By Keith Mickler, county coordinator and agriculture agent
Granulated Ambrosia Beetles are an early season threat to our trees and shrubs. Granulate ambrosia beetles are tiny (<1/8″) wood-boring insects that attack the trunks of young and weakened trees and shrubs. Ambrosia beetles tunnel into stems and construct galleries where they raise their young. Beetles carry on their bodies a fungus that grows in these galleries producing ambrosia which feeds both adults and larvae. Ambrosia beetles can also carry the spores of disease pathogens that infect the tree. The growth of these fungi leads to weakening or death of the tree.
As the beetles tunnel, they push sawdust out through their entry hole. This sawdust can cling together forming short ‘toothpicks’ sticking from the infested stem. These toothpicks make it easy to identify ambrosia beetle attacks. Wind or rain may destroy these toothpicks leaving just the small holes and scattered sawdust from the beetle. Since the entry holes are only about the size of a #2 pencil lead, close inspection is necessary to detect these attacks in time to treat. Ambrosia beetles attack many types of trees and shrubs including crape myrtles, cherries, oaks, sweet gums, pecans, peaches and others.
The ambrosia beetle’s first flight occurs with mild weather typically in late February to early March. Cold weather will put them off for a while, but we usually see a big emergence of the granulate ambrosia beetle in early March for our area in north Georgia.
A simple method to determine adult activity in the area is using alcohol and a bolt of wood about 2 to 4 inches in diameter and 2-feet long. Any hardwood species such as maple will work for building a trap. A half-inch diameter hole is drilled into the center of the wood bolt, about a foot deep, and filled with alcohol and then close the opening using a stopper cork. Ethyl alcohol or grain alcohol with 95-percent alcohol content (190-proof) can be found at most liquor stores. Hang several bolt traps along the border of your property at waist height to determine beetle emergence and activity. Sawdust toothpicks (Fig. 1 above) begin to appear on the bolt when they are infested with adult beetles. Once toothpicks are detected on the bolt trap, spray immediately using a pyrethroid insecticide on the trees.
Young trees in the landscapes (less than three years old) are vulnerable to attack even if they are not obviously stressed. This is especially true during the green-up period. Prompt action can save these trees if the number of attacks (“toothpicks”) is less than 4 – 5 per tree.
Trunk sprays containing a pyrethroid insecticide applied now will provide some protection. If applied in time, insecticides such as bifenthrin, cyfluthrin, or permethrin can repel the beetles even after plants are attacked. You should apply the insecticide now and repeat it every 10 – 14 days until the plants are completely leafed out.
Spray affected trees making sure to cover the trunk completely. Monitor the trees for signs of wilting of the new leaves, a sign that a pathogen has been introduced. Once the wilting starts, the tree will probably die. However, do not assume the attack will be fatal, trees may recover. Also watch treated trees to see if new toothpicks develop – a sign that the ambrosia beetles are still active.
If you see toothpicks on larger trees, the tree is probably severely stressed. If attacks are confined to one limb, pruning is an option. If the attacks are on the main stem, prepare to remove the tree if it dies.
Source: Will Hudson, and Shimat V. Joseph UGA Extension Entomologist
Keith Mickler is the County Coordinator and agriculture agent for The University of Georgia/Floyd County Cooperative Extension. Located at 12 E. Fourth Ave., Rome, GA 30161 (706) 295-6210.