Pesky Garden Pests: Spider Mites, Fungus Gnats, and Caterpillars

Spider Mites

Spider mites are small, sucking pests that can be difficult to see without the aid of magnification. They are usually brown or translucent, yellow or green in color, sometimes red, and are as small as 1/50^th of an inch (0.5 mm). A 10x hand lens or greater is usually required to positively identify spider mites. Both the adult mite and their immature progeny can do extensive damage to plants. The most common species encountered by the indoor or home gardener is the two-spotted spider mite, Tetranychus urticae Koch. They are known to affect hundreds of species of garden, field, greenhouse and hydroponic crops.

The life cycle of spider mites is very rapid. It can vary some between species and varies with temperature, but with all mites, it includes the egg stage, the larval stage, two nymph stages (protonymph and deutonymph) and the adult stage. At about 80˚F, spider mites evolve from eggs into adults in as little as five days. This process can take several weeks to occur if temperatures are cooler. When left untreated, many generations of spider mites occur per year. Adult females live for up to one month and can lay several hundred eggs during their short lifespans.

Mites do their damage to plants by sucking out the sap from the leaves. Their pattern is to start on the undersides of leaves and then move from leaf to leaf as they drain the plant of its vital liquid. Meanwhile, the mite populations continue to explode. Damage to plants from mites appears as yellow-speckled or mottled leaves, appearing on the undersides of leaves or needles first, but becomes quickly evident on the tops of the plants as well. If unchecked, the leaves eventually turn yellow or brown and the entire plant will die due to its inability to make food for itself.

To test for the presence of spider mites, place a sheet of white paper under the leaves of your plants and tap or shake the leaves. If there are mites present, some should fall on the paper and appear as moving specks across the paper. Black specks of frass (fecal material), amber-colored eggs and white skin-castings are another sign that mites are present.

Mites do best in warm and dry conditions. If possible, your grow area should be kept as humid as possible, or at least as much as your indoor plants and crops can tolerate. This can be done by periodically misting your plants, especially the undersides of the foliage with a spray bottle. Make sure also you do not let your soil get too dry between waterings either as this can promote the spread of mites as well.

Spider mites are not true insects. For this reason alone, many gardeners fail to control them with widely commercial insecticides. They are arachnids and can only be controlled with miticides or those formulations and controls that are meant to kill or disrupt the biological activities of spiders or ticks. If possible, take your plants outside to treat them so you are not just knocking the mites off only to have them return to their original host or move on to another. If this is not practical, the same effect can be achieved by taking your plants into the bathtub to treat them so that any mites knocked loose simply disappear down the drain.

This option should not be used with chemical insecticides as they will eventually find their way into the water supply. Regardless of your choice for organic, natural or synthetic control, it is wise to treat only a small area of an infected plant first to check for phytotoxicity, which is damage to the plant by the remedy. Some formulations can actually burn leaves of sensitive plants, so read the entire label to make sure the plant or crop you wish to treat is appropriate for the pesticide selected.

Some natural or organic remedies for control include using a forceful stream of water to dislodge them from your plants, or applying insecticidal soap or horticultural oils (these should be at about 2% oil so they do not burn the plant foliage). Homemade insecticidal soaps can be created with 2 tsp. of mild dish soap mixed with 1 gal. of slightly warm water. A disposable paint brush or tooth brush can be used to spread the soap onto the leaves, ensuring contact with the mites and additionally knocking them off.

Neem oil is an effective pesticide that also has fungicidal properties as well. Soaps and oils should be applied about once per week until no further damage is noted. As spider mites hide out mostly on the undersides of leaves, it is important to make sure both sides of the leaves are treated.

Predatory mites such as Phytoseiulus persimilis and Amblyseius californicus can be released in your grow areas to kill spider mites. These ‘good guys’ do not bite people and will stay put where you release them. When they have done their job, they too will die as their food source disappears. It is more effective to release them in indoor growing areas than outside growing areas.

Fungus Gnats

Fungus gnats are a fly species that typically do more damage to indoor crops than outdoor crops. The adult gnats, similar in appearance to a small mosquito, do little to no damage to plants; it is the larvae that live in the soil that does the damage. The larva feed on plant roots and root fibers. Because it is difficult to know you have fungus gnat larvae in your soil, control is often aimed at the adults.

Yellow, sticky cards are an effective, non-chemical control of fungus gnats, but should not be the only approach to controlling these flying pests. Cards should be hung in or near plants so as to be seen by the gnats. Plants can also be sprayed with a pesticide containing pyrethrins, a botanically obtained insecticide from the chrysanthemum plant. There are organic and synthetic versions of this material, so be sure to read the label if you are looking for the organic version.

Killing the soil-dwelling larvae should be done in tandem with controlling the adults. Beneficial bacteria (Bt) can be applied as a soil drench to kill the larvae (more on Bt below). Beneficial nematodes can also be applied to the soil. Nematodes are microscopic worms. The species Steinernema feltiae is one of the ‘good guys’ that will kill fungus gnat larvae by injecting their eggs inside of the gnat larvae. Once the eggs hatch and the young nematodes are ready to start feeding, they do so by consuming the fungus gnat larvae from the inside out.

Cultural controls can also control the spread of fungus gnats. Fungus gnats prefer to lay their eggs in moist soils. Allow your plants to get dry in between waterings if they can tolerate it. Make sure to get rid of dead and infected plants and try to keep all areas around your plants as dry and clean as possible.

Caterpillars

Unlike many plant pests, caterpillars are large enough to be seen without magnification. Caterpillar is a generic term for the larval stage of numerous butterfly and moth species. This is the life stage between egg and pupae for these flying insects.

These crawling pests can chew holes in leaves and even consume whole leaves of some plants. Other caterpillar species will eat flowers, fruits and even chew into the woody stalks of your plants. There are species of caterpillar that will consume almost all crop species, but there are many that are host-specific. For instance, there are those that feed on members of the Solonaceae family, like the tomato hornworm, and those that feed on members of the Brassiceae family, like the cabbage looper. Regardless of which type of caterpillar you have, their biology is similar and can be treated in largely the same manner.

If any of your outdoor crops succumb to caterpillar damage, they are easy to spot and can be hand-picked off. They also have many natural enemies including spiders, parasitic wasps and birds. If they appear on any indoor crop, you can spray your plants with a beneficial bacterium that only kills caterpillars.

Bacillus thuringiensis kurstaki or Bt (or Btk) is readily found in commercial formulations that can be applied as a foliar spray. It is important to fully cover your plants and make sure you spray as soon as damage appears. Bt is more effective on young caterpillars than mature ones. It can be sprayed on any outdoor crop, including ornamental plants, but be aware that it will kill beneficial pollinating butterfly larva as easily as it will kill crop pest larva. Use on outside crops with discretion.

In the final part of this three-part piece, we will detail damage and control for snails, slugs and other pests, as well as a look at how to attract beneficial insects to your growing areas.

Missed part one? Find it here.