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Past Issues

Seasonal Timing Determines Best-Management Tactic for Ground Squirrels

A proactive, year-round strategy is the best approach for managing ground squirrels in almond orchards. UC IPM Wildlife Pest Management Advisor Roger Baldwin reported at The 2012 Almond Conference that almond growers need to take a season-long view and plan for what control tactics work best at certain times.

Start After Harvest

Growers can start thinking about ground squirrel control immediately after harvest, using bait to knock down populations going into the winter and following up early the next spring with burrow fumigation.

“After harvest is not necessarily the time to forget about squirrels, and is actually a good time to put together an integrated management plan,” Baldwin said.

There are some important facts to consider when putting that plan together:

  • Squirrels hibernate and will not be controlled with fumigation or bait during periods of inactivity in the winter.
  • Squirrels have seasonal eating habits, so control strategies should be timed to coincide with those preferences.


After winter hibernation, ground squirrels move above ground and begin actively feeding on surface vegetation. Burrow fumigation works well during this period in early spring and anytime enough soil moisture is present, either through precipitation or irrigation, to hold gases in the burrow system.

Aluminum phosphide is the most effective, least costly material registered for ground squirrel burrow fumigation, providing efficacy rates of 97% to 100%. As a restricted material, however, there are several hurdles that must be cleared before applying aluminum phosphide. Newer restrictions on aluminum phosphide include increased buffer zones around occupied structures and new 48-hour posting requirements after application. Gas cartridges, which primarily produce carbon monoxide, provide 75% to 80% efficacy, with fewer restrictions.

“If you just have a few burrow systems to treat, it is easier to use gas cartridges, but if you have a lot, the cost and efficacy benefits of aluminum phosphide will outweigh the negatives,” Baldwin said.

Late Spring Baiting

By late spring, as vegetation begins to senesce, ground squirrels switch their diet to eating seeds, and bait becomes a more logical control choice. Baiting should be done before nuts fill on trees, as they provide a preferred food source to baited materials.

Setting out bait stations should be done before nuts fill out;
otherwise, the nuts become the preferred food source. Photo
courtesy of Roger A. Baldwin, Ph. D.
Anticoagulant baits must be consumed several times over a three- to five–day period, and it can take up to two weeks before a lethal dose is consumed. They are also now a restricted-use material. Anticoagulants do, however, provide the advantage of good bait acceptance.

Zinc phosphide can yield poor bait acceptance and is only registered for a single application per year, so Baldwin suggested growers do prebaiting prior to applying zinc phosphide to see if squirrels will accept the bait.

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