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Olive bark beetle, a threat to state’s trees?

September 16, 2017
Martha Proctor

Olive growers beware! An insect that is a well-known pest of the olive tree (Olea europea) was obtained from an olive tree at a grape vineyard in Riverside County this past fall. It’s the first record of the presence of Phloeotribus scarabaeoides, the olive bark beetle (OBB), in the Western Hemisphere. This beetle was also found at a residence and three nurseries, all in Riverside County.

So far, surveys of olive trees at other nurseries in other counties have not detected is presence. However, the species is widely distributed around the Mediterranean basin (including Europe, North Africa and parts of Asia). Because we also live in a Mediterranean climate region, authorities agree that OBB is highly likely to spread throughout California.

It is possible that OBB could be more widespread in California than now is thought. A native bark beetle, (Hylesinus californicus) that sometimes attacks stressed olive trees was initially called the olive bark beetle of California. However, its common name was changed to Western ash bark beetle to reflect its typical host, the ash tree (Fraxinus), another member of the olive family, Oleacae. Consequently, it is possible that OBB is more widespread in California and with the confusion in naming, damage caused by OBB may have been attributed instead to the Hylesinus californicus bark beetle.

This tiny pest attacks both weakened and healthy hosts. Two types of damage occur. In the first type, damage is caused by larval feeding and by adults, which attack the already weakened stems and branches of debilitated olive trees, increasing damage and causing further decline and sometimes the death of trees. In the second type of damage, losses are due to adults producing incisions on small, healthy branches when feeding causing the tips to dessicate or are due to adults damaging axillary buds when trying to initiate small galleries or holes in which to hibernate. Although total yield loss is variable, the effects of damage on an individual tree is cumulative and could potentially leave a tree unproductive within five years. Studies reveal that fruit production of infected trees is reduced by 60 percent; OBB-infested trees can be killed in five years.

Adult beetles are attracted to chemicals emitted by the plant after injury takes place, including after pruning. Adult females bore through bark and excavate a transverse tunnel on either side of the entry point. Inside the twig/branch the female lays up to 60 eggs and, as larvae hatch, each larvae bores up or down from the entrance tunnel underneath the bark. This feeding causes partial to complete girdling of the twig/branch; thereby structurally weakening it as well as damaging the vasculature i.e., the system which moves water and minerals throughout the tree.

Larvae pupate inside the feeding galleries. OBB has two to four generations a year. During the spring and summer, adults tend to lay eggs in prunings and olive wood stacked as firewood rather than in living trees. In addition to olive trees, OBB feeds on oleander (Nerium oleander) and occasionally on ash (Fraxinus excelsior) and lilac (Syringa vulgaris). The fact that adult beetles can fly and all life stages can be transported long distances when olive wood or infested plants are moved opens the possibility of spreading the pest’s impact beyond Southern California.

The OBB is considered a serious pest of the olive that can cause heavy losses of young shoots, flowers and fruit. The beetle is expected to increase crop production costs for olive growers as they implement management strategies. In regions with established OBB populations, growers are forced to alter cultural practices by moving olive prunings and wood far away from groves to reduce damage.

There are no known effective or approved treatments available yet; a parasitic wasp is being tested in the Mediterranean.

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