- Author: Pamela Kan-Rice
Monitor for rodent activity and use bait stations before the growing season to prevent problems, UC ANR scientists recommend.
Roof rats are running rampant in California orchards this year, according to UC Agriculture and Natural Resources scientists.
“In pistachio and other nut orchards, roof rats are burrowing and nesting in the ground where they're chewing on irrigation lines, causing extensive damage,” said Rachael Long, UC Cooperative Extension advisor. “They are also nesting in citrus trees, feeding on the fruit and terrifying field workers when they jump out as people are picking fruit. The chewing pests are also girdling citrus limbs, causing branch dieback.”
Holes in the ground around the base of pistachio trees throughout a Yolo County orchard puzzled the grower.
“We looked for ground squirrels, but never saw any,” Long said. “We set up game cameras, but only got birds and rabbits. We put rodent bait in the holes, but the digging didn't stop.”
Long, the pest detective, cracked the case by consulting Niamh Quinn, UC Cooperative Extension human-wildlife interactions advisor based in Irvine. “She informed us that the damage we were seeing was from roof rats.”
Burrowing roof rats sounds like an oxymoron. While roof rats generally don't burrow in urban environments, their country cousins have been known to burrow.
“It's not true that they don't burrow,” Quinn said. “When I worked as staff research associate for Roger Baldwin, UC Cooperative Extension wildlife specialist, that is mostly what we studied, burrowing roof rats in orchards.”
Baldwin said, “It seems to be a good year for rats in a number of different areas and crops throughout the state. I've received more questions and comments about rats this year than perhaps the last 10 years combined. As for bait application, putting bait down burrow systems for rats doesn't usually work too well, so I'm not surprised that approach didn't work. Growers will likely have better luck with bait stations in the trees.”
Because the rats climb, Baldwin suggests attaching bait stations to tree branches.
“In addition, elevating the bait stations will eliminate access to bait for many protected mammal species, such as kangaroo rats,” Long said. “The bait diphacinone grain can be purchased from some ag commissioners' offices. This is what Roger Baldwin said they tested and it worked.”
As for the bait stations, they should be designed so that there isn't any spillage for nontarget animals to eat, Long said.
When roof rat outbreaks occur, rodenticides are often needed to prevent crop damage. However, timing is critical as diphacinone use is highly restrictive and not allowed during the growing season, which is beginning as the weather warms.
“Check the product label for application instructions,” Long reminds growers. “It's the law.”
Identifying the pest
“Roof rats can forage away from their nest, so you won't likely find signs of their activity, such as rat droppings outside their burrow, to help identify them,” Long said.
Ground squirrels are active during the day, so they are more likely to be seen, dig holes about 4 inches in diameter and forage above ground near their burrows. Vole and mouse holes are 1- to 2-inches in diameter. Roof rat holes are typically 3 to 4 inches in diameter and might have nut shells in front of them, for example pistachio or almond shells. Rabbits will feed on seedling crops, but do not dig burrows.
“Rats are sneaky and hard to spot,” Long said. “If you see damage, including digging in the soil but no wildlife, suspect rats.”
For more information on controlling roof rats, download Quinn and Baldwin's free UC ANR publication 8513, Managing Roof Rats and Deer Mice in Nut and Fruit Orchards at http://anrcatalog.ucanr.edu/Details.aspx?itemNo=8513.
- Author: Pamela Kan-Rice
“Given California's drought and the need to use all available water supplies, even those of marginal quality, there will be great interest in Ken Schmidt's and UC Cooperative Extension advisor Blake Sanden's talks about Valley water supplies and quality,” said Louise Ferguson, a UCCE specialist in the Department of Plant Sciences at UC Davis and organizer of the event.
Sanden, who is based in Kern County, will give a presentation on his research on the effects of using saline water for pistachio irrigation on crop yield and soil quality.
“In 2014, there were problems of fruit set and pollination,” Ferguson said. She expects there will be strong interest in the talk about the effects of climate and other factors on pollination requirements and fruit set by Gurreet Brar, UCCE advisor in Fresno County.
An emerging problem that growers have been seeing in California and Arizona in the past three years is what scientists are calling Pistachio Bushy Top Syndrome in clonal UCB1 rootstocks. Affected trees are short and stunted, have closely spaced internodes, exhibit bushy growth and twisted roots. The cause is unknown, but scientists have found it to be associated with the bacterium Rhodococcus.
Jennifer Randall, a professor in the Department of Entomology, Plant Pathology and Weed Science at New Mexico State University, will deliver the first public presentation of research results on the "bushy top" syndrome.
A full day of research presentations are scheduled.
Themis Michailides, a researcher in the Department of Plant Pathology at UC Davis, will give an update on pistachio diseases.
David Haviland, UCCE advisor in Kern County, Kris Tollerup, UC IPM advisor, and Bob Beede, emeritus UC Cooperative Extension advisor will discuss management of navel orangeworm, Phytocoris, leaf-footed bug and stink bugs.
Brad Higbee, director of entomology research for Paramount Farming Company, will discuss how winter sanitation of orchards can decrease pest pressure and, in turn, reduce the need for pesticides.
Joel Siegel, USDA-ARS research entomologist, will explain how to how to anticipate pest pressure based on past infestation levels.
Patrick Brown, professor in the Department of Plant Sciences at UC Davis, will discuss nutrient management in pistachios.
The 2015 Statewide Pistachio Day will be held from 8 a.m. to 4:30 p.m. at the
Visalia Convention Center. For more information, visit http://ucanr.edu/sites/pistachioday.
For more than 100 years, the University of California Cooperative Extension researchers and educators have been drawing on local expertise to conduct agricultural, environmental, economic, youth development and nutrition research that helps California thrive. UC Cooperative Extension is part of the University of California's systemwide Division of Agriculture and Natural Resources. