White spots on cactus indicate scale infestation
Q: I don't know what is on the beavertail cactus. It just appeared this summer. It's about seven years old and I've never seen this white stuff before. The cactus doesn't have a water source -- no bubblers, no overspray -- it's completely on it's own. I sent you a picture. What should I do?
A: This problem is called cochineal scale. This is a tiny insect that produces a white covering over its body to protect itself from the environment.
At the University of Nevada Cooperative Extension Master Gardener Orchard in North Las Vegas we grow the prickly pear cactus, or beavertail, as a food for both its pads and its fruits, which Mexicans call nopales and tunas respectively.
Some of the cochineal scales produce a red dye that can be seen when the insect is physically harmed. These insects can crawl from place to place but once they find a location they like and set up house by producing that white outer covering they usually stay put and begin feeding. They feed by sucking plant juices from the cactus. When hundreds of them find homes on a cactus they can cause enough damage to kill the cactus.
You have so many of these scale insects that they merely cover the entire plant and its pads.
It is possible that a bird could have visited another cactus that was infested and transported them to your cactus. But it does not have to be birds, it could be people, insects or anything else that could transport them to your location.
They are difficult to control. We usually control these insects with a high-powered stream of water through a sweep nozzle attached to a hose. The stream of water or simply blasts the scales off all of the plant. During the heat of the summer, this has to be done about twice a week to keep them off.
Soapy water also can be used, but it seems not to add much of an advantage over straight water.
The insecticide Sevin also can be used but only after the scale insects have been swept off of the pads with a hose. However, you might be able to add an additive to the spray called a spreader and get adequate penetration of the white covering to control the protected insect. I'm sure it will take several applications of this insecticide each year to control this insect once the plants have been infested.
Some people believe that only stressed beavertail cactus are infested. I have seen both stressed and nonstressed plants heavily damaged by this insect.
Q: Our once-thriving and lovely 15-year-old or more Mimosa tree is still dying after watering with shock stuff and trimming dead branches.
A: The usual problem with Mimosa is mimosa wilt and, unfortunately, there is not much you can do about it. The disease is fungal and plugs the water-carrying system inside the tree.
This is a common problem with Mimosa, or silk tree, and is probably the reason we do not see very many old ones around. It is recommended to remove the tree and replace it.
The disease spreads from the soil into plant roots, particularly when the tree is in a weakened state. Mimosa trees should never be planted in rock mulch or in a dry area of a desert landscape.
Mimosa should be planted in the wetter areas of a desert landscape and would be best served with an organic mulch, regular watering and adequate fertilizer applications.
Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at morrisr@unce.unr.edu.
