Oaks ‘n’ Voles

Oaks ‘n’ Folks – Volume 10, Issue 1 – April 1995

Vole predation on oaks can be a major problem. A few years ago researchers wrote in these pages (Oaks ‘n’ Folks, Oct. 1988) an article titled “Valley Oak Regeneration: Rodents May Cause Problems”. The specific problem discussed was the complete wipe-out of an oak planting by voles. Although we seldom catch them in the act, there are ways to determine vole predation, and there are remedies. But just what is this strangely named creature the vole?

Voles are mouse-sized rodents with small ears and short tails commonly found in grassy settings, and often confused with the mouse clan. They are even usually called “meadow mice”, but they are actually more closely related to lemmings and muskrats. The voles most likely encountered on hardwood rangelands in California are Microtus californicus. They will clip off leaves and twigs of seedlings, clip the smallest seedlings at ground level, and girdle the stems of larger seedlings and even sapling oaks as large as 2.5 inches in diameter.

You can detect the presence of Microtus by the silver-dollar sized entries to their burrows and by their surface runways and tunnels in thick, grassy cover. Parting this vegetation, you can follow these foraging pathways, often paved with little green droppings and newly clipped foliage. Unlike mice, which prefer a diet of seeds, voles eat green vegetation too; including the bark of young oaks. Seedlings and saplings with the bark stripped up to five inches from the ground are a sure sign of voles. Smaller seedlings may be entirely gnawed off, as if by a miniature beaver, leaving a pointed stub. (Gophers may also feed this way, but can tackle larger stems and usually bite the stems off bluntly below ground, and leave large tooth marks.)

The real power of voles lies in their numbers, for they are among the most prolific of mammals. Peak populations may number up to 450 individuals per acre, and females are capable of bearing as many as six litters a year, ranging from 5 to 11 young. And they may begin breeding when only two weeks old! Fortunately, these population bursts are only reached at three to five year intervals. Predators help keep populations in check. Nature rewards reproductive excess with a population crash, but peak populations can cause serious problems to orchards and other planted trees.

Last year during late summer at the Sierra Foothill Research and Extension Center (SFREC), we were amazed to discover a large number of sapling-sized oaks 6-8 feet tall severely damaged by voles. We had not controlled the vegetation around these plantings the previous spring, thinking they were large enough to no longer require weed control. Not only were they girdled at their base, but many were severely stripped of bark as high as 4.5 feet. It’s not the usual case for Microtus to climb trees, but with the combination of heavy weed cover, large populations, and late summer scarcity of green food, they will scramble up to each branching node which forms a platform for them to strip bark to get at the nutritious tissues just underneath. Girdled oaks resprout from their base, but years of growth are wasted.

The easiest way to prevent vole damage is to deny them cover. Clean cultivation to bare earth or severely mowing a 4-6-foot diameter circle around the base of each tree discourages them, for they prefer to forage close to their runways. At SFREC we have also been able to virtually eliminate our vole problems by using four-foot-tall plastic tree shelters sunk a few inches into the ground. Saplings protected this way can easily be given weed-free maintenance with herbicides, and voles fail to burrow beneath or climb them. This coming year we plan to test some other protective treatments, including various guards around the base of seedlings.

Vole problems happen when grassy cover is too thick, and when planting areas are adjacent to unmowed and ungrazed areas like road-sides and field margins. We often exclude cattle from areas where we are trying to grow oaks, and the resulting tall grass invites voles. Possibly controlled cattle grazing could be helpful to discourage this serious rodent predator of young oaks, as long as the trees can also be protected from the livestock.

prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc