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Common Cactus Problems

Author: Cathy Cromell
Issue: August, 2014, Page 124

How to inhibit pests and diseases and keep your desert plants healthy

Cacti are so resilient that their ability to survive the desert’s challenging growing conditions is something most people take for granted. However, like any other plant type, stressed cacti are more susceptible to attack by pests or diseases. Although exceptionally drought tolerant, landscape cacti still benefit from occasional watering to reduce stress. Keep an eye out for the following insects and diseases to inhibit their spread.

Cactus Longhorn Beetle
This beetle (Moneilema gigas) is found on varied common landscape cacti, including barrel, cholla, prickly pear and young saguaro. Monitor for hard, shiny black beetles about 1- to 1 ¼- inch-long, with white markings on long antennae.

Adult beetles eat the edges of prickly pear pads and terminal buds on other cacti. They also lay eggs on cholla stems. Larvae (pale grubs with brown heads) hatch and burrow into the stems or roots, eating plant tissue. Telltale larvae signs include holes with mushy green waste (frass) that dries to a crusty black. Handpicking beetles is the recommended control method. They are most active in the early morning or late evening, especially after summer rains.

Cochineal Scale
Prickly pear pads and cholla stems often are dotted with a white, cottony-looking substance. This is the waxy protective secretion of tiny sucking insects called cochineal scale (Dactylopious coccus). Bright-red fluid exuded from the female insect has been harvested for centuries to make a natural red dye. In fact, British “Redcoats” were dyed with cochineal scale. Today, cochineal is farmed on spineless prickly pear in Mexico for use in foods and cosmetics.

If kept in check, cochineal scale does minimal harm to cacti. Hose off the white fluff periodically with a blast of water. Because tiny insects are embedded among the cacti’s spines and glochids and the barbed bristles on the plant’s areoles, it is impossible to wash it all away in one attempt. Scale reproduces several times per year. Whenever white splotches reappear, hose them off.

Pad Spot
Black lesions on prickly pear pads are caused by a fungus. Its small black reproductive structures appear as spots on the pads’ surface. Dripping water or windblown rain spreads the fungal spores to pads of the same plant or nearby prickly pear.

You may notice that lower pads have more lesions than upper ones. This is because humidity and moisture remain longer in the lower portion of the plant after it rains. When pads dry out, the fungus becomes inactive. To inhibit fungal spread, remove pads or entire plants that are severely infected. Place them in tied plastic bags, and dispose of them in the trash; be sure to disinfect tools after use.

Photos - Clock-wise from top left: Cochineal scale insects exude a substance used to make a natural red dye. • White markings on the cactus longhorn beetle’s antennae help to identify it.

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