Landscape Pests Including Gophers, Squirrels Moles, Voles, Wood Rats and Their Control and Related Laws
By Dr. R.S. Kaae and Patricia Kaae
CHAPTER 1-The Pocket Gopher
The pocket gopher is one of the most serious and difficult to control vertebrate pests that the landscaper or pest control operator may encounter. There are at least 5 species of pocket gophers in the west, with Thomomys bottae having the widest distribution, occurring throughout this area except in the northernmost extremes. The remaining species are confined to the Northwest. Within their ranges, these species are found in most situations, except for very rocky areas, parts of the dry desert and the highest mountain ranges.
Damage from these pests is multifold. In agricultural and urban situations they commonly attack a variety of crops and landscaping plants by feeding on roots, stems, foliage and bark. They have been known to kill young trees by entirely girdling the bark. Their burrowing activity and removal of vegetation adds to the possibility of slope failure in landscape situations.
In turf they kill the grass, leave unsightly above ground mounds (Figure 1) and cut below ground irrigation wires. Their earthen mounds dull or destroy the blades of lawnmowers. There have been many lawsuits due to someone’s tripping on a gopher hole in a yard, park or athletic field.
Figure 1. Typical pocket gopher damage in turf. All this damage is due to one gopher.
ANATOMY AND SENSES
The pocket gopher (Figure 2) is named for its external fur-lined pouches on both sides of the head. These pouches are used exclusively for either carrying or temporarily storing food. Because they live an almost entirely subterranean life and rely little on vision, they have very poor eyesight. Gophers have a keen sense of smell and are extremely sensitive in the perception of ground vibrations. This is a sense that comes in quite handy when being stalked by a cat or overly zealous pest control technician. Vibrations are sensed by their oral vibrissae (whiskers) and elongated tactile hairs on the body. Pocket gophers have narrow hips and well-developed shoulders. The narrow hips allow them to maneuver and turn easily in their small burrows while their strong shoulders enhance their digging ability.
Figure 2. An adult pocket gopher, Thomomys bottae.
Pocket gophers lead an almost completely subterranean existence, venturing above ground only to push dirt out of their burrows, seek new territory, or to graze on plants immediately adjacent to the burrow entrance. The burrow system typically consists of 2-to-4 inch diameter tunnels and can be quite extensive (Figure 3). Depending on the time of year, the main tunnel (run) is situated from 3 to 18 inches (or more) below and parallel to the soil surface.
During the winter, gopher tunnels are typically found at shallow depths (4 to 8 inches). During the summer months when soils tend to dry out and temperatures rise, these critters dig deeper. Lateral runs are dug at varying angles to the surface from the main run and are used to push out loose dirt accumulated by burrowing. Once removed from the tunnel, soil appears as the large mounds (Figure 4) that are characteristic of gopher activity.
These mounds are normally horseshoe in shape, with the high side of the mound furthest away from the plugged opening of the lateral run and the lowest side nearest the plug. The shape of the mound becomes important when attempting to locate the main run (discussed later in control). The number of above ground mounds produced by an individual gopher varies with the season. During the winter gophers produce a daily average of 3 to 4 mounds, while during the summer they produce an average of 1 mound every 2 days. Of course in irrigated areas or areas nearly barren of plants, these figures can change drastically. For example, gophers tend to dig more when looking for food in areas with few plants and, as a consequence, many above ground mounds may be produced in one day.
Figure 3. Well-developed burrowing system of a pocket gopher.
Figure 4. A typical large above ground gopher mound.
Pop holes (Figure 5) lead straight up to the surface from the main run and are used as access to foliage growing closely to the opening. Pop holes and lateral runs are nearly always filled with loose dirt, but appear at the surface as small, circular plugs surrounded by denuded foliage. If a gopher dies, irrigation or rain can wash the loose soil plugs from lateral runs and pop holes, which leaves open tunnels. Keeping the pop holes and lateral runs plugged excludes potential predators such as snakes and weasels.
