Granulated Ambrosia Beetles

— Written By and last updated by

Our plants are susceptible to damage from many sources including insects. The granulated ambrosia beetle (Xylosandrus crassiusculus) was introduced into South Carolina from Asia in the early 1970’s. It is currently active in Lenoir County.

It is a potentially serious pest of woody ornamentals, nut and fruit trees and is reported to damage over 100 species of trees. Some shrubs including azaleas have also been damaged. Granulated ambrosia beetles emerge in early spring and attack thin barked, deciduous trees. The activity seems to begin in March and peaks in April but activity does continue through the summer and into the fall.

Adults and larvae bore into twigs, branches or small trunks of woody host plants, excavate a system of tunnels in the wood or pith. The female beetles inoculate the trees with ambrosia fungus on which they feed. The boring and introduced fungus can damage and block the xylem vessels of the plant interfering with vascular transport. Boring can also serve as an entry point for pathogenic fungi such as Fusarium. Visible symptoms include wilted foliage and strands of boring dust protruding from small holes. Ultimately part of the plant or the whole plant can be killed usually during the leafing-out stage.

Infestations normally can be identified by toothpick-like spines of boring dust protruding from holes made in the host plant by females excavating their galleries. The strings or spines of boring dust may be up to 2 to 3 inches long but are fragile and easily broken off by wind or rain. Unlike other scolytids, which normally attack only stressed or damaged plants Asian ambrosia beetles attack apparently healthy plants. Individual plants may host up to 50 beetles. It is almost impossible to save heavily infested plants.

Granulate ambrosia beetles are active year-round during warm periods when they mate, lay eggs and rear young within the galleries excavated by the females. They breed in host material from 1 to 12 inches in diameter, although smaller branches are most commonly attacked first. All life stages can be found inside the galleries. When mature, females leave infested plants and fly to new host plants. Males are rare, small and do not fly. There are two generations per year in North Carolina.

Preventative applications of pyrethroid insecticides can protect trees by preventing the beetles from excavating galleries. Once beetles are inside trees they cannot be killed with insecticides and fungicides are ineffective against the ambrosia fungus. Therefore, the timing of preventative insecticide applications is crucial to protect trees from damage by the pest. Use monitoring traps to time applications for the best results. Heavily infected plants or plant parts should be removed and destroyed.

Keep trees healthy and avoid any unnecessary tree stress. Find recommendations for monitoring traps and products available on the Internet at www.ces.ncsu.edu/depts/ent/notes/O&T/trees/note111/note111.html