Thrips Damage To Cactus

Trying to eradicate every single insect from cultivated cacti is not only an impossible task that destroys the fun of growing plants, it would also indiscriminately kill benign or even beneficial insects along with those that truly are pests. Also, good cultivation practices make cacti less attractive to six- and eight-legged pests. Pests are more trouble to weak, unthrifty plants. Successful growers satisfy themselves with control rather than cure. Pest control must become pest management. This is achieved by careful observation to identify pests and their approximate numbers. Strategically placed yellow and blue sticky cards will capture the flying insects, and a good magnifying glass will help find the crawlers.

All new plants should be isolated before joining the rest of a collection. Three weeks is usually adequate to detect pest problems. If problems are detected, then the isolated plants can be treated. New plants should also be repotted before leaving isolation to rid them of potentially infected soil.


There are hundreds of kinds of thrips but they all damage cacti similarly. Thrips feed by scraping and rasping tender stem surfaces; however, their favorite food is pollen. The best way to look for thrips is to tap a flower or blow on it. Thrip-infected flowers will show a flurry of activity as the fast-moving insects scurry out of pollen sacs to the edge of a petal. Next they will swing over the edge to find a hiding place underneath. Other signs are blotches or streaks of pollen grains on flowers, as well as deformed and undersized blossoms. Cactus epidermis will appear finely speckled with many tiny yellow spots. There may be a scattering of tiny black specks, thrip feces, on flowers.

Thrips are about 0.1 in (2 mm) long and move rapidly for their size. To the naked eye they may look like a short worm with legs. Both larvae and adults have a similar appearance, except adults have wings. During summer thrips are found almost everywhere outdoors and they enter houses and greenhouses through doors, windows, and vents. They can crawl through normal screening, and pets, people, and fresh flowers and fruits may introduce them into the plant growing area. Thrips are prolific. Reproduction can occur with or without mating, and each female will lay 50-200 eggs. Eggs hatch in a few days, and larvae begin feeding on plants immediately. They become adults in 7-10 days and the cycle begins again.

The key to thrip control is early detection. Thrips prefer warm temperatures and low humidity. They do the most damage when temperatures are 90°F (32°C) and higher. Many species of thrips live part of their life by pupating in soil, and I prefer the precision of applying a granular systemic insecticide to the soil.


Spider mites are the most common pests of plants cultivated indoors. Mite-infested plants will be freckled with little yellow marks where mites have been sucking plant juices. In heavy infestations there may be fine webbing on plant tops or between spines. Heavily damaged plants will probably scar with a light tan to brown callus. Mites are so tiny they can float on wind currents. They may also be introduced indoors on clothing or the coats of pets. They are about the size of a pinhead and can best be found with a lOx lens. The most common species is the two-spotted mite. It is yellowish tan to greenish in color and has two dark spots, one on each shoulder. The older the mite, the larger the spots.

At 70°F (21°C) a generation of spider mites is completed in about 2 weeks, and above 86°F (30°C) it just takes a week. Below 50°F (10°C) mites go dormant. Hibernation is also triggered as days become short in the fall. They crawl off to sleep through the winter in cracks and crevices. A warm, heated greenhouse will counteract this impulse in a few mites, so some damage maybe seen during winter.

A more recently introduced chemical to manage mites is made from cinnamon oil. It controls all stages, including eggs. Field trials show that it kills most mites the first day and also provides fungicidal activity. Powdery mildew is controlled for many weeks after just one application.


Mealybugs are scale insects that look like tiny wood lice and are about 0.25 in (6 mm) long. They are covered with a fluffy wax that waterproofs them and makes them resistant to many water-based insecticides. Mealybug nests are small masses of woolly, cottonlike substance tucked away in cracks and crevices of plants or their surroundings. Nests may be found on spines, between tubercles, or among dead flowers. Some mealybugs prefer living in soil among roots, and others complete their lives out in the open on plant surfaces. Eggs and a few stray adults may be located in areas away from plants, leading to reinfection. Male mealybugs have wings and can fly, so an infestation can spread rapidly. Control rather than cure is needed.

I prefer treating for mealybugs with a systemic insecticide, which controls any soil dwellers as well as those found topside. All insecticides are dangerous and should be used with caution. If the same insecticide is used again and again, surviving mealybugs will become resistant. A biological control marketed for mealybugs is Cryptolaemus montrouzieri, a black ladybug beetle from Australia. Cryptolaemus worked poorly for me on cacti because the spines prevented them from reaching their prey easily.


Fungus gnats are tiny, brown to black flies that cruise the airspace around plants. Fungus gnats can be distinguished from whiteflies by their habits in addition to their color. Fungus gnats do not spend as much time resting as whiteflies often do, but sticky yellow cards are a good way to detect an infestation.

