US mandarin boom forces citrus pest management to evolve

Since the 1990s, mandarin oranges have become the 'Rising Stars of the Fruit Bowl'. In the US, 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 the establishment of integrated pest management (IPM) guidelines specifically for mandarins. Instead, growers use tried-and-true approaches for larger navel oranges.

Unfortunately, IPM that works for oranges 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”.

“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.”

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.

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.

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.”


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