Harvesting nut crops followed by good orchard sanitation will deprive rodents of a preferred food source and drive them to poison baits far more easily in the fall, said Roger Baldwin, UC Davis wildlife specialist.

Ground Squirrels

California ground squirrels are the primary vertebrate pest in many tree nut orchards, and fall presents a good opportunity for population control. With their primary food sources, nuts and vegetation, removed, squirrels are more likely to eat toxic baits. Multiple-dose anticoagulants can be applied in bait stations or spot treatments near burrows or broadcast where squirrel activity is present. The UC Pest Management Guidelines note that for multiple-dose baits to be effective in vertebrate pest control, they must be consumed over a period of days. If spot or broadcast treatments are used, multiple applications may be necessary.

California ground squirrels generally hibernate, although in warmer areas they may be active year around. Those that do hibernate emerge in mid-winter as temperatures warm. They live in underground burrows 2 to 3 feet below the surface. The burrows can have several openings marked by scattered soils. Burrows are often dug beneath trees, but they will travel 100 yards or more to feed.

Severe infestations of ground squirrels can not only affect production, but can result in girdled tree trunks and scaffold branches. Squirrels can also damage tree roots and irrigation lines. Extensive burrowing can divert irrigation water. Large and numerous burrow openings pose safety issues for orchard equipment and workers.

Monitoring squirrel numbers and recognizing their feeding habits will help with control decisions.

Baldwin said rodenticides are a good choice for fall control of vertebrate pests as there are fewer restrictions on use following harvest. Reading the label is always important, he added.

Use of first-generation products which are multiple feed materials may take longer to have an impact on pest populations. These anticoagulants can be applied by broadcast or spot treatments placed near burrows. Bait stations can also be used to protect the materials, ensuring they will be viable for longer periods. Zinc phosphide is an acute toxicant that can kill after a single feeding and can reduce numbers quicker than an anticoagulant. Baldwin noted that this chemical does have a distinctive odor and taste. They may only eat a small dose and become sick then develop an aversion to the product.

Gophers

Once rains return, it’s a great time to reduce gopher numbers, Baldwin said. Moist soil makes probing the tunnels easier and increases the frequency of mounding. An effective fall knockdown campaign prior to gopher winter reproduction can control numbers. When possible, destruction of burrows will slow down re-infestation. New orchard sites, where there is extensive evidence of burrows, should be ripped to destroy existing burrows.

Like squirrels, pocket gophers will gnaw on tree roots and trunks when green vegetation is not available. A fall knockdown campaign should be centered around poison bait and trapping.

Fresh mounds of dirt in an orchard are a sign of gopher activity. Determining active tunnels will help with bait placement.
First-generation anticoagulants, zinc phosphide and strychnine are the primary toxic bait materials, although strychnine has generally proven most effective.

All pocket gopher bait is applied below ground. There are three primary methods for baiting: 1) Hand baiting via the funnel and spoon method, 2) An all-in-one probe and bait dispenser, and 3) A mechanical burrow builder.

The hand baiting system is not practical for large acreages. The probe and bait dispenser can work well, but the person doing the work needs proper training to know if the tunnels where the bait is being deposited are active tunnels.

The mechanical burrow builder is an implement that creates tunnels through the soil and automatically drops a measured amount of poison bait in the tunnel. The tunnels intercept existing gopher tunnels and they enter to eat the bait. Success with burrow builders, Baldwin said, depends on the right amount of soil moisture and getting the depth of the tunnel right. If the tunnel is too deep or too shallow, it will not be used.

Tunnels will also need to be checked to make sure the bait is deposited. Burrow builders work best where gopher tunnels are extensive. This method is less time consuming and can achieve a quick knockdown, but efficacy is not always consistently high. Monitoring and use of other control tools, including trapping, may be necessary.

Additional Vertebrate Management

Voles, roof rats and deer mice are other vertebrate pests that can damage trees by girdling, particularly young trees. Vole populations can increase rapidly and they prefer sites with cover crops. Their burrow openings are much smaller than those of gophers and their runs through vegetation can be observed. These rodents spend more time above ground but have underground burrows. Broadcast rodenticides are the primary control method and should be applied prior to leaf out as they are generally not labeled for use during the growing season.

If zinc phosphide is used, the key is to avoid moisture as that will start off-gassing and voles will avoid the bait.

As for biological control, Baldwin said that while barn owls are adept at killing vertebrate pests, there is no way of knowing if the barn owls observed on site are hunting there or elsewhere. They likely won’t eliminate huge pest populations. Prey composition varies across sites based on rodent availability, but is largely composed of gophers, voles and mice.

Resources for control of vertebrate pest species can be found at the UCCE Vertebrate Pest Control Education web site. For pocket gophers go to the UC IPM Pocket gopher Pest Note at http://ipm.ucanr.edu/PMG/PESTNOTES/pn7433.html. Ground squirrel management can be found at groundsquirrelBMP.com.