Integrated pest management (IPM) practices that work for navel oranges may not be best for mandarins, suggests a new study in the Journal of Economic Entomology. Research by scientists in California’s mandarin-growing region indicates that IPM tactics for traditionally grown oranges may not be most effective for optimum mandarin production. The scientists noted, in fact, that “there is likely an overuse of pesticides in some mandarin species.” (Photo credit: Bodil N. Cass, Ph.D.)

By Ed Ricciuti

Since the 1990s, mandarin oranges have become what The New York Times headlined in 2016 as the “Rising Stars of the Fruit Bowl.” In the United States, mandarins are almost all grown in California, supporting a $2 billion industry and accounting for almost half of the nation’s citrus production. The surge in growing this small fruit, native to Asia, has outpaced establishment of integrated pest management (IPM) guidelines specifically for mandarins. Instead, growers use tried-and-true approaches for larger navel oranges (Citrus sinensis).

Ed Ricciuti

IPM that works for oranges, though, may not be best for mandarins, suggests a new study published in late May in the Journal of Economic Entomology. Research by scientists in the San Joaquin Valley, heartland of California’s mandarin country, indicates that IPM tactics for traditionally grown oranges may not be most effective for optimum mandarin production. The scientists noted, in fact, that “there is likely an overuse of pesticides in some mandarin species.”

The study, conducted by researchers at the University of California’s Davis and Riverside campuses, starts filling in the blanks, providing a first look at arthropod pest densities and damage in commercially grown mandarins in California. It revealed that mandarins in groves studied suffered considerably less damage from pests than did comparative navel oranges, a finding that could substantially impact how growers plan pest control.

“It has taken decades to establish guidelines for C. sinensis oranges,” says Bodil N. Cass, Ph.D., postdoctoral scholar at UC Davis and lead author on the study. It is difficult for researchers to keep up with new types of fruit being grown to meet consumer demand, new pests on the scene, and changing environmental conditions, she says. “We’d been assuming or hoping that the IPM guidelines that were carefully established for oranges are also effective in the different mandarin species that are now commonly being grown. Here we’ve seen in some cases a very different picture emerging in mandarins.”

California citrus is marketed as fresh fruit, so it needs to appeal to the eye as well as the taste buds. The thin skin of mandarins makes them easier to peel than standard oranges, a selling point, but potentially more vulnerable to blemishes such as scarring from pests. Even a small amount of insect scarring renders mandarins unmarketable, except in some cases for processing as juice, hardly an economic option.

Using data mining techniques of information science—termed “ecoinfomatics” when applied to environmental subjects—the researchers compiled and analyzed mandarin production and pest management records, from 2003 to 2012, in 202 groves in Fresno and Tulare counties. Their aim was to determine the density of pests and their relation to fruit damage. The results surprised. “When we first ran the analysis of the data sourced from commercial operations, the result of much lower scarring in Citrus reticulata mandarins was so striking and unexpected that we wanted to see it for ourselves,” says Cass. So they went into groves and checked mature fruit on the tree for scarring.

By combining ecoinfomatics and old-fashioned field research, the scientists used tools both new and old. Traditionally, researchers assess pest damage by experiments in small field plots, providing a snapshot in time that by extrapolation they hope can apply to the much larger realm of commercial production. By adding information science to analyze years of records, they can construct a much more accurate picture.

“It has taken decades to establish guidelines for [navel] oranges,” says Bodil N. Cass, Ph.D., postdoctoral scholar at UC Davis and lead author on a study comparing pest management in navel and mandarin oranges. “We’d been assuming or hoping that the IPM guidelines that were carefully established for oranges are also effective in the different mandarin species that are now commonly being grown. Here we’ve seen in some cases a very different picture emerging in mandarins.” (Photo credit: Bodil N. Cass, Ph.D.)

The forktailed bush katydid (Scudderia furcate, nymph stage shown here) threatens both mandarin and navel oranges in California citrus groves. (Photo credit: Bodil N. Cass, Ph.D.)

Damage resulting from the forktailed bush katydid (Scudderia furcate) can render both mandarin and navel oranges unmarketable for California citrus growers. (Photo credit: Bodil N. Cass, Ph.D.)

Damage resulting from citrus thrips (Scirtothrips citri, adult female shown here) can render both mandarin and navel oranges unmarketable for California citrus growers. (Photo credit: Bodil N. Cass, Ph.D.)

Damage resulting from citrus thrips (Scirtothrips citri) can render both mandarin and navel oranges unmarketable for California citrus growers. (Photo credit: Bodil N. Cass, Ph.D.)

Infestation by California red scale (Aonidiella aurantii) threaten both mandarin and navel oranges in California citrus groves. (Photo credit: Bodil N. Cass, Ph.D.)

The study indicated that the density of at least some insect pests and damage differs between standard oranges and the four citrus species commonly marketed as mandarins, Citrus reticulata, C. clementina, C. unshiu, and C. tangelo. Perhaps not surprisingly, fruit damage varied between citrus species studied, with the commonly grown C. reticulata ” [appearing] to express full or partial resistance to the entire suite of direct pests that attack citrus.”

The researchers tested not only whether mean densities of fruit-damaging pests differed between those on standard oranges and the four major mandarins but also how pest density impacts damage to fruit. In the San Joaquin Valley region, several early-season pests feed on very young fruit, including the forktailed bush katydid (Scudderia furcate), citrus thrips (Scirtothrips citri), and some caterpillars, especially Egira curialis, sometimes known as citrus cutworm. In addition, California red scale (Aonidiella aurantii) infests all aerial parts of the tree. including fruit.

The research highlights how many issues must be considered when planning IPM. Forktailed bush katydid densities, for example, differed according to year and location, while they increased with acreage and decreased with tree age. Caterpillar densities and scarring were higher in larger groves.

C. reticulata groves had lower densities and less damage from California red scale and citrus thrips compared to navel oranges. The difference might be due to better control measures or perhaps even a natural resistance, according to the researchers. They urged more research into whether C. reticulata requires less frequent monitoring and control than IPM guidelines for navel oranges would suggest.

The apparent vulnerability of thin-skinned mandarins to scarring seems offset by a resistance to pests, the study suggests. Scarring of C. reticulata and, less so, of C. clementina was much lower than expected given the density of katydids in the groves studied. While mandarins are so valuable than any damage can be costly, the researchers wondered if the katydid “is causing any economic damage to these citrus species.”

The low degree of damage, the scientists said, could have several causes. Perhaps the katydids simply avoid the two mandarin species. Or else, the fruit may be capable of regenerating skin, hiding scarring. Another hypothesis suggested is that the mandarin trees drop scarred fruits, putting energy into these that remain whole. “This is where we’re focusing our future research efforts, to understand what is the mechanism underlying the low scarring,” says Cass.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.