A reader asked for detailed information on an insect problem she’s been having in her garden for several years. Josefa Price has 10 prickly pear plants on her 2.5-acre property west of Highway 99 in Fresno that have severe, repeat infestations of cochineal scale, a soft-bodied scale.
Price correctly identified the scale herself by crushing a few of the insects to produce a bright red liquid (the carmine dye used by native peoples in the Southwestern states and in Mexico). She’s also been correctly treating for the scale by daily spraying them off the cacti with water from a hose and by applying neem oil in an effort to protect beneficial insects that prey upon scale insects from being harmed by chemical pesticides. Unfortunately, the scale infestations are so heavy that washing them off has only a temporary effect. Neem oil applied during the heat of summer burned the outer surface of the cactus paddles. Cutting off severely infested paddles slows the decline of the entire plant, but does not permanently reduce the cochineal populations. Cold winter weather, which usually knocks down soft-bodied pest insect populations, also had little effect.
Price is concerned that continued heavy infestations will eventually kill many cacti in the area but she was unable to find suggestions for other control methods online.
This is a rare instance when the UC Davis IPM website does not provide comprehensive information on the identification and treatment for a pest insect in California. The University of Arizona Cooperative Extension websites: www.cals.arizona.edu and www.extension.arizona.edu. recommend exactly the treatments that Price has already tried. Our local UC Cooperative Extension, Fresno, horticultural adviser had not yet heard of severe cochineal scale problems and will be contacting UC Davis Integrated Pest Management staff. We should have an update soon.
Cochineal colonies look like small, soft, cottony tufts on the surface of the plant. They are generally host-specific, feeding in this case on prickly pear and cholla cacti. The nymph stage of the scale are carried by the wind to neighboring plants.
Cochineal scale insects are sucking insects. Like aphids, cotton-cushiony scale, whiteflies and mealybugs (which cochineal resemble), they pierce the outer tissue with their mouth parts and feed on sap and nutrients. Extremely heavy, persistent infestations weaken and can kill affected plants.
Systemic insecticides applied as a soil drench will kill sucking insects, but are not recommended for edible plants and Price enjoys eating the fruit of her prickly pears. Some websites suggest mixing chemical insecticides (i.e., malathion) with a sticker soap solution and spraying the entire plant. Many beneficial insects including syrphid fly larvae, lacewings and lady beetles feed on cochineal scale and should be able to reduce populations. It’s extremely important in a case like this to protect the beneficial predators from broad-spectrum insecticides.
Horticultural oils sprayed in winter (to avoid plant tissue burn) will smother most adult soft-bodied scale insects as well as overwintering aphids, mealybugs and whiteflies. That treatment would be worth a try.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).