Chapter 55 Combating Larger Animal Pests

Insects and fungal/bacterial diseases are the most common problems we encounter, but occasionally larger animals will attack trees.

55.1 Slugs and Snails

Slugs and snails do not target any particular tree species, but feed opportunistically on a variety of deciduous trees. One or two can simply be picked off, but if they are appearing more frequently, use diatomaceous earth to repel them.

Image 1. A typical slug. Image 2. A slug feeding on a leaf. Link to original image 1; image 2.

Diatomaceous earth is the finely ground shells of diatoms. The diatoms’ shells are nearly pure silica, which means diatomaceous earth is essentially powdered glass. DE works as an insecticide and mollusc repellent because of its abrasive and absorptive properties. The fine powder strips lipids from the waxy outer layer of the exoskeletons of many species of insects so that they desiccate and die. DE also repels slugs and snails but mainly because of irritation, not dehydration.

Diatomaceous earth is not toxic, but avoid breathing the dust. Chronic exposure can cause the lung disease silicosis.

55.2 Voles, Chipmunks, and Rabbits

Voles and chipmunks clip off very small trees (pencil-sized or smaller) just above the soil line. On larger trees, voles will gnaw away or pull off strips of bark to reach the green cambium. Unchecked, voles can girdle trees in the ground and kill them.

Chipmunks tunnel up from underneath and clip roots off, or if the tree is small enough, pull it underground and chew away the root mass entirely. Voles tend to take the blame When a tree or bedding plant is eaten from underground, but more often it is a chipmunk; voles tend to feed above the soil line.

Like voles, rabbits will gnaw away or pull off strips of bark to reach the green cambium of trees. One rabbit can girdle a tree in the ground and kill it in 1-2 nights. I have had rabbits damage trees up to 2 inches in trunk diameter. They seem to prefer apples, crabapples, and native maples (but not Japanese maples.) They are more likely to gnaw on trees in winter, but can attack trees in the ground anytime.

Bark stripped off of the base of a tree by rabbits. Link to original image..

The best way to distinguish which animal is causing damage is to look at how high and how extensive it is. Chipmunks and voles tend to clip bark within a few inches of the ground; voles tend to remove bark systematically, while chipmunks feed randomly. Rabbits will strip the bark systematically like voles, but reach much higher on the trunk. Rabbits also may clip low branches that they can reach.

Voles are very timid so relatively easy to discourage; wrapping the trunk in aluminum foil often is sufficient. Chipmunks can be stubborn pests once their tunnels reach your trees. To discourage tunneling, plant trees in a larger than needed hole. Place 2-3 inches of 1:1 mix of loose nursery soil and Permatill in the bottom of the hole, and 2-3 inches out beyond the edges of the root ball. Place regular nursery soil around the tree’s roots.

Alternatively, line the planting hole with 1/2-inch mesh hardware cloth. Let the mesh extend at least 6 inches above the soil line.

When you find chipmunk tunnel openings in your growing area, toss 3-4 mothballs into the hole, and cover it up. The chipmunks will be repelled by the odor but not harmed otherwise. Over time they will begin tunneling in a different direction.

There are several chemicals available that are marketed as rabbit repellents, but in my experience rabbits seem oblivious to chile powder, rotten eggs, and predator urine.

Keep rabbits away from trees with a 3-foot high cage of 1/2 inch wire screen placed around the base of the tree. Make sure the cage is at least 1 inch away from the trunk on all sides. Anchor the cage to the ground using several landscape staples.

55.3 Squirrels

Usually squirrels are not a problem unless you are trying to grow oaks or other trees from seed. If you plant oak acorns, gingko nuts, beech nuts, or other meaty seeds, squirrels will start digging them up almost immediately. Squirrels will even dig up 1-2 year seedlings of these trees and eat the meaty heart of the taproot. I have lost entire crops of live and English oak seedlings this way.

To keep the squirrels out, I built a wire cage out of scrap 2x4 treated lumber and 1/2-inch mesh screen. The cage is about 3 feet high, and has a heavy top that I keep in place with a couple bricks. All of my oaks and nut tree seedlings stay in the cage for at least 2 full years. If the tree is growing slowly, I may keep it there for 3 years.

During nesting season squirrels clip branches from trees. They tend to go back to the same tree over and over, so if you find a few clipped branches, move the tree immediately somewhere like a well-lit garage where the squirrels cannot get to it. They will stop clipping branches once they have built their nests, and you can move the tree back out.

