Cactus Pests

We have been fortunate not to have many insect problems with our plants. The following information has been gleaned from various sources and the internet. If you are considering using pesticides to cure an insect problem, please read and follow all manufacturer instructions. Remember pesticides are designed to kill.

[Mealybugs] Mealy Bugs: white/gray insects about 0.1 inch (3mm) in length. Mealy bugs dine on cactus sap and reproduce rapidly laying their eggs underneath a cotton-like elliptical covering. This white covering also serves to protect the insect from predators and pesticides. A plant infested with mealy bugs is covered with these white spots and sticky honeydew. No parts are immune to their attack; while most often you will find them on the stem, we have also found them adhering to the spines of the plant. Left unchecked, infested plants will stop growing, take on a sickly-deformed appearance, and start to shrivel. Weakened plants often succumb to fungi and rot.

Minor mealy bug infestations can be handled by mechanically removing the pests (i.e. using fingernails, scrub brushes, tweezers, and/or water pressure). Dabbing the insects with a cotton swab previously dipped in rubbing alcohol is the preferred treatment. Alcohol dissolves the waxy coat and kills the bug.

Systemic or contact insecticides may also be used to control widespread mealy bug attacks. Malathion and dimethoate are often recommended. Before applying insecticides, insure that the plant has been well watered a day prior and always read and follow instructions carefully. Weekly re-applications are sometimes required for complete control.

Mealy bugs may also attack the roots of a cactus causing the plant to appear sickly. If you can't locate any insect above ground on sickly plants, root mealy bugs may be feasting on the plants roots. To eliminate, unpot the plant and rinse off all of the white spots and soil using warm water. Remove any suspicious looking roots and allow the plant to dry. After the plant has dried, repot into a fresh soil mixture.

To be absolutely safe, you may want to take cuttings from an infested plant, apply an insecticide to the cuttings and start off some new plants.

Red Spider Mite: minute, reddish insects less than 0.2 mm long. Often these pests are found in their whitish webs where they dine on cactus sap. Pale yellow/white spots which later turn rusty brown appear on plants infested with red spider. Damage occurs mainly at the top of the plant. Weakened plants are also suseptible to viral, bacterial, or fungal infections. Dry indoor air exasperates the problem.

Overhead watering is both a preventative and cure for red spider. As the plant permits, a strong stream of water should be used when watering. The use of a miticide is recommended for widespread problems.

Scale: pinhead-size insects that present themselves as raised tan/brown spots. These spots, resembling marine limpets, are actually hard shell coverings that protect the insect or eggs underneath. Scales reproduce like rabbits, prefer columnar and rock cacti, and dine on plant juices. Infected plants appear yellow and weakened. Left untreated, scale can cause bud and shoot drop. Scale are treated similar to mealy bugs. Minor infestations may be remedied by washing the plant with a weak detergent solution or by mechanically removing insects with a toothpick. Malathion may be used to control major infestations. When fighting scale, be sure to cover the soil, otherwise, these pests will just drop off and then get right back on the plant.

Snails/Slugs: these familiar insects are great plant eaters. Seedling or adult, these insects do not discriminate. Careful examination of plant and pot is the best preventative. Be sure to examine the undersides of pots before bringing plants indoors. These pests are messy but you can manually remove them. To treat a widespread problem, first sprinkle metaldehyde bait on the soil then water the soil.

Thrips: thrips leave behind telltale yellow or white spots on plant leaves. To control, spray with a solution of nicotine sulfate or if necessary use a miticide.

Nematodes: microscopic wormlike animals living in the soil. Nematodes burrow themselves into plant roots causing root damage. Plants infected with nematodes take on a stunted or pale look. To remedy a nematode outbreak, unpot your plant and remove all soil from roots. Carefully cut off all damaged roots, or all roots to be safe, and let the plant or cutting dry for several days. Repot the plant or cutting in sterile soil. Systemic insecticides are also effective against nematodes.

Ants: the familiar ant does its share of cactus damage. Often nesting in the soil, the ant dines on young stems, grains of seeds from ripening fruits, and stamens of flowers. The ant is often an indicator of an aphid problem. These two insects are allies; ants often eat the honeydew excreted by aphids. To remove ants, wash them off with a strong lukewarm stream of water.

Aphids: these minute green insects often inhabit the soft parts of the cactus. We have found that blooms and blooming parts of the cactus are often targets. A colony of aphids appear as tiny green spots. The pesticide Malathion may be used to control.


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