Agriculture and Natural Resources Program

Each year, shipping containers full of Christmas trees arrive from Oregon and Washington State. And each year, Guam Customs and Quarantine find hitch-hikers in the containers: insects and other alien animals which potentially put Guam's agriculture and ecosystems at risk (Google Search). Of major concern is potential for the accidental introduction of yellow-jackets and other wasps.

The Guam Department of Agriculture requires that trees must be "mechanically shaken for a period of time sufficient to dislodge any hitch-hiking insects and other contaminants" prior to shipment. And all shipments must be a phytosanitary certificate from a state inspector certifying that the shipment is free from pests (See the 2011 Christmas Tree Shaking and Treatment Compliance Agreement from Oregon State.)

In 2009, several Hawaiian entomologists (Hollingsworth et al. 2009) published a research project in which they examined the effectiveness of shaking Christmas trees. They found that shaking alone reduced the risk of importing hitch-hiking yellow-jacket queens by only about 50%. However, their project showed that if the trees were sprayed with pyrethroid insecticides several weeks prior to harvest, risk of shipping live hitch-hikers dropped to almost zero with no effect on the quality of the Christmas trees. Pyrethroids are the class of insecticides commonly used in indoor household insect sprays. It is interesting to note that Mexico took note of this research. Trees shipped to Mexico from Oregon or Washington State must be treated with a pyrethroid three to six weeks prior to harvest. Might be wise for Guam to adopt the same requirement. Otherwise, it is probably only a matter of time before the island gets invaded by yellow-jackets or other pests hitch-hiking on Christmas trees.