Citricola Scale

Citricola scale is a soft scale. Crawlers of the citricola scale appear from June through August. They settle primarily on the underside of leaves, but in severe infestations they also settle on the upper leaf surface and on twigs, rarely on fruit. Young scales are flat and almost translucent; they grow slowly over the course of the summer and fall, molting only once during that period. By November, second-instar scales turn a mottled dark brown color and begin migrating to twigs; this migration peaks in February and March. Once on twigs, they develop faster than they did on leaves and they turn a gray color. By late April, citricola scales molt and mature into the adult female stage. Females lay 1,000 to 1,500 eggs during the time from early May to early August. Eggs hatch after 2 to 3 days and crawlers move to leaves. There is only one generation a year, and there are no males.

Brown soft scale, another soft scale that is similar to citricola scale, may be found in the same areas as citricola scale but it has multiple generations and its colonies are composed of mixed instars and adults.

Citricola scale can be a serious pest of citrus in the San Joaquin Valley. A severe infestation may reduce tree vigor, kill twigs, and reduce flowering and fruit set. As they feed, citricola scales excrete honeydew, which accumulates on leaves and fruit. Sooty mold grows on honeydew and interferes with photosynthesis in leaves and causes fruit to be downgraded in quality during packing.

Citricola scale is completely controlled by Metaphycus and Coccophagus parasites in Southern California and is almost never seen. Even though these parasites are established in the San Joaquin Valley, biological control is not effective there, and insecticide applications may be necessary in groves where broad-spectrum pesticides are not regularly used to control other pests. In groves practicing biologically based pest management (e.g., releasing Aphytis melinus for California red scale control), growers may consider withholding broad-spectrum citricola sprays until Aphytis activity is over in the fall (e.g., late October or November).

Introduced and indigenous parasitic wasps, Metaphycus luteolus, M. stanleyi, M. nietneri, M. helvolus, and Coccophagus spp., control citricola scale in Southern California. Several of these parasites occur in the San Joaquin Valley but are unable to control citricola scale except in groves near urban areas or in those with high numbers of brown soft scale, which serves as an alternate host for the parasites when citricola scales are not in the stage that the parasite attacks.

Use biological control and organically approved petroleum oil sprays, such as 440 oil PureSpray Green in organically managed orchards. Oils alone only suppress citricola scale so multiple applications may be required.

Oil is the most selective pesticide available for control of citricola scale. However, oil simply reduces the scales' overall numbers and in many cases must be applied 1 or 2 times every year. The organophosphate malathion and the carbamate carbaryl (Sevin) are broad-spectrum and toxic to most natural enemies. The neonicotinoids thiamethoxam (Actara) and acetamiprid (Assail) are fairly broad-spectrum (i.e., toxic to most natural enemies). The insect growth regulator buprofezin (Centaur) is a fairly selective pesticide, but it does affect vedalia beetles.

IIn the San Joaquin Valley, 40% of populations of citricola scale have been found to be resistant to organophosphate insecticides. There is likely cross-resistance to carbamates. Thus, low rates of these insecticides would be ineffective, and high rates only suppress citricola scale for a single year. Growers experiencing resistant scale should use other insecticides (buprofezin, thiamethoxam, or acetamiprid).

Citricola scale is only a problem in the San Joaquin Valley. It is very sensitive to organophosphates and carbamates (if not resistant) and generally does not become a problem until growers stop using these insecticides for control of other pests. Check for citricola scale at all times of the year when monitoring for other scales, but look especially closely at the twigs in April and at the underside of leaves in late July.

Be sure to distinguish the citricola scale from brown soft scale. Brown soft scale has multiple generations and all stages will be present on leaves and twigs year round, whereas citricola scale has only one generation and is found on leaves only in the summer and fall, and the nymphs will be uniform in size.

April-May Sampling

Examine one 24-inch twig on the northeast side of 10 trees in each of four rows (for a total of 40 twigs) distributed throughout the orchard. On each twig count the number of scales and determine the average number of scales by dividing the total number in the sample by 40. If there is more than an average of one scale per twig and heavy production of sooty mold is occurring, the orchard may require an immediate pesticide application. If the population on twigs or leaves is observable but sooty mold is not a problem, then it is best to postpone pesticide application until fall when scales are small, positioned on leaves on the outside of the tree, and generally easier to control.

July-September Sampling

To sample for citricola scale in summer:

1. Walk down four evenly spaced rows of the block.
2. In each row, pick one leaf from the northeast side of 25 trees.
3. Examine the scale on the underside of the leaf to determine if they are alive or dead.
4. Count the number of leaves in the 25-leaf sample that are infested with live scale (presence-absence sampling). Record results (example form—PDF)

A spray is warranted if one or more of the four rows has five or more leaves infested with live citricola scale in a 25-leaf sample. If four or fewer leaves are infested, then a pesticide application could wait until the following season. Alternatively, count the number of nymphs on those 100 leaves (4 rows x 25 leaves) and if there are more than 0.5 nymphs per leaf then treatment is needed.

Acetamiprid (Assail), thiamethoxam (Actara) and flupyradifurone (Sivanto) are the most effective insecticides for citricola scale control.

Spring Treatments

Assail and Actara are the only insecticides effective against adult scales in spring before petal fall. However, they are fairly broad-spectrum and at this time of year only suppresses the females. Thus, they should not be used unless there is a severe problem that cannot wait until a summer treatment.

Summer through Fall Treatments

Most foliar insecticides are applied during late July through September because at that time of year the female scales have died, the nymphs are small and located on the outside leaves of the tree, and temperatures are warm, which makes the insecticides more effective. Monitor eggs under the adult females during June and July and apply the insecticide after hatch is completed.

About 40% of populations of citricola scale in the San Joaquin Valley have been determined to have resistance to organophosphate insecticides. Avoid malathion, dimethoate, or cabaryl insecticides if resistance is known to be a problem. The insect growth regulator buprofezin (Centaur) is fairly selective (toxic only to vedalia beetles) and will suppress citricola scale during the season that it is applied. The foliar neonicotinoids thiamethoxam (Acatara) and acetamiprid (Assail) and flupyradifurone (Sivanto) are fairly broad-spectrum and will control citricola scale for 1 to 2 years. Foliar neonicotinoids are more effective against citricola scale than systemic neonicotinoids. Citricola scale populations grow more rapidly and survive the summer better when the San Joaquin Valley experiences a cool, wet spring. When these conditions occur, the higher rates, careful spray coverage of the tree, and properly timed applications (late July to early August, when eggs have completely hatched and the nymphs are small) are most effective.

• Citricola scale monitoring form (PDF)
• Photos of citricola and brown soft scale