Cacti tap into their own immune system to fight infections and insect attacks. But when a cactus plant is in poor health because its growing requirements haven't been met, it loses the battle. Provide your cactus the care recommended for its species and cultivar to prevent insect damage. If, despite your effort, the plant succumbs to bug infestations, identify the bug and follow the steps to eliminate the pest.
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Handpick cactus longhorn beetles off plants. Drop them in a bucket of soapy water to kill them. The insect, which usually frequents cacti early in the morning or late in the evening, is black and about 1 inch long. Its antennae display white markings and they are also often longer than the bug's body. Cactus longhorn beetles eat the edges of prickly pear pads and the buds on the tips of other cacti.
Scrape cochineal bugs off cholla and prickly pear cacti. Use a flat, dull blade, a butter knife, for instance, to prevent injuring the cactus skin. Drown the bugs in a bucket of soapy water. Wear thick gloves to protect your hands from the thorns. Alternatively, dislodge and kill the bugs with a strong jet of water. The pressure, however, might also break the plant. When both methods fail to eliminate the cochineal bug infestation, spray them with insecticidal soap, following the application rates listed on the product label. As a last resort, treat the cactus with a synthetic insecticide containing malathion as the active ingredient. Select a brand and use the product according to the manufacturer's instructions. Cochineal bugs are scale insects that protect themselves with a waxy cottony cover. Identify the insect by crushing it. Cochineal bugs release a red fluid.
Treat insects known as plant bugs (Caulotops barberi) with insecticidal soap. Spray the cactus according to the manufacturer's instructions. Caulotops barberi is about 1/2 inch long, brown with black eyes that protrude to the sides. This insect does enough damage to kill the plant.