Figure 5. Earthen filled pop hole with signs of gopher feeding around perimeter.
Drainage tunnels, as the name implies, are used for water run-off, which is mandatory during heavy rains or excessive irrigation. There may be one or more drainage tunnels extending downward many feet off the main run and there may be one or more “potty holes” which extend a few inches below the main run. Other parts of the system typically include food storage areas and a nesting area for sleeping and/or rearing of young (Figure 3).
The length and complexity of any one-gopher burrow system depends on a number of factors. First of all these systems are not static but instead they change daily. In well-irrigated areas a gopher may not need to dig much in search of food; but when food is sparse, a gopher is likely to dig considerably, moving into new areas and filling the old tunnels with the earth that it uncovers in doing so.
Because gophers are extremely territorial and the regions of any 2 rarely overlap, it follows that in heavily infested areas the tunneling system of a mature gopher is much shorter than the same-aged gopher in a lightly infested area. Of course it is quite logical that newly formed burrow systems are much shorter and less developed than older more established systems. While a newly formed burrow system may be only several feet in length, it is not uncommon to find well-developed systems beginning in one lawn and extending under a street and continuing a few hundred feet into several other properties on the block.
Mature gophers normally are solitary except at mating time or before the females wean their young. If 2 adult gophers encounter each other at any time other than the above 2 situations, they will fight, occasionally to the death of one.
In warmer areas (e.g., Southern California), these pests remain active throughout the year in irrigated landscaping. Under these conditions, they have a tremendous reproductive capacity, with females capable of bearing 3 broods per year (an average-size brood is 6 to 7 young). In non-irrigated areas, (e.g. natural fields), their reproductive cycles correlate with rainfall; hence, under these conditions, there is typically only one generation per year (during the spring months). Many authorities believe that gophers do not hibernate, but may have a dormant period in non-irrigated areas during the hot, dry summer months.
Even though gophers are typically found underground, they are capable (much to the landscaper’s or homeowner’s disgust) of traveling above ground to new areas. Excavation connected with construction projects or disking of fields is commonly believed to uproot large numbers of gophers that subsequently move into adjacent areas. However, in the authors’ opinion, the opposite is true and that most are likely killed by these operations, especially with deep plowing.
We once had a 40-acre control project that was heavily infested with gophers and squirrels. It was surrounded by homes and there was considerable controversy as to what would happen to the gophers once the property was plowed in order to develop an auto center. As a result our bid included monitoring and controlling any gophers that moved out of the field into the surrounding lawns once it was plowed. We successfully controlled the squirrels prior to plowing, but because the soil in the field was bone dry and there was little rain that year, we had little success in controlling the estimated 3,000+ gopher population. It was quite surprising that after plowing, less than a half dozen of these gophers migrated into the surrounding lawns and an adjacent park and the deep plowing left none in the 40-acre field.
Heavy rains may force gophers from their tunnels. Finally, because these animals are territorial, a mother will force weaned young from her burrow, which then may move above ground up to 1000 feet or more to new territories.
A number of factors determine potential infestation of a given area. Again, unsettled areas are more likely to have more gopher problems than settled areas. Slopes, also, are more prone to infestation than are flat areas. This undoubtedly is associated with better drainage and possibly with the looser soil in filled areas.
Finally, and most importantly, the types of plants used in landscaping are closely linked to potential gopher infestations. Most ornamental trees and shrubs do not readily support large gopher populations and there is a tremendous variation in the attractiveness of various other plants to these pests. Based on several years of gopher control, we find most major gopher problems in areas planted with, gazanias, alfalfa and other legumes such as O’Connor’s legume. In landscaping those ground covers not commonly infested include honeysuckle, ice plant, red apple and ivy to mention a few.
All ground covers have advantages and disadvantages. However if plants such as O’Connor’s legume or gazanias are planted on slopes near undeveloped areas, the landscaper can expect a gopher problem to develop and should be well versed in control techniques.
There have been various studies trying to determine what is the maximum capacity or the largest number of gophers a given area can support. Of course this would be dependent on a number of factors (e.g., slope versus flat ground, type of plants present, soil type, amount of moisture). Indeed this is difficult to determine as these animal live below the ground. However a few hundred per acre would be a conservative figure. I once saw a worker empty a large watering truck into one area trying to drown the gophers out at a school where the gopher population had to be at nearly peak capacity (a mound every 2 feet or so). Several gophers surfaced from the soil in about a 30-foot square area in an attempt to avoid drowning.
Gas.Three techniques commonly used in gopher control are: fumigation, trapping and poison baits. Several types of gas bombs are commercially available; however, most studies indicate that most fumigants of this type are not as effective as trapping or baits. Gophers have a well-developed sense of smell and it is believed that they can easily detect most fumigants and then plug off that portion of the tunnel from where the gas emanates. If used, fumigants work best in moist clay soil and in lesser-developed burrow systems of young or newly established gophers.
The one notable exception to the ineffectiveness of fumigants for gopher control is aluminum phosphide (Brand names-Fumatoxin, Phostoxin). This is a highly toxic material that comes in a variety of formulations. Tablets are the most common formulation being used for vertebrate pest control. This material is currently under heavy scrutiny and may its use in the future mat change drastically. Aluminum phosphide, when in contact with atmospheric or soil moisture, releases a very toxic phosphine gas. These tablets break down quickly, generally within 3 hours under normal atmospheric conditions and much faster with high soil moisture. This material is packaged in an airtight aluminum canister. When not in use, the lid to the canister should always be kept tight to prevent the loss of gas from the tablets. Cotton or leather gloves should always be used when handling aluminum phosphide. Avoid opening the canister at face level. and with a cross wind if possible.
This is an extremely effective material when used properly in certain environmental conditions. Ideal conditions for effective control include adequate soil moisture, heavy clay soil and access to the entire length of a gopher’s tunneling system.
Aluminum phosphide is generally less effective in sandy soil. The logical reason for this is that in this type of soil, a certain amount of gas escapes from the runs. If sandy soil exists, this problem can be partially alleviated if there is a high level of moisture in the soil, presumably sealing the runs and releasing the gas at a faster rate.
If the entire length of a gopher’s run is not treated, the gopher may escape the gas by plugging off one end of the tunneling system with soil. Best results are achieved by first placing a tablet at each end of the system and then as many as needed at intermittent intervals in between. For effective control, tablets should be applied at the rate of 2 to 4 per burrow system. With very heavily infestations (e.g. a mound ever 3 or 4 feet) the applicator really doesn’t know how many gopher systems there are in a given area. In this situation we have found treating the burrows with one tablet every 15 feet in a grid-like fashion typically will give good control.
Treatment consists of opening up the main runs with a metal probe and dropping a tablet into the tunnel and then closing off the opening. Some county Departments of Agriculture or other regulatory agencies insist that the tunnel be reclosed with a wad of paper and then a barrier of soil.
Traps. Trapping, although effective, is time consuming and best used for small areas with light populations. In heavy infestations there may be hundreds of gophers per acre. Considering the reproductive capacity of these pests, it would be difficult (if not impossible) to control large populations with traps alone. A good trapper typically can set and later check 8 or 9 in an hour. An average catch rate would possibly be 50 %. Assuming there is a large area (10 acres) with a heavy gopher population and taking into account that the average female will produce 6 to 7 young every 4 months who within a few months will also begin reproducing, the numbers become astounding. In this type of situation it would take (according to my math) 3 people trapping 8 hours a day, 30 days a month to stay even with the population. Maybe my math isn’t that good but the point should be made. Traps do not give effective control with big populations of gophers!
One of our studies compared the effectiveness of several models of box and snap traps-the 2 basic trap types (Figure 6). As can be seen from the results in Figure 7, there was a large variation in the degree of effectiveness from model to model.
Figure 6. The two commonly used types of gopher traps. Image courtesy of California Department of Agriculture.
