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Common Pests

Spider Mites

Spider mites are spiders, not insects. They have 8 legs; insects have only 6. Regardless, they are a serious pest both indoors and out. Spider mites are very small, only about 1/50 of an inch long, and so are difficult to see. They have piercing-sucking mouth parts for feeding on plant sap. They can multiply rapidly and in large numbers, cause leaves to take on a dusty, dull appearance. Leaves then yellow and drop or turn brown or tan.

Symptoms and Diagnosis
Since the mites are so small, the first sign is generally a plant that looks dull or in poor health. Leaves may appear stippled and curled. Fine webbing may also be evident under the leaves or between the leaf and the stem. When a leaf or branch is tapped over a white sheet of paper, small specks that appear as dust or pepper may be seen to move.

Life Cycle
Spider mites can go from egg to mature adult in less than two weeks. Indoors as well as outdoors, several generations occur each year.

Integrated Pest Management Strategies
1. Remove mites. Dislodge as many mites as possible using a strong stream of water. Done on a regular basis, this can reduce populations dramatically. Wash the plants either outdoors or in the shower. Protect the soil so it is not washed out of the pot by the spray.
2. Use insecticidal soap. Many insecticidal soaps are also effective in controlling mites so check the label. Other soap products may actually be labeled as miticidal sprays. These soaps are specially formulated to kill mites and not damage plants. They can be very effective if used frequently until the problem is under control.
3. Use superior horticultural oil sprays. Highly refined oils sold as superior or horticultural oils are also very effective in controlling mites. The oil suffocates the mites. Unlike dormant oils, these oils are highly refined and under proper conditions, can be applied to plants in foliage without damage. Follow label directions to avoid damage to some plants that may be sensitive. Superior oils are also considered nontoxic and are less harmful to beneficial insects. When spraying indoors, protect surfaces that may be damaged by an oil residue.
4. Use chemical miticides. Many miticides registered for use indoors are available. Follow directions and if possible, spray out-of-doors or in a garage, weather permitting. Check label for control of spider mites.
5. To limit future problems, inspect plants regularly. With regular inspection, pest problems can be caught when just beginning and control is easier. It is also recommended to isolate newly acquired plants for 2–3 weeks to limit introducing pests indoors. Bringing plants indoors in the fall is another way of introducing mites indoors.

Organic Strategies Strategies 1 and 5 are strictly organic approaches. For organic approaches to Strategies 2 and 4, consult the Organic Materials Review Institute (OMRI™) for appropriate insecticidal soap and miticide products.


A common pest found in greenhouses and indoor/ outdoor gardens, thrips damage plants by sucking their juices and scraping at fruits, flowers, and leaves. Plant leaves may turn pale, splotchy, and silvery, then die. Injured plants are twisted, discolored, and scarred.
Adults are very small (less than 1/25 inch) straw-colored or black slender insects with two pairs of feathery wings. Without the use of a hand lens, they resemble tiny dark threads.

Extremely active, thrips feed in large groups. They leap or fly away when disturbed. Host plants include onions, beans, carrots, squash and many other garden vegetables, and many flowers, especially gladioli and roses. Both adults and the wingless larvae are attracted to white, yellow and other light colored blossoms and are responsible for spreading tomato spotted wilt virus and impatiens necrotic spot virus.

Life Cycle
Adults and pupae overwinter in garden soil. In spring, newly emerged females insert eggs into the tissues of flowers, leaves or stems. (They do not need to mate for reproduction.) Each female can produce up to 80 eggs, which hatch within days in warm weather, or weeks to months in colder weather. They become wingless larvae (nymphs), which feed on plant sap. After two or more nymphal stages, many thrips drop to the soil to pupate. Emerging adults fly to the plant and repeat the cycle. There may be 12-15 generations per year with the entire cycle from egg to adult requiring less than 16 days in warm weather.
Thrip management is a matter of garden maintenance — reducing the places where thrips may breed — and requires removing plant debris while it’s still on the ground and green. Thrips lay their eggs in slits they cut in live plant stems. Vigilance — spotting problems early and responding to them — is also required. Check your plants for damage and clusters of the pests at the place where leaves are attached to stems. Don’t wait to take action. Take the measures listed below. And be sure to use the safest, most proven products.
To get rid of thrips remove weeds and grass from around garden areas to eliminate alternate hosts. Clean up crop debris in the garden, especially onion leaves after harvest. (Dry mulch will not attract thrips. Green mulch will.) Inspect all plants you import into the garden for signs of thrips or their damage. Discard any infested plants by securely bagging and putting in the trash. Blue sticky traps are helpful for monitoring adult populations. If found, use the Bug Blaster to hose off plants with a strong, encompassing spray of water to reduce pest numbers. Release commercially available beneficial insects, such as minute pirate bugs, the effective thrips predator (feeds on eggs and larvae before they can become adults), ladybugs, and lacewing, (especially effective in green houses) to attack and destroy all stages of this pest. For best results, make releases after first knocking down severe infestations with water spray or other method. Severe populations may require a least-toxic, short-lived botanical insecticide (pyrethrin) to reduce pest numbers. Follow-up with predatory insects to maintain control.
Safe, smothering insectical soaps made from naturally occurring plant oils and fats, are also effective for knocking down heavy infestations (and won’t harm most naturally occurring beneficial insects). Spinosad, and neem oil can be used to spot treat heavily infested areas.

Tip: Thorough coverage is necessary when using natural contact insecticides, especially on the undersides of leaves and where leaves attach to stems, a favorite place for thrips to congregate.

Fungus Gnats

They are small, grayish to black flies that are 2 ½ millimeters long and resemble tiny mosquitoes (minus the bloodsucking). Their legs are long and slender, and their skinny antennae are usually longer than their heads. Their wings are shades of gray. Fungus gnats are relatively weak fliers and generally remain near potted plants, often running or resting on soil or leaves.

What kind of damage do fungus gnats do?

They may not threaten human health, but with fungus gnats you get a triple whammy: Not only do people hate to see them in general, the pests can be vectors for plant diseases – not good! “What’s a vector,” you ask? It’s an organism that transmits a pathogen, so if you have a sick plant, fungus gnats can spread it to all your healthy neighboring plant friends. They can also vector several different fungal root rots, including ones called Fusarium and Pythium, and even foliage pathogens like Botrytis. And as if that’s not bad enough, fungus gnat larvae make breakfast, lunch and dinner out of your plant roots.

So there’s good news, and there’s bad news: The good news is adult fungus gnats only live about one week. The bad news is that in this short time, the female will deposit 100-150 eggs on your plant’s soil surface. These eggs are laid in strings of three to 40 and can hatch within four days of being laid!

The emerging larvae are clear to creamy-white and can grow to about 5 ½ millimeters long. They have shiny black head capsules. The larvae feed on tasty root hairs in the upper 1 centimeter of the soil, then work their way up into the plant stem. (They also love to feed on the roots of your newly planted seed, so watch those seed-starting trays in spring!) The larvae feed on highly organic soils, too. After feeding for approximately 14 days, the larva pupates. In about three and a half days, an adult will emerge from the case. The total life cycle takes two to four weeks.

How do you control fungus gnats?

They key is prevention, and you can do this two ways: The first is to avoid overwatering your plants. Overwatering, to fungus gnats, is like laying a big steak on the floor in front of a starving dog – they can’t resist it. The second way to prevent the problem is to inspect the soil of a plant before bringing one home. Do you see gnats buzzing around it? If so, that’s not a good sign. Put the plant down and just walk away.

Some good monitoring methods can help cut down fungus gnat issues, too. Yellow stick cards (small, yellow cards with sticky adhesive on both sides) often do the trick. These can be purchased online. Many insects, including flies, are attracted to the color yellow. So upon seeing the yellow stick card, they’ll mindlessly fly right into it, and SPLAT! The adult fly is stuck. Ta-da! These cards are most effective when placed horizontally near the surface of potting soil. You can use popsicle sticks or straws to hold the cards, or some come with sticks in the package. Keep in mind, however, that this control method only traps adults – not the larval stage of fungus gnats.

Another way to control these plant invaders is with the potato trap method: Cut chunks of potato into 1 ½-inch-square pieces. Place them on the surface of your potting soil. This is like a chuck wagon call for fungus gnat larvae! They’ll head straight for it and start munching. Leave the potato for a few days and then lift it up – you’ll quickly discover if you have larvae in your soil. (And obviously remove the potato once you’re finished with it – no one wants rotting potatoes in their pots … or anywhere in the house, for that matter.)

Controlling fungus gnats through biological means is a relativity easy thing to do as well. Currently two different biological control agents are available: beneficial nematodes and predatory mites.

There are several species of beneficial nematodes that can be found through the Internet. These nematodes are so small, they can’t be seen with the naked eye. (If you get real close and look hard enough, you can see them with a 10X hand lens.) Steinernema feltiae is the most commonly used nematode species for fly larvae control. Nematodes are simple to apply – just water them into the soil.

One predatory mite you might like to try is Hypoaspis miles – a brownish soil-dwelling mite less than 1 millimeter in size (now that’s really small). This predatory mite inhabits the top layer (at a depth of 1-4 centimeters) of the soil and feeds on harmful soil insects, such as fungus gnat larvae and thrips pupae. It takes an average of 18 days for this mite to reach adulthood. Adult mites can kill up to seven fly larva per day. One of the favorable characteristics of this particular mite is its high tolerance to starvation. Newly emerged adults can survive for three to four weeks without food. (If fed, adults can be active for four to five months.) These, too, can be purchased through the Internet.

A fungus gnat outbreak can cause quite a headache, but by following proper water-management practices and treating the pests once they’re found in the soil, they can be easily controlled.