A handful of insect pests account for most trouble on a cactus, and the common ones can often be removed by hand if you find them early.
The short version
- The most common pests are mealybugs, root mealybugs and scale. Spider mites and fungus gnats also turn up.
- White cottony tufts that smear when pressed are mealybugs. Small shells stuck to the skin that pry off with a fingernail are scale. Touch each one with a cotton swab dipped in rubbing alcohol and wipe it away.
- A pale plant that puts on no new growth in warm weather may have root mealybugs. Slide it out of its pot and look for white deposits on the roots. Wash them off and repot in fresh potting mix.
- Webbing and small brown dots on young growth are spider mites. Wash them off with a firm stream of water.
- Small black flies over the soil are fungus gnats. Let the surface dry between waterings and hang yellow sticky traps.
- Sprays may damage a cactus. Test a spray on a small patch first, keep soap and oil sprays for days below 90 F, and follow the label.
- Keep a new plant apart from your others for 4 weeks. Check it on arrival, at 2 weeks and at 4 weeks.
Checking a plant
Check in good light, with a magnifying glass if you have one. Pests hide in these places:
- around the areoles, the small felted pads the spines grow from
- in the grooves between the ribs and where a branch meets the stem
- at the growing tip, where the skin is youngest
- at the soil line, under the rim of the pot and around the drainage holes
A soft, dark, wet patch on the stem is rot, and it needs prompt attention. See cactus rot.
Mealybugs
Mealybugs are slow-moving insects up to about 1/5 inch long, with a waxy, white, cottony look. They gather in clusters along the spines and in sheltered spots where a branch meets the stem.
In many kinds the eggs sit inside the cottony tufts and can hatch in as little as 5 to 10 days, so swab the tufts as well as the insects. Check again every week or so and swab whatever has appeared, until you stop finding any. Mealybugs also shelter under the lip of the pot and in its drainage holes, and they can live 2 to 3 weeks off a plant, so wipe those places with the same alcohol swab.
Root mealybugs
Root mealybugs live in the mix, the potting soil in the pot, and feed on the roots. They are 1/16 to 3/16 inch long and coated in powdery white wax, so a colony looks like white deposits on the roots and between the roots and the pot. Above the soil the signs are slow growth and a pale or yellowish color. The damage also leaves the roots more open to rot.
- Take the plant out of its pot and throw the old mix away.
- Wash the roots under running water to remove the insects.
- Clean the pot before you reuse it, or use a new one.
- Repot in fresh dry mix and wait about a week before you water. See cactus pots and repotting.
Root mealybugs spread in draining water and on ants, and the young ones walk from pot to pot. Keep an affected plant where its drainage water cannot reach another pot. One generation takes about 2 to 4 months, so look at the roots again in a couple of months.
Scale
Scale insects look like small cottony or dome-shaped shells attached to the stem. A mealybug moves, slowly, and smears when pressed. A scale insect stays put and comes away as a shell when you scrape or pry it with a fingernail.
When there are only a few, rub or pick them off, or remove them with a cotton swab dipped in rubbing alcohol. Check again every week or so until you stop finding any. If they cover more of the plant than you can clear by hand, see the section on sprays below.
Spider mites
Spider mites are about 1/50 inch long. The first signs are usually webbing and small brown dots where they have damaged the skin, especially on younger growth. They prefer hot, dusty conditions, and a plant that is short of water is more open to them.
Cover the soil in the pot, with a plastic bag for example, so it is not soaked or washed out. Then wash the plant with a firm stream of water. Repeat if you see fresh webbing.
Fungus gnats
Fungus gnats are black flies 1/16 to 1/8 inch long that look like small mosquitoes. The adults do no damage. Their larvae live in the mix and eat organic matter and roots, and cacti are sensitive to that injury.
Gnats breed in moist organic matter, so they often point to a mix that stays damp. Let the surface dry out between waterings. Watering cactus explains how to tell when a pot is dry. Yellow sticky traps catch the adults. If the gnats persist, garden stores sell 2 biological controls for fungus gnat larvae, both watered into the mix: bacterial products, and beneficial nematodes, tiny worms that kill the larvae.
Sprays and other products
Start with a swab or water. A cactus has natural oils and waxes on its skin that may make it prone to damage from insecticidal soap.
- Alcohol spray. A spray of 1 part rubbing alcohol to 3 parts water may help with pests hidden where a swab cannot reach, but it may damage the skin of sensitive cacti.
- Insecticidal soap and horticultural oil. Do not use either when the temperature is above 90 F or on a plant that is short of water, one that is shriveled or has gone a long time dry. Oils should not be used on waxy succulents or other plants the label lists as sensitive.
- Systemic insecticides. The plant takes these up through its roots. For home use they are sold as granules or stakes that go into the mix, and they are meant for pests that sprays do not easily reach.
- Test first. Spray soap on one small patch and wait a day. With other products wait several days. Go on only if the skin is unmarked.
- Follow the label. Check that the label says the product is safe for your plant, use the rate it gives, and wear the protection it calls for.
The wax on mealybugs makes them hard to control with pesticides, so keep checking by eye after any treatment.
Keeping new plants apart
Infestations are almost always started by a plant that was recently brought home. A separate room is best for the 4 weeks apart. The first check is described in after your cactus arrives.
Two less common pests
Cactus longhorn beetle. In some desert areas, a shiny black beetle 1 to 1 1/4 inches long, with white markings on its antennae, feeds on the growing tips of cacti. Pick the beetles off by hand in the early morning or late evening, when they are most active.
Harrisia cactus mealybug. This mealybug attacks several kinds of cactus and distorts the plant. Branches curl, and new growth comes in deformed. The USDA lists it as a reportable pest. If you see mealybugs together with that kind of twisted growth, keep the plant apart from your others.
Viruses and clean tools
Cacti can carry viruses, usually without any visible sign. They can be passed from plant to plant on cutting tools, so clean your blade before you move from one plant to the next. Wipe off any sap and dirt, then wipe the blade with 70% isopropyl alcohol so it stays wet for at least 30 seconds. Let it air dry.
If you are unsure what you are seeing, email us with photos at support@thoughtfulpricks.com.
Sources
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