What are those Giant Bugs?

California prionus.
Photo by Wendy Hanson Mazet, UNCE

It is soon after sunset. You are enjoying the lovely evening, when whoosh, something flies by your head making you exclaim, “What the heck was that?” You have just experienced a fly-by of the California prionus, a large (up to 2¼ inches in length) reddish brown beetle with long antennae as big as or bigger than many hummingbirds.

California prionus has a host range that includes most deciduous trees and shrubs and some conifers. It can also attack grapes, hops, berries and fruit trees. The cream- to brown-colored larvae can be 3 inches in length with large chewing mandibles (mouthparts). It is at the larval stage that the prionus is destructive, since adults do not feed.

Adults emerge from the soil from June to early August. They fly at night searching for mates. The females are larger than the males with ovipositors (egg-laying appendages) on their hind end. After mating, the female lays eggs ½ to 1½ inches below the soil surface near trunks of trees. She can lay 150 to 200 eggs in her 10- to 20-day lifespan. After the eggs hatch, the larvae chew deep tunnels and furrows into the roots and region of the trunk near the soil surface (crown) of the host tree. This feeding and tunneling can kill the roots and hurt the crown, reducing water and nutrient uptake, reducing growth and possibly killing parts of or the entire tree. In addition, disease organisms can infest the damaged roots furthering weakening and harming the tree.

Symptoms to watch for include dying branches or sudden loss of vigor.

California prionus.
Photo by Wendy Hanson Mazet, UNCE.

Although the prionus isn’t a significant pest in our area, it doesn’t hurt to manage trees and other plants as if it were. Avoidance and prevention are the best strategies. Basically, avoid stressing trees and other susceptible plants. Provide enough water on a regular basis to keep trees healthy. This includes watering in the fall, winter and early spring, not just during the growing season. Prune carefully, making proper pruning cuts. Avoid pruning conifers until late fall or winter. Fertilize appropriately at the right time of year. Avoid damaging trees with lawnmowers and string cutters.

If you have questions about proper pruning, fertilizing and care, contact your local University of Nevada Cooperative Extension office, 887-2252 in Carson City and 782-9969 in Douglas County. Email me at skellyj@unce.unr.edu.

Remember our next free Grow Your Own class, “Insect and Critter Control” on Monday, July 30 from 2 to 4 p.m. at 2621 Northgate, Suite 15 or 1329 Waterloo Lane, Gardnerville.