Normally traps are not baited with any food to attract the gopher. Gophers are normally attracted by light or run into the traps randomly. Typically, snap-type traps (Macabee, Victor) are best set in the main run while box traps are set in the lateral run or in the main run, if there is room (Figure 8).
|Snap Traps*||No. of sets||Percentage Catch|
|Go For Steel||292||16.1|
|* One set of a snap traps is defined as one trap set in both directions of the main run.
**One set of a box trap is defined as a single trap set in the lateral run.
Figure 7. Comparative efficiency of various commercially available gopher traps.
Figure 8. Typical locations when setting snap or box type gopher traps and proper placement of baits=bait should be placed in the main run (right). Image courtesy of California Department of Agriculture.
The trap or traps should be placed as far as possible into the main run for best result. Any loose soil should be removed from the tunnel prior to placement. It’s best to work the trap down a little into the soil whenever possible. Snap traps should be secured to a stake with a small chain or wire. If not, a trapped gopher will likely drag the unsecured trap deep into its tunnel before dying. Once in place, soil should be used to close off the tunnel behind the trap. In the case of snap-type traps it is preferable to leave a pinpoint of light shining through this earthen closure. As we have already discussed, gophers readily close off any light entering their tunnel system. The theory is that if too much light (behind the trap) enters the tunnel, the gopher pushes dirt to close off the tunnel and merely sets off the trap with this soil On the other hand, if a pinpoint of light is left behind the trap, the gopher will think (?) the opening is further down the tunnel and thus more readily blunder to the trap in its attempt to reach the distant opening.
Most box traps already have a hole in them to allow a certain amount of light into the tunnel. In the case of the Guardian box trap, there is a baffle at the back of the trap that covers the hole and permits a minimum amount of light into the tunnel but which supposedly allows a draft to enter. The theory behind this trap is that the draft attracts the gopher.
Baits. For years baits have been the standard for gopher control. There are 4 toxicants, or active ingredients, that are commonly used in gopher baits: strychnine alkaloid, zinc phosphide, chlorphacinone and diphacinone. These toxicants are impregnated on some type of grain (milo, kernelled wheat, barley) or in some case mixed with and pelleted in a dried alfalfa type mix.
Of the 4, strychnine alkaloid bait is most commonly used and based on our experiences the most effective. Strychnine alkaloids baits are formulated with anywhere from 0.1% to 3% active ingredient (refer to product label). Generally, the higher the concentration of strychnine, the more effective is the bait. Strychnine is one of the most toxic pesticides known to humans and other vertebrates; consequently, extreme care should be taken to follow all label directions precisely.
It is quite important that strychnine bait or any other bait is placed below ground in the main run. Bait placed below ground in a lateral run is not only more difficult for the gopher to find, but is also more easily pushed above ground, becoming more accessible to non-target organisms (including children). Most poisoned gophers die underground, but in large-scale control programs, a few possibly will die above ground. In this case, the possible poisoning of either a domestic or wild animal after eating a poisoned gopher found on the surface should be considered when selecting bait.
Zinc phosphide is commercially available in a 2 % pellet and an alternative to strychnine alkaloid baits. Zinc phosphide is also highly toxic to mammals but is considerably less so than strychnine. It has the additional safety factor of being an emetic and consequently will be regurgitated if consumed by non-target organisms.
Chlorophacinone and diphacinone are anticoagulants and work primarily by preventing coagulation of the blood and rupturing capillary blood vessels. In order for this type of bait to be effective, a gopher must eat bait containing either of these toxicants over several consecutive days. These materials are quite effective for squirrel control but have limitations in gopher control considering where the baits have to be placed. The baits typically rot while in the ground and must be replaced on a daily basis. There are some new formulations available that can possibly eliminate this problem. The big advantage of these baits is that they are much safer to use than the others.
