The Almond Board of California recently released a staggering figure: more than 100,000 acres across the southern and western San Joaquin Valley have experienced a roof rat infestation.
This infestation causes widespread damage and economic losses in those almond orchards. The Almond Board of California says an economic analysis conducted by CDFA estimates on-farm losses between $109 million to $311 million, with drip line replacements accounting for between $56 million and $168 million of that figure. Drip line replacement accounts for about $43 million to $129 million of the total loss and yield losses from disrupted postharvest irrigation is estimated to cost growers between $43 million to $129 million.
“Field observations indicate that these rodents are using irrigation canals and other waterways as corridors, enabling them to rapidly spread between orchards and diverse agricultural fields,” according to the organization. “This mobility is exacerbating the extent of damage and complicating containment efforts.”
Causes
Roger Baldwin, Extension specialist in wildlife, fish and conservation biology with the University of California, Davis, says rodent damage in orchards is nothing new, with ground squirrels, gophers and voles all causing some sort of issue. But roof rats is a new phenomenon.
And while roof rats are arboreal, researchers have reported these rats have burrowed in almond and other tree nut orchards.
“Roof rats are really kind of the critter that’s been getting most of the attention here over the last year and that’s because of the rapidly expanding roof rat populations in these orchard systems,” he says. “These rodents are now causing quite a bit of damage in a variety of different crops, and in particular pistachios and almonds, with damage estimates dramatically increasing over the last year.”
Roof rats cause damage through girdling small and large limbs, burrowing in the root zone which also disrupts and damages irrigation systems, equipment and other orchard infrastructure. Rats also consume nuts grown in the orchard.
Baldwin says it’s not exactly clear why roof rats have suddenly infested almond orchards, but he does suspect a few different things. First, many tree nut growers abandoned orchards in the Central Valley due to a lack of water. And, although these orchards have been abandoned, rainfall in the last few years has helped these orchards survive in some instances, thus being a decent host for the roof rats.
“They might be producing a little bit of a crop out there, which, of course, is a good food resource for these animals, too,” Baldwin says.
Then, if a grower does decide to remove an abandoned orchard, the disruption forces roof rat populations into existing ajacent orchards.
“Rodents do generally respond positively to greater rainfall amounts, because that leads to higher quality habitats, and we’ve had pretty good rainfall the last few years, so that may be contributing to this,” he says.
Another contributing factor may be regulations on some of the pesticides, rodenticides and burrow fumigants that control roof rats.
“It’s possible that some of those changing regulations may have reduced the ability of growers to utilize some of those products, which might have led to an increase in problems as well.
Control measures
Baldwin says growers seeking to control roof rat populations should utilize an integrated pest management approach, using a variety of tools such as burrow fumigants, trapping, rodenticides, and biocontrol agents such as barn owls to help keep populations low. (UC Davis has put together a resource titled Managing Roof Rats and Deer Mice in Nut and Fruit Orchards).
In particular, there’s an opportunity to utilize predation and other control methods.
“In nut orchards, the fact that they are a deciduous crop means that they don’t have leaves on the trees for a certain period of time, which means the rats are going to be more susceptible to predation,” he says. “There’s a potential that we could utilize barn owls as an effective strategy for trying to remove some of the rats out there. That’s an area that we’re currently trying to get some funding to look at the utility of barn owls to help manage roof rats in these nut orchards.”
The other thing Baldwin thinks is important to point out is that the roof rat is an invasive species.
“They have been here for a few centuries, so they’re nothing new, but they’re also not native,” he says. “So, because of that, you know, they can cause a lot of damage to our native ecosystems, and so that just kind of adds to that whole desire to really try to substantially reduce their populations.”
