Specialists advise how to protect drip tape from rodents

University of California researchers advise an integrated approach to minimize rodent damage to drip irrigation lines, which is a major disincentive to investing in this method of delivering water to crops.

Installing owl boxes, trapping and killing rodents, using thicker irrigation lines, and wetting the soil enough to minimize cracking are all among the candidates for this tool box.

"It's worth trying to save your investment by using your drip line for 10 to 12 years," said Konrad Mathesius, UC Cooperative Extension agronomy farm advisor in Yolo, Sacramento and Solano counties.

Mathesius discussed strategies for helping buried drip systems last as long as possible with the growers and pest control advisors at the Cooperative Extension Rotational Crop Meeting in Woodland.

Gophers are notorious for feeding directly on some crops, especially alfalfa, but they and other rodents also chew holes into buried drip irrigation tape.

"One answer is thicker tubing," he said. "Mice and rats cause less damage to 15-mil tape. If you spread out the cost over five years, the 15-mil could be saving $25 an acre."

While investment in thicker lines should reduce damage by all rodent pests, many management strategies depend on knowing which sort of critter is doing the damage.

"It helps to identify whether voles, mice or some other rodent are doing the damage," Mathesius said. "The control you use depends on which rodent you have. A motion-activated game camera can help. Mice have larger ears and longer tails."

Gophers may be hard to identify because they stay out of sight in underground tunnels, but UCCE wildlife specialist Roger Baldwin advised they do leave distinct visual clues.

"Gopher mounds are plugged and you rarely see them above ground," he said. "It is important to know the difference from moles, because gophers are herbivores and feed directly on the crop. Damage can range all the way up to more than 8 percent for alfalfa. Gophers also feed on irrigation tape, and that is one of the main disincentives to adopting buried drip."

Baldwin advised an integrated approach using a number of gopher-control tools, rather than just one.

Flood irrigation works, but can be an impractical strategy in situations where water is in short supply.

Deep ripping fields that have been infested is also helpful because it upsets the gophers already in the ground, and makes life harder for those yet to come.

Owls can also control modest numbers, and Baldwin is taking a closer look at their effectiveness, but there is no evidence they can knock down a population that has already grown large.

"It looks like barn owls can keep gophers under control if the populations are low, but can't do much if the population is already high," Baldwin said.

There are many poisons available for killing gophers, and they do help, but a variety of factors can limit their efficacy.

"Dew or fog make zinc phosphide ineffective," Baldwin said. "You're only going to get a day or two out of zinc phosphide. It's hit or miss."

Strychnine is likely the most widely used gopher poison, but it is not always an effective choice, he said.

"Gophers can develop a resistance to strychnine if it is used over and over for years," Baldwin said.

Traps may be the single most effective gopher management tool, but they are time consuming and take a commitment to coming back to a field a second time in a week or so to work best.

"We can get 92 to 93 percent removal after a second trapping throughout the field," Baldwin said. "For gophers, you have to commit to a second trapping after about a week."

These underground rodents are best controlled before they have a chance to multiply and build large populations.

"Gophers are around year-round, so you can manage them year-round, but the best time is winter to early spring," Baldwin advised. "The ground is soft, so you can probe it more easily, and the population is lower than it will be after the spring. Use a multi-tool approach."

(Bob Johnson is a reporter in Davis. He may be contacted at bjohn11135@aol.com.)