I finally got my courage up and picked and some grapefruit a few weeks ago.
They had become a beautiful shade of coral pink and the sour memory of the first harvest earlier in the summer has faded from memory. Our citrus tree has multiple grafts of lemon, orange, and grapefruit.
This tree was planted when we first moved here more than 12 years ago, when our back yard was not landscaped and we thought 3 feet from the fence would be plenty of space for the tree. Needless to say, 3 feet is not enough space and the grapefruit graft grows right up to the fence.
We try to keep the tree canopy low so the fruit are easier to reach and to help shade the trunk from the sun. The low branches make getting around the back side of the tree a challenge. I maneuvered my way under the branches and as I reached for the first grapefruit, a white cottony mass on a leaf caught my eye.
Sandwiched between the fence and the tree, I was trying to bend and stretch to get a good look at the pest. It was cottony cushion scale (Icerya purchasi). In years past, I have seen it on the lemon part of tree and on the Japanese aralia (Fatsia japonica), but I usually found only one mature adult that I removed by hand.
This branch had the heaviest infestation I have found in our garden, along the veins of several leaves and even a few small branches. My first reaction was to just scrape off all the insects. But I resisted the urge and decided instead to research the pests to see if that was an effective solution. I picked my grapefruit, extricated myself from the tree and went to learn how to control this pest.
The first step in managing any pest problem is to identify the pest. A mature female cottony cushion scale is very distinctive in appearance. It has a hard, ridged, yellow-tan shell that is approximately one-eight inch in length and covers an elongated cottony white egg sac. The egg sac can grow to one-half inch in length and contain up to 800 eggs. Long white hairs protrude near the edges of the shell.
The adult male cottony cushion scale is rarely seen. It is very small, red in color, and has wings. Eggs hatch quickly in warm weather, but may also hatch in winter. Newly hatched scale nymphs are called crawlers because they have legs.
Unlike most scale, this species retains its legs throughout its life cycle. Cottony cushion scale nymphs begin life as red with black legs and molt several times developing a cottony appearance before producing the hard scale shell. Adult females are typically found on twigs, branches, and trunks of trees while the developing cottony crawlers are found along the veins of leaves.
Cottony cushion scale feed by sucking vital fluids from the host plant and can cause the leaves to drop. It can infest many species of woody ornamental plants such as pittosporum, nandina, and citrus. Severe infestations can cause twig and branch dieback, weaken the plant, and reduce fruit production. They excrete honeydew which supports the growth of sooty mold and attracts ants.
The scale pests are normally controlled by their natural enemies, the vedalia beetle (Rodolia cardinalis) and parasitic fly (Cryptochaetum iceryae). The natural enemies can be disrupted by ants, insecticides, and excessive dust in the environment. Keeping the dust down, controlling ants with baits and barriers, and not using insecticides will allow the natural predators to control the scale.
The vedalia beetle was introduced to the United States from Australia in the 1890s and saved California’s citrus industry from destruction from scale infestation. The mature vedalia beetle appears similar to the lady beetle we see in the garden (red with black spots), but smaller in size. The female beetle lays reddish, oval shaped eggs on the outside of the cottony cushion scale egg sac. When the vedalia beetle eggs hatch, the larva burrow into the egg sac and feed on the scale eggs. Adult vedalia beetles also feed on all stages of cottony cushion scale.
The parasitic fly inserts its eggs into the cottony cushion scale, where the scale insect is destroyed by the developing fly larvae. The adult flies emerge from the scale by creating a hole in the shell.
Armed with this knowledge, I returned to the grapefruit tree to look for natural enemies. The infestation initially looked bad and I was not optimistic. But, we do not use pesticides, have little dust, and there are no ants in the tree.
I brought a hand lens and even though it was the middle of the day, the area between the fence and the tree was shady so I brought a flashlight. I sat down under the infested branch, turned on the flashlight, and examined some adults . . . vedalia beetle eggs studded the scale egg sacs! I could not believe my eyes! I did not find any mature adult vedalia beetles, but I found empty pupal skins which is evidence of the larva maturing to adults. Relieved to know that nature was going to take its course, I left the beetles to do their work.
Pesticides are not recommended for the control of cottony cushion scale and I can see why. It has been several weeks since my first observations and the amount of cottony cushion scale is declining. I am sure in a few more weeks the scale will be gone and the vedalia beetles will move on to find more food in the neighborhood.
For more information on managing ants and numerous other pests, visit the University of California Integrated Pest Management Web site: http://www.ipm.ucdavis.edu or contact the Solano County Master Gardeners Office located at 501 Texas St. in Fairfield.
- Sharon Leos