In autumn I sometimes have problems with squirrels hiding their nuts in the soil around trees or, more often, in flats of soil where I am starting seeds. To discourage them, I get a large shaker of ground black pepper and sprinkle it liberally on the soil of the containers. Some people recommend red chile powder for this, but I prefer black pepper for two reasons. First, black pepper has a stronger odor; squirrels do not have to dig into the soil to come in contact with it. Second, I recycle a lot of my soil; red chile powder retains its heat for longer, so I risk getting it into cuts or my eyes later.

55.4 Deer

If you want to see me foam at the mouth, let me catch a deer in my yard. White-tailed deer have become so numerous that they are regular vistors in most backyards. They are not particularly fussy about what they eat. If food is scarce they will try to eat almost anything that is not viciously thorny. Several times I’ve had browsing deer destroy years of development growth in a single night. In autumn, males also may use trees in your collection to rub the skin off their antlers. Larger trees may be used for territory marking.

Image 1. Deer damage caused by bucks rubbing their antlers. Image 2. Deer feeding damage to a hemlock. Unlike rabbits, deer tend to feed 2-4 feet above ground level. Link to original image 1; image 2.

Suburban deer are more accustomed to people, so are not as easily spooked as deer living in deeper woods.

Since we cannot shoot or physically harass deer in the city limits, I resort to chemical warfare. More specifically, stink warfare.

Even suburban deer are very wary of certain smells, especially moth balls an Irish Spring soap. Hang laundry bags with 3-4 moth balls or a half-bar of Irish Spring soap in several places around your trees. Replace them when you cannot smell them anymore.

The next step up is rotten egg solids. Several years back Rocky Mountain States Cooperative Extension conducted a head-to-head test of deer repellents. The results of their trials showed deer would almost never enter an area or eat a plant sprayed with a 5% suspension of rotten egg solids in water. This mixture kept even high-risk vegetables deer-free. It works because deer have a keen sense of smell on par with dogs. Even if we cannot smell it, the deer still avoid it. Rotten egg solids also can discourage rabbits.

There are two ways to apply rotten egg solids. If you have a strong stomach (or plugged sinuses), most hardware stores sell Liquid Fence, a spray-ready suspension. I use this routinely, and trust me when I say wear old clothes that you can let air out before washing.

Alternatively mix 3-4 beaten whole eggs into a gallon of water. Within a day of spraying, beaten fresh eggs will start smelling just as badly. On the day you plan to spray, whip the eggs in a small amount of water, strain the mixture through cheesecloth or a strainer to remove any remaining solids, then pour it into a spray bottle. You have about 24 hours before the eggs begin to decompose, and the stink cloud begins to rise.

This spray schedule works very well, and actually trains deer and rabbits to stay away from your trees. It is quite safe. The egg solids are a source of both nitrogen and sulfur, and the hydrogen sulfide is not toxic in the quantities produced.

  1. Before you spray the first time you spray, move all the trees you want left alone as close together as you can. If they have different light requirements, collect them into groups in each area.
  2. Lightly spray all surfaces of the leaves, plus the pots, benchtops, and ground immediately around them.
  3. One week later, repeat the overall spray.
  4. The third week, spray the bench and the soil around it. Skip the trees themselves.
  5. For the rest of the season, you should only need to spray a ring about 12 inches in diameter around the trees. Local deer will have learned to avoid the entire area, and will not bother checking what is on the other side of the stink line.
  6. As deer migrate during summer you may have an untrained one that starts munching on a previously protected tree. At the first sign of damage spray the trees directly again. Do not delay; deer tend to go back and feed repeatedly on the same source until it is completely depleted.

I have used this method several times, and the only time it fails is when I do not keep up with my spraying. Do not be too worried about the odor; it falls below our ability to smell it in a few hours to a day, but deer can smell it for a week or more, even after a rain.

While I have no personal experience, I have heard that deer also will avoid anything that smells of human feces. Before you poop in the pots though, try milorganite. This natural fertilizer is produced from dried human sewage that has been composted so as not to contain hazardous bacteria. It is very safe, but supposedly smells very strong. A product that both fertilizes and foils varmints too seems too good to be true, but I have heard that deer and rabbits avoid it just as much as rotten eggs.

Remember deer are afraid of anything new, yet learn quickly and adapt to one repellent. Plan to use several repellents and rotate them.

To learn more about deer repellents, read this article by Dr. Leonard Perry, Extension Professor at University of Vermont.