Many factors influence the success of control with baits. Dry, sandy soil collapses easily, burying the baits so it’s difficult for the gopher to find. Also in dry soil, gophers may not show signs of activity until moisture is available. On the other hand, excessive moisture can cause the bait to mildew quickly, making it unpalatable to gophers. In our experience, the types of available plants affect how quickly gophers accept bait. For example, gophers are controlled more easily with baits in turf than in O’Connor’s legume, as the latter is a preferred host.
Finally, gophers may become “bait shy” if they ingest sub-lethal amounts of a bait and become sick. Because the animal associates the sickness with the taste of the bait, it will no longer feed on it. Once this occurs, other types of bait or alternative control method should be used. Bait shyness may develop rapidly in a few gophers; however, typically it takes many months, or even years, of repeated applications to occur in many individuals of a large population.
Exclusion. Although not a means of controlling a gopher population, exclusion, as the name implies, is a technique that can be used to prevent gophers from reaching a property or plants in the first place. The use of this technique is typically limited to small-scale locations of relative high value. The most commonly used method of excluding gophers from a property is burying ¼-inch mesh hardware cloth to the depth of 2 feet around the entire perimeter of the property. A 6-inch high wall of the cloth should extend above ground to prevent gophers attaining access via this route. Of course the amount of screening necessary will vary tremendously depending of block fencing or other barriers. The bottom of the hardware cloth should be bent at 135-degree angle away from the property to be protected to reduce the possibility of a gopher working around the barrier. This is not a permanent solution but such a barrier will last several years or more depending on environmental conditions.
FACTORS LEADING TO UNSUCCESSFUL CONTROL
Probes. Mechanical handheld probes are most commonly used for injecting poison baits into the gophers’ burrows. Most mechanical probes have a large-diameter injecting end that can clog readily with damp soil. A plugged tip may go unnoticed, resulting in non-baited applications. The large tip can also make it more difficult to determine the location of an open tunnel as opposed to a tunnel that has been refilled with soil. Many commercial vertebrate pest control companies use a non-mechanical gopher probe with a narrow, more sensitive end to find the tunnel (Figure 9). Once it is located, the tunnel can be opened wider with the opposite end of the probe and bait can be poured into it with a long handled spoon or then injected by mechanical probe.
Figure 9. A non-mechanical gopher probe. Image courtesy of California Department of Agriculture.
Proper Placement of Bait or Aluminum Phosphide. It is essential to place bait or aluminum phosphide in a main run (Figure 8) rather than in a lateral run. This normally can be accomplished by probing 10 to 12 inches in front of the earthen plug of the large mounds. The lateral runs are partially filled with loose soil and placement here (in the soil) will prevent the gopher from finding the bait or prevent the gas from being released into the tunnel. Non-mechanical probes are more sensitive than mechanical ones.
Dry Soil. Adequate soil moisture is essential for effective gopher control. If there is not enough irrigation or rain, an attempt to control any sizable population of gophers is usually worthless. Without moisture the runs do not maintain their integrity, quickly collapse and cover baits and fumigant pellets. Gophers under these conditions also burrow deeper and make it difficult to find active systems.
Soil Type. It is harder to control gophers in very sandy soil. Aluminum phosphide and other fumigants typically do not work as well as they
Competitive Plants. In situations where the preferred host plants such as O’Connor’s legume and gazanias are present, baits may be less effective than when used in other types of ground cover. In these cases the gophers may prefer to eat the plants rather than the baits. In such situations aluminum phospide is by far the preferred product for control.
Workers. Undoubtly one of the major reasons why many gopher control programs fail is due to the personnel who are performing the work. In very large projects gopher control can be very hard work and unless one is especially motivated, the work is performed incorrectly or not at all. It is especially worthwhile to spend the extra money to hire someone who will do a good job.
GIMMICKS AND FOLKLORE
Electrical devices and whirling yellow pansies. There is and have been a number of probe-like electrical devices that produce a sound or beep and when inserted into the ground, are supposed to drive away or eliminate gophers (Figure 9a). We are not aware of any scientific evidence that any of these work. I once walked into a customer’s backyard that had 8 of these devices placed in various locations in her backyard. The whole yard was beeping and the gopher was doing quite well, thank you. It should be mentioned that there are similar devices that can be used in the home for control of any of a variety of insects and vertebrates. Again, in the authors’ opinion and that of many other scientists, these devices are of little, if any, value. Whirling yellow daisies, sunflowers and happy faces, although attractive in the yard, pretty much have the same result.
