I consider gophers to one of the primary causes of alfalfa stand decline in this area, especially in lighter soils. About 70% of the gopher's diet consists of underground portions of plants and damage to alfalfa is primarily cutting the tap root. Damaged plants may not die immediately, but might barely hang on until moisture stressed later in the season. During the first year of the stand, adjacent plants may compensate for stand thinned by gophers, but in subsequent years, losses due to gopher damage averaged 20% per cutting in some studies. At this point, even if gophers are controlled, the stand will not recover because of the lost plants.
Pocket gophers live most of their life entirely underground except when the young leave the nest to look for a new place to live. Recent studies on the habits of gophers in alfalfa have shown that the gophers invade new alfalfa fields from undisturbed breeding areas such as older alfalfa, irrigated pasture, some orchards, and natural areas. They travel from these "source areas" using established travel routes, such as canals, roadways, etc. and enter new fields at the point where these linear travel routes meet the new field. They travel above ground and will not cross standing water (such as a drain). They, like many other pests, start at the margins of the fields and move in. By concentrating control methods at gopher points of entry at the corners of new stands, you may have a better chance of keeping a field from becoming badly infested.
It may take two to three years for a new alfalfa field to become completely saturated with gophers. Because they are highly territorial, only a certain number of gophers can fit into a field. If the established adult population is eliminated, young, hungry gophers driven out from other areas can quickly take up residence, and the new population can be higher than the original because the younger gophers need smaller territories. Populations in alfalfa may reach 50 gophers per acre.
Mounds are not necessarily an exact indicator of gopher activity in established stands because digging gophers may not only bring excavated soil to the surface, but will also use the soil to backfill old tunnels. High populations can exist with relatively few mounds. During winter rains, mounding activity may increase as gophers expand their living spaces in preparation for the breeding. In alfalfa, gophers may produce up to three litters per year because food is so plentiful.
There are two main options for controlling gophers: toxic baits or traps. Both methods may be used at any time of year but work best when the soil is not too dry and hard. Baits may be applied either mechanically with an artificial burrow maker or with a hand probe. For hand probes to be effective, the bait must be placed in a main tunnel, which is not usually the one associated with the mound. Artificial burrow makers may be effective but have disadvantages such as damage to the stand, collapse of the artificial burrows, and gophers using the artificial burrows as runways to invade new areas of the field once the bait is gone. Traps are effective for small infestations but are impractical for large areas. More information on these methods is available from our office.
There has been considerable interest recently in using barn owls to manage gopher populations in orchards and vineyards, and this should work equally well in alfalfa. Each adult barn owl will consume one gopher per night, while a nest of seven-week-old young will eat as many as eight each night. There is no lack of food in this area to support a larger barn owl population, however there is a lack of nesting sites. Barn owl nest boxes are not difficult or expensive to make, and the owls are free. Since owls may fly a considerable distance to hunt each night, there is no guarantee that providing nesting sites on your property will control your gophers, but it sure couldn't hurt, especially if your neighbors do it too. Instructions for building and mounting barn owl nest boxes are available from our office.
November 5, 1999