With the emergence and spread of the polyphagous shot hole borer through Los Angeles, Orange and San Diego counties, researchers at the University of California Riverside examined means of reducing the spread of the beetle when transporting wood from infected trees.
While the beetles prefer living trees in which to bore and build their galleries, females will lay viable eggs in cut logs thus permitting the emergence and spread of the beetles from infested wood. In fact, felled infected wood can remain a source of beetles for several months. Thus, Drs. Michele Eatough Jones and Timothy D. Paine examined the effectiveness of chipping and solarization methods in reducing the survival rate of shot hole borers in cut wood.
The researchers conducted two chipping trials — one in fall and the other in spring — using a commercial chipper. Chips were sorted into three categories — coarse (>5 cm), medium (2.5 - 5 cm) and fine (<2.5 cm) and then placed in covered buckets with emergence chambers where the beetles’ boring activity and emergence were monitored for about 4 months.
The researchers also conducted four solarization trials in spring, summer, fall/winter and summer. Each trial tested both black and clear 6 mil polyethylene plastic sheeting. To solarize the logs, a single layer of cut logs was placed on plastic sheeting and then the sheeting was folded over the logs with the edges rolled and secured by binder clips. The trials ran for 10 – 14 weeks.
The results indicated that both chipping and solarization significantly reduced — but did not eliminate — the beetles. Although the beetles did survive the chipping process, most likely due to their small size, chipping was determined to be a “very effective method of sanitation” by the researchers if covered and left on site (the researchers did not test whether leaving the chips uncovered would have the same results). Chipping was most effective for chips < 5 cm.
As for solarization, this process was most effective in the hot summer months, especially when temperatures were greater than 35˚C (August). In summer, beetle activity under clear sheeting was eliminated after week 6. In spring, the elimination of beetle activity did not occur until late June when daily max temperatures averaged 34˚C. For all seasons — except fall — using clear plastic was sufficient to stop all beetle activity. In contrast, zero beetle activity only occurred during one of the summer trials when the researchers used black sheeting.
In order to kill the beetles, the researchers recommend solarizing for 4 – 6 weeks when temperatures under the clear plastic sheeting will exceed 55˚C. The most effective solarization method was determined to be dry logs under clear plastic in summer (preferably August). If growers are removing infested wood in non-summer months, it is recommended that the wood be chipped; if it cannot be chipped, the logs should be covered in clear plastic and left on site for at least 6 months.
The complete research article, “Effect of Chipping and Solarization on Emergence and Boring Activity of a Recently Introduced Ambrosia Beetle (Euwallacea sp., Coleoptera: Curculionidae: Scolytinae) in Southern California is available online.