Brood XXIII of the periodical cicada is due to emerge along the Mississippi River Valley (Figure 1). In Kentucky, the bulk of this 13-year brood should occur west of I-65; the insects should be most abundant in counties along the river.
Prior to emergence, cicada nymphs living in very wet soil may build mud chimneys similar to those made by crawdads (Figure 2). The chimneys allow the nymphs to move to drier air in order to molt, while also remaining protected from predators and desiccation. Nymphs can begin to leave the ground in late March to find vertical surfaces to cling to while molting to the adult stage. Soon after emergence, the cicadas will accumulate in the tree tops and begin their rhythmic chorusing that may continue into early June.
Egg laying is the real danger from periodical cicadas, so their emergence is the signal to begin watching to determine if plant protection will be needed. A week or so after emergence (Figure 3), females are ready to lay eggs.
Periodical cicadas feed little, but in large numbers, they can be very destructive. Physical injury occurs when females slit the bark on pencil-sized twigs and lay their eggs inside the wounds (Figure 4). They prefer grapevines, oak, hickory, apple, peach, and pear trees about the diameter of a little finger; however, they are not limited to those species. The damage causes “flagging” due to broken twigs that die and turn brown.
Young trees may be severely misshaped from leader or branch breaks. More mature trees and shrubs usually survive even dense emergences of cicadas and continue to grow during subsequent years. This can be difficult to believe in the month or so following a large emergence because branch terminals of many deciduous trees will turn brown due to injury.
Cicada nymphs spend years feeding on sap taken from the roots of trees and shrubs. Over time, large numbers of them can take a toll. During the first few years, there usually are no apparent signs. However, the nymphs grow and gradually take increasing volumes of sap as they feed. During the last few year of their development, this can affect tree and shrub growth and vigor, and production of fruit trees. The impacts are subtle and probably would be noticeable if there were nearby uninfested trees.
Plants can be protected from cicada damage in three ways: (1) covering, (2) spraying, or (3) pruning. There is no effective means of controlling nymphs in the soil.
Small trees can be covered with protective netting or cloth. Be sure to secure the bottom around the trunk. This covering will have to stay on for the next 4 to 6 weeks, or until egg laying is complete.
Spraying with Insecticides
Insecticides may not provide much protection against cicada damage, especially for trees near heavily wooded areas that provide a continual supply of replacement cicadas. Sprays that hit the insects directly are likely to be more effective than residues on treated surfaces. Several synthetic pyrethroid insecticides are labeled for landscape trees and shrubs. Often, these insecticides may have a repellent effect that causes insects to leave treated surfaces shortly after landing.
Nurseries under a routine spray schedule should be sprayed according to intensity of the outbreak, which can range from a few cicadas in some areas to massive numbers in other areas. During low to moderate outbreaks, twice weekly applications may be enough. During massive outbreaks, damage will potentially occur even with daily applications. Continued cicada flight to landscapes and nurseries from surrounding woods keeps re-infestation pressure high.
The most serious consequence for nursery plantings will be the injury to branches that provide the basic plant shape and structure. Increased pruning may be needed if cicada damage destroys the current year’s growth. One or 2 years of growth may have to be removed in order to have quality trees to harvest. In extreme cases, some trees near the end of their production cycle may be unmarketable the fall after emergence or during the next year.
By Lee Townsend, Extension Entomologist