Controlling Black Scale in California Groves

Black scale (Saissetia oleae) is one of the more dam­ag­ing pests in California, prey­ing on the leaves of the olive tree, which reduces its vigor and pro­duc­tiv­ity while leav­ing it prone to other infec­tions.

While the impacts of cli­mate change on the Golden State may favor the inva­sive insect’s poten­tial to spread, experts advise that farm­ers can get ahead of the prob­lem by mon­i­tor­ing for the pest.

Black scale is native to Africa and is found in most Mediterranean and semi­trop­i­cal regions. In addi­tion to olives, host plants include almond, apple, apri­cot, cit­rus, coy­ote brush, fig, Fuschia, grape, grape­fruit, ole­an­der, Peppertree, plum, prune and rose.

The insect is known to be sen­si­tive to hot tem­per­a­tures and dry con­di­tions and can, there­fore, be man­aged with agro­nomic prac­tices to increase canopy air­flow.

“Black scale is loca­tion and time spe­cific,” Kent M. Daane, an envi­ron­men­tal sci­ence, pol­icy and man­age­ment spe­cial­ist at the University of California – Berkeley, told Olive Oil Times.

“If you were in Bakersfield, there is enough sum­mer heat that smaller stages of the pest die off because they can’t deal with the hot tem­per­a­tures,” he added. ​“However, in places like Madero, Modesto and Corning, black scale sur­vives and thrives in milder weather.”

The black scale does not directly harm the olives or reduce the qual­ity of the oil, but large infes­ta­tions can lower olive yields. As a result, grow­ers are advised to mon­i­tor for black scale closely.

Infestations of the insect com­monly occur when the canopy becomes too dense, and a mild sum­mer allows the pest to repro­duce faster than its nat­ural ene­mies.

“With new grow­ing meth­ods, we’re see­ing more olive groves with trees planted closer together to facil­i­tate mechan­i­cal pick­ing,” Daane said. ​“More stud­ies are needed to deter­mine how this grove plant­ing design may affect the envi­ron­ment for black scale.”

While black scale was the key pest of olives in California from 1970 to 2000, the focus shifted to the olive fruit fly in the late 1990s. The per­ni­cious olive pest caused many com­mer­cial olive farm­ers to resort to annual pes­ti­cide treat­ment.

In California, olive fruit fly gets the most press. The olive fruit fly is inva­sive and has the poten­tial to destroy crops. The fly lays its eggs under the fruit’s skin; when they hatch, the lar­vae feed on the flesh of the olive drupe, result­ing in poor-qual­ity oil.

“Black scale does not affect olive taste or oil taste and qual­ity,” Daane said. ​“It stresses the tree and reduces the yield. In heavy infes­ta­tions, the olive might be smaller, and the tree has less vigor.”

Both black scale and the olive fruit fly tend to affect areas of California with cooler cli­mates and milder weather.

According to a lit­er­a­ture review of black scale pro­grams and car­baryl alter­na­tives, the best meth­ods to con­trol, pre­vent and respond to black scale require direct mon­i­tor­ing, with grow­ers look­ing for the tell­tale signs of black scale: a hon­ey­dew liq­uid.

Black scale feeds by insert­ing its mouth­parts into a leaf or twig. As the insect feeds, it excretes car­bo­hy­drate-rich hon­ey­dew that accu­mu­lates on leaves. The hon­ey­dew pro­vides a sub­strate for sooty mold, which can form a dense black cov­er­ing, shad­ing leaves and reduc­ing pho­to­syn­the­sis and res­pi­ra­tion.

This com­bi­na­tion of insect feed­ing and build-up of the sticky fluid can reduce bud for­ma­tion, cause leaf drop, and ulti­mately reduce the crop in the fol­low­ing year.

The pres­ence of hon­ey­dew droplets on olive leaves may first appear in early spring, such as in March and April. A rapid increase in scale size can be an early sig­nal of increased scale den­sity in an orchard.

According to the University of California Division of Agriculture and Natural Resources, mon­i­tor­ing and con­trol should begin with check­ing about 40 trees per block for hon­ey­dew. With new infes­ta­tions, the scale may only be found in a few trees. Since the scale pop­u­la­tion tends to build in the inner canopy, farm­ers should check this part of the tree.

If hon­ey­dew is found, treat­ment pro­gres­sively increases with the degree of infes­ta­tion.

For light infes­ta­tion, no treat­ment is needed. Closed canopies can be pruned to increase air cir­cu­la­tion and raise tem­per­a­tures.

For mod­er­ate infes­ta­tion, ini­tial dam­age may be lim­ited, but off­spring of adult scales can sub­stan­tially dam­age the crop. To reduce black scale den­si­ties, farm­ers may con­sider apply­ing a dor­mant oil, prun­ing orchards with closed canopies, or apply­ing an insec­ti­cide.

Heavy and severe infes­ta­tions are known to cause eco­nomic dam­age. If the orchard has not been pruned for years, an in-sea­son insec­ti­cide can be used to pre­vent fur­ther dam­age at har­vest.

After har­vest, open the canopy to cre­ate an envi­ron­ment less hos­pitable to scale. For severe infes­ta­tion, eco­nomic loss can be exten­sive.

Several com­mon myths per­sist about the black scale. ​“Three big myths about black scale are that it shows up all at once, that mon­i­tor­ing is not needed and that one-time appli­ca­tions oils and soaps will resolve the prob­lem,” Daane said.

“Black scale does not show up overnight, but devel­ops over time, and can take three to four years to build up a high enough pop­u­la­tion to cause severe dam­age,” he added. ​”You could have sev­eral years of mild weather and think that you escaped, only to have the pop­u­la­tion build up into a heavy infes­ta­tion.”

Monitoring is advised as the best way to catch the prob­lem early. ​“Involve your har­vest crew and prun­ing crew, and walk through the groves,” Daane said. ​“Catch prob­lems early and prune out infested branches.”

If han­dling black scale organ­i­cally, Daane noted that farm­ers often need to apply soaps and solu­tions mul­ti­ple times through­out the grow­ing sea­son to be fully effec­tive.

He said it would be ben­e­fi­cial for fur­ther research in iden­ti­fy­ing car­baryl alter­na­tives, test­ing and devel­op­ing other mate­ri­als that may be effec­tive against black scale.

Many nat­ural ene­mies may pro­vide bio­log­i­cal con­trol, and fur­ther research must explore con­trols that might pro­mote or at least not lower nat­ural enemy activ­ity.

In look­ing towards the future for olive grow­ers and black scale, Daane reflected on National Oceanic and Atmospheric Administration research and the emer­gence of global cli­mate fore­cast­ing mod­els.

“Many mod­els for California are indi­cat­ing a milder win­ter with less snow­fall and more late rain­falls in January, February and March,” he said, ​“If these con­di­tions occur, we could be look­ing at an increase of black scale due to the lack of cold win­ter tem­per­a­tures which knock the black scale pop­u­la­tion back.”

“The weather kills about 80 per­cent of black scale,” he added. ​“The best cul­tural con­trol avail­able involves reg­u­lar prun­ing to open the canopy and expose the scales to higher tem­per­a­tures and drier con­di­tions.”

“To update this prac­tice, fur­ther research needs to include under­stand­ing how the rel­a­tively new hedge style of pro­duc­tion influ­ences black scale pop­u­la­tion devel­op­ment,” Daane con­cluded.

• Olive Oil Commission of California
• University of California Integrated Pest Management