Figure 9a. One of many electrical devices for control of gophers and moles.
Other Remedies.Gopher purge is a plant that is commonly planted in yards with the idea it will drive gophers away. Undoubtedly gophers do not prefer or will not eat these plants, but as far as driving gophers from a property, this seems very unlikely.
There are a number of home remedies that, again, have no validity but are at least interesting. One is that if a person catches a gopher, kills it and then puts it back into its tunnel, it will drive others away or keep gophers from reinfesting the same tunnel. It is true that gophers do reinfest abandoned tunnels. Generally speaking this occurs but typically it takes months. Also, as we know, there is normally only one gopher in each tunnel system. As a consequence, if you kill the only gopher in the tunnel and put its dead body back in, this will not drive away the other gophers, as there are none there in the first place. Additionally a dead gopher in a tunnel rots and is eaten by ants and other insects within less that 2 weeks. Other remedies include putting a bottle in the tunnels. The theory here is that if a gopher sees its own reflection it will run away. Wrigley’s spearmint chewing gum, banana peels, mothballs and ground glass placed into the tunnel are equally worthless for control
Placing a water hose in a gopher tunneling system may or may not produce results. If it is a new gopher with a relatively short tunneling system, this may drive the gopher above ground. If it has a well-developed tunneling system with drainage tunnels this typically has no effect as the water merely sinks into the soil, drains down the tunnel while the gopher may retreat to its nest, which is located above the main run and the water. Because excess water has to go somewhere, as it washes out at the other end of the tunnel, it carries dirt with it. This can undermine the tunnel, which sometimes collapses. This can lead to sunken areas on flat ground and devastating slope loss on hillsides. Placing several road flares in the tunnel has been used for many years and is fairly effective when favorable conditions are present for the use of fumigants. Unlike the small incendiary gas bombs that are available at many garden centers, road flares produce a large amount of gas. These smaller gas bombs are typically only effective on newly established gophers with small runs. Of course the use of road flares over a large area is would be very costly.
Finally the authors are aware of more than one situation where a homeowner has poured gasoline down a gopher tunnel and then igniting its. In one situation the result was setting a field and a neighbor’s fence on fire and in another case (4 gallons were used in this case) the individual basically blew up his backyard and ended up in the hospital. I don’t know if the gophers survived or not but my guess is they did just fine (i.e.-Caddy shack).
Questions for Continuing Education Test. Chapter 1.
1. The number of generations of young produced by gophers is often dependent on location, with 1 generation per year in non-irrigated areas and 3 in irrigated areas.
2. Gas bombs and aluminum phosphide are less effective for gopher control in dry or sandy soil.
3. Other than aluminum phosphide the currently registered fumigants are not generally considered very effective for gopher control.
4. Baits containing single dose toxicants such as strychnine tend to be more effective for gopher control than baits containg multiple dose anticoagulants while the latter is considered safer from the standpoint of possible toxic effects on human and other non-target organisms.
5. The tunneling system of a mature gopher may extend the length of a few average-sized homeowner properties.
6. A pop hole is a tunnel that typically leads directly up from a main run and functions as a temporary feeding site.
7. Experts in the field consider whirling yellow daises and electrical devices that produce a beeping sound in the gopher tunnel ineffective for gopher control.
8. Under normal conditions and excluding the unweaned young, several gophers are found in a single tunneling system.
9. Conditions that typically lead to heavy gopher infestations include slopes, unsettled urban situations and the presence of plants such as O’Connor’s legume and gazanias.
10. Gophers almost always keep their tunnels plugged with earth. On occasion they will open up the system during periods of high moisture to allow for drying out.
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