may be possible to manage the beginnings of a mite problem simply by removing parts of a plant, or a single plant amongst many.

Biological Controls for Spider Mites

Many pests are effectively controlled by their natural enemies, which eat them and reduce their numbers significantly, serving as biological control agents.

Two-spotted spider mites are controlled by several biological control agents including:

• Lacewings
• Tiny mite feeding ladybirds (Stethorus spp.)
• Predatory gall-midges (Feltiella spp.)
• Various predatory mites, including the Persimilis predatory mite (Phytoseiulus persimilis) which controls spider mites, and the Californicus predatory mite (Neoseiulus californicus) which controls spider mites, broad mites and cyclamen mites.

Using cultural practices (cultural controls) such as increasing humidity around plants during extremely hot or dry periods can create conditions that favour these predators and disadvantage the pests, leading to much better pest control outcomes.

Commercial growers can purchase these predatory mites and introduce them into their crops to help control spider mite populations without chemicals.

Home growers can encourage the naturally occurring predatory mite populations that are present in many gardens, which are usually quite effective at controlling two-spotted spider mites and keeping their populations quite low all year-round. This can be done in two ways:

• Avoiding using broad-spectrum pesticides. These are non-specific chemicals that kill a broad range of pests, and include both the nasty synthetic, highly toxic commercial pesticides, as well as safer organic ones such as pyrethrum.
• Creating a favourable habitat for them. They way we can do that by maintaining a diverse garden with many perennial plants (ones that don’t die down in winter) to provide a home for them, and a good layer of mulch over the soil, which some of them may also inhabit.

Safe and Low Environmental Impact Organic Chemical Controls for Spider Mites

There are various chemical controls that can be used to control two-spotted spider mites, which are safe, approved for use in organic garden, with very low environmental impact.

These safe chemical controls can be used as a last resort, as they will affect predatory mites also. They can be used together with the other control methods discussed to increase the effectiveness of the overall pest control strategy.

The organic chemical controls for two-spotted spider mite are insecticidal soap, horticultural oil, neem oil and wettable sulphur.

Insecticidal soap and horticultural oil work by smothering the pests so they can’t breathe, effectively suffocating them. Since these products are not toxic, they are contact sprays that only work when sprayed directly a pest to cover its body.

To ensure good control, complete coverage of the plant is necessary. Spray both upper and lower leaf surfaces, branches and stems to run off. It’s best to spray in the morning or early evening when temperatures are cooler. Avoid spraying insecticidal soaps and horticultural oils on days when temperatures are around 30°C (86°F) or higher, as the leaves may be burnt from the heat more easily. Also, don’t spray when soil is dry, and plants are suffering from moisture stress.

Since spider mites breed so quickly, to interrupt their life cycle it’s necessary to apply a thorough spray over the entire plant once they’re detected, and then again 2–3 days later to control any newly hatched mites that may have survived.

You can also make your own horticultural oil, see the article – How to Make Horticultural Oil Spray for Organic Pest Control

Insecticidal soap is nothing more than natural organic liquid soap, which is made commercially by reacting potassium hydroxide with a natural fat or oil. That’s why the labels list the ingredients as potassium salts of fatty acids or something similar. You can use unscented organic liquid Castille soap which can be purchased from an organic shop as a hand soap. Use 20mL (4 teaspoons) per litre of water.

Note: Don’t use non-organic dishwashing liquids or hand soaps, these are actually detergents, which are not soaps, they’re harmful to plants because they strip the protective waxes from the leaf surfaces!

Neem oil is another natural plant-based insecticide derived from the neem tree (Azadirachta indica). It contains the active ingredient azadirachtin, which acts as an anti-feedant, stopping insects from eating, which eliminates them after around three days, and it also disrupts the life cycle of insects, preventing them from maturing into adults and breeding. Some neem oil products are also combined with insecticidal soaps. As there are many different neem oil formulations, follow the product label for the correct application rate.

Wettable sulphur is an organically approved miticide used to eradicate pest mites and is also used as a fungicide. This fine yellow-brown powder is a colloidal form of elemental sulphur which dissolves in water and is designed to be sprayed. It’s not the same as agricultural sulphur, which is used as a soil amendment, is not soluble in water and can’t be sprayed.

Sulfur acts as a miticide in two ways, the first mode of action is via direct contact. Physical contact with the sulfur directly damages a mite’s exoskeleton (outer protective layer), causing water loss and death due to dehydration. The second is through inhalation. When sulfur is present in the air, it can be inhaled by the mites through the tiny openings in their bodies known as spiracles, where it interferes with their respiratory system, leading to their death. Sulphur also inhibits key enzymes required for various biochemical reactions, interfering with the mite’s cellular metabolism, disrupts their normal functioning, leading to their eventual death.

What Is Mite Flare and How Can It Be Prevented?

When toxic pesticides are used, the pests inevitably develop pesticide resistance, as it only takes one survivor to pass on their genes to create a resistant population. Spider mites are notorious for building up resistance to toxic chemical pesticides when the same product is used repeatedly. This can lead to excessive use of toxic pesticides to control them.

When toxic broad-spectrum chemical pesticides are used to control other insects, or the spider mites themselves, the pesticides kill the beneficial predatory insects and predatory mites that would normally have kept the spider mites in check. The beneficials are much more sensitive to the pesticides than the pests are and therefore are harmed much more by their use. This allows the spider mites to reproduce without hindrance, leading to a condition known as mite flare, where mite populations surge, exploding into huge numbers rapidly, making the problem worse than it was before.

Mite flare can be exacerbated because some insecticides can cause mites to reproduce more quickly than they normally would! That’s because these insecticide compounds kill insects, and spider mites are not insect, so they have a different effect on them.

When using chemical controls, is best to use the safer and more environmentally friendly pesticides that are less harmful to beneficial insects and predatory mites.

References

• Victoria State Government, Agriculture Victoria, Two-spotted mite, <https://agriculture.vic.gov.au/biosecurity/pest-insects-and-mites/priority-pest-insects-and-mites/twospotted-mite&gt;
• Spider mite pests of Western Australian plants, Agriculture and Food. Government of Western Australia. <https://www.agric.wa.gov.au/plant-biosecurity/spider-mite-pests-western-australian-plants&gt;
• Clemson University Cooperative Extension Service, BEAN & SOUTHERN PEA INSECT PESTS, Factsheet HGIC 2201, Updated: May 5, 2021. <https://hgic.clemson.edu/factsheet/bean-southern-pea-insect-pests/&gt;
• How to recognize and manage spider mites in the home garden, Oregon State University, OSU Extension Service. <https://extension.oregonstate.edu/gardening/techniques/how-recognize-manage-spider-mites-home-garden&gt;
• University of Florida IFAS Extension Sarasota County, Edible Gardening Series: Question of the Week – spider mites by Carol Wyatt-Evens and Sarah Bostick <https://blogs.ifas.ufl.edu/sarasotaco/2021/04/08/edible-gardening-series-question-of-the-week-spider-mites/&gt;
• PlantVillage, Beans, <https://plantvillage.psu.edu/topics/bean/infos&gt;
• UNIVERSITY of NEBRASKA–LINCOLN, Nebraska Extension, Institute of Agriculture and Natural Resources, Cropwatch, Insects: Bean Leaf Beetle and Mites, https://cropwatch.unl.edu/soybean-management/insects-bean-leaf-beetle