Owls Continued from Page 6
“The problem is there are pockets where it’s really bad, and in other locations, it’s kind of hit and miss,” Baldwin said. Not only do roof rats cause econom- ic damage by directly consuming nuts, but they also can injure trees by girdling branches, he said. One of the most com- mon and costly types of damage is de- stroyed irrigation lines. The California Department of Food and Agriculture in March estimated rat damage to almonds ranged from $118 million to $297.7 million, while losses in pistachios were between $48.1 million and $275.5 million. The estimate, which covers about 112,000 acres of almonds and 116,000 acres of pistachios in Fresno, Kings and Merced counties, does not include pest control expenses. Baldwin said he and Johnson suspect much of the rat problem is tied to the in- crease in abandoned almond orchards, which provide food and habitat. “You go two to three years without any management, and you start to see a build- up of rats on the property, and they spread out to adjacent properties,” he said. The problem may multiply when trees are tak- en out and the rats seek new habitat. Baldwin also pointed to other pos- sible causes, including further restric- tions on chemical rodenticides and in-burrow fumigants that many growers have relied upon. Vicky Boyd is a reporter in Modesto. She can be reached at agalert@cfbf.com.
“But there’s so much we don’t know,” Baldwin said. During his years of vineyard studies, Johnson found barn owls hunt primarily using acoustics or sounds, allowing them to locate prey under mowed cover crops and other natural protection. Previous work by Johnson and Baldwin noted that a single barn owl family can kill 3,000 to 4,000 rodents annually, frequently reducing rodent populations. In vineyards, field mice, voles and gophers are the dom- inant prey. More recent work by Johnson, Baldwin and Cal Poly, Humboldt, doctoral student Karen Gallardo Cruz found that barn owls also create a fear factor, with rodents show- ing 16% to 38% increased fear of predators. For their upcoming project, Johnson said they’re seeking cooperating nut pro- ducers willing to volunteer at least 40-acre blocks. The orchards would need what he described as the “Goldilocks” of rat issues. They would need enough rats so re- searchers could observe potential popu- lation reductions with the different treat- ments. But the blocks couldn’t have so many rodents that the producers would feel a need to use a chemical toxicant to prevent significant economic damage. He said they hope to have 12 participat- ing blocks. Of those, four will be untreated controls, four will have only owl boxes, and four will have both owl boxes and traps. To
Roof rats have become a problem for many nut producers by directly consuming nuts, girdling tree branches and chewing through irrigation systems.
learn how to participate, contact Johnson at matthew.johnson@humboldt.edu. The researchers will begin this fall and spend the first year collecting baseline data on resident rat populations. To do this, they’ll use tracking tunnels and ink pads, among other methods. Johnson said they’ve already placed an order with a nonprofit organization that builds barn owl boxes and erects them. The nesting sites will be installed before the second year and monitored through the third year. Using activity cameras and other types of surveillance, the researchers will deter- mine owl predation levels, nesting success
and progeny produced. They’ll also iden- tify the types of prey brought back to the nest boxes. Known scientifically as Rattus rattus, roof rats are agile climbers and frequently build nests in trees, attics or other elevat- ed locations. They’re not a new problem either, having caused isolated infestations in rural and suburban locations for years, Baldwin said. It’s only been the past two or three sea- sons that nut producers in some regions have experienced population explosions and related economic damage. The prob- lem isn’t widespread throughout the Cen- tral Valley but is a scattered issue.
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8 Ag Alert September 16, 2026
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