On closer observation, a few fungus gnats can be seen emerging from the soil. Adults do not feed on plants or damage them, but larvae do considerable damage. Larval forms look like tiny worms and live in the top 2 in (5 cm) of potting media. They wiggle through the soil, feeding on organic matter, including plant roots. Large plants with complex root networks suffer little harm from this root pruning, but for seedlings root loss can be devastating. Fungus gnats can destroy a seed flat in a few weeks. Those seedlings that are not killed will have stunted growth. The best control is achieved with an insecticidal soil drench, which attacks the problem at the site of plant damage.


Whiteflies look like tiny white moths resting on plant surfaces. They rarely fly unless disturbed, and then they just hover in the air for a while before landing. Larval forms will also be found on plant surfaces and look like glistening small white scales. All stages from larvae to adults suck juice from the host plant. Most of it is defecated onto plant surfaces as a sticky foul material called honeydew. Black sooty mold grows rapidly on this honeydew and can also harm the plant.

Two species cause the most distress, the greenhouse white-fly and the sweet-potato whitefly. It is easy to identify each by carefully examining the wings of adults. The greenhouse whitefly's wings are arranged flat on its back parallel to the ground; the sweet-potato whitefly's wings are attached to the sides of its back at a 45° angle to the ground. It is important to know the species because sweet-potato whitefly is harder to control and may require chemical application for a longer period of time. A systemic insecticide applied to the surface of the soil is often used for control.


Caregivers of plants must on occasion use pesticides as part of a control program. Pesticides are usually classified as poisons and present health risks for the three p's—people, pets, and plants—if they are handled carelessly. Take time to do the job properly.

Spraying should be accomplished with the smallest droplet size possible. If a sprayer cannot deliver a fine spray, replace it. Research has shown that the coverage of a spray increases eightfold each time droplet sized is halved. One drop of liquid properly spread covers 0.3 square inch (2 square centimeters); the same amount of liquid delivered as a fine aerosol will cover up to 10 square inches (65 square centimeters). Delivering spray as a fine mist will cover more area with less pesticide, not only reducing health risk and saving money but also doing a better job of getting the spray into cracks and crevices.

When spraying, skin should be covered. Wear a hat, mask, long sleeves, and full-length trousers. Hands should be covered with rubber gloves, either when spraying or when applying solid pesticides. Disposable gloves and masks can be purchased inexpensively at pharmacies. Though you are protected, other people, pets, and plants are not. Use good judgment and foul the environment as little as possible.

Whenever mixing sprays, use either distilled water or water purified through reverse osmosis to dilute chemicals. Hard water of the wrong pH destroys many pesticides before they are applied. For example, organophosphates or carbamates diluted with alkaline water are hydrolyzed, rendering them ineffective as pesticides.

There are products that improve the efficiency of sprays, called spray adjuvants. (1) Buffering compounds change the pH of solutions. Most often water is too alkaline, so acids are used to lower the pH. (2) Spreaders, sometimes called wetting agents, allow sprays to cover plant surfaces more thoroughly. Some pesticide concentrates already contain a spreader, so read the label before adding more of something that may already be there. (3) Spreader-sticker is even more efficient, acting not only as a wetting agent but also encapsulating the chemical and holding it against the plant surface. Spreader-sticker prevents the pesticide from washing off in rain or during overhead watering and is needed when spraying outdoor plants. (4) Extenders act as a sunblock, preventing ultraviolet light from destroying chemicals before they have done their job. Extending the period of chemical control in this way will allow spraying less often.

The benefits of pesticides outweigh their dangers when they are used properly and safely. Each wise plant caregiver should follow 10 commandments:

1. Read the label. Be sure the chemical is effective for the problem, that it is diluted properly, and used at proper intervals.

2. Agitate. Periodically shake the spray bottle or tank when spraying. Most pesticides are in suspension and will settle to the bottom. That thick slurry would damage plants.

3. Do not mix pesticides. Mixing several pesticides together instead of applying them separately will often damage plants.

4. Time the application properly. Apply sprays early in the morning, after sundown, or on cloudy days to reduce wastage and the possibility of harming plants. Do not apply chemicals if air temperature is warmer than 85°F (30°C).

5. Spray carefully. Avoid drenching the soil surface with foliar sprays because many are not intended for use around roots. This is particularly true for seedlings.

6. Avoid moisture stress. Plants should be recently watered. Water-stressed plants are more easily damaged by chemicals.

7. Do not fertilize. Do not apply pesticides with fertilizer.

8. Mix small amounts and use immediately. Most products cannot be stored after dilution because they break down and become ineffective.

9. Do not use damaged products. Do not use a wettable powder if it fails to break up and suspend properly, and do not use an emulsifiable concentrate if it fails to form a milky suspension.

10. Ventilate. Mix chemicals in a well-ventilated area.

Continue reading here: Propagation

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  • mindy
    Do thrips prefer "weak" plants?
    6 months ago
  • semira
    Do thrips affect cacti?
    11 months ago