JEFFERSON CITY, Mo. – This year’s summer green is being interrupted by brown patches in Missouri’s trees, and it’s not just because of the heat.
Brown leaves can appear on trees in many patterns for many reasons. Common this summer are small clusters of wilted, brown leaves scattered throughout tree crowns and around yards. These dead tree-branch tips are cases of “branch flagging,” which can be caused by insects, disease or weather-related injury.
This year, the prevalent branch flagging in many parts of the state is largely a result of two insects: the 13-year periodical cicadas that emerged in May and much less obvious insects called Kermes scales, which appear more like galls or buds than insects.
“In years when periodical cicadas emerge, branch flagging can be widespread and dramatic,” said Missouri Department of Conservation (MDC) Forest Entomologist Rob Lawrence. “Branch flagging in these years results from the cicadas’ breeding and egg-laying behavior.”
Once periodical cicadas emerge from the ground, shed their exoskeletons and dry their wings, they mate and lay eggs in the twigs of trees. Female cicadas use a saw-like appendage on the abdomen to slice into the underside of twigs and deposit their eggs. The weakened twigs are often broken by wind, causing bunches of leaves to dangle from branches or fall to the ground.
Cicada-damaged twigs can be identified by a line of small slits on the underside. MDC recommends only very light pruning in the summer if necessary, with heavier corrective pruning as needed when trees are dormant in the winter.
Branch flagging can occur due to causes other than cicadas. A main source of tree damage in southwestern Missouri has been Kermes scales, sap-feeding insects that damage oak trees. These tan to reddish-brown insects, less than 1/4 inch wide, feed by inserting tube-like mouthparts into plant tissue and sucking out fluids. Kermes scales can be treated in late fall with systemic insecticides labeled for use on soft scales on oaks, or they can be left for their natural enemies to control.
Other possible causes of branch flagging include insects such as twig girdlers and twig pruners, or Botryosphaeria cankers caused by a fungus.
Another more serious concern is thousand cankers disease of black walnut, which has not yet been detected in Missouri but can cause wilted, brown leaves to remain on branches and eventually kill walnut trees.
“For all causes of branch flagging, reducing further stress on the tree provides the best chance for it to recover,” Lawrence said. “This might include supplemental watering during drought, avoiding injury to trunk and roots with mowing equipment and avoiding excess pruning and other damage.”
For information on how to identify and address tree-health issues associated with branch flagging, visit the MDC website at www.mdc.mo.gov and search “Tree-Branch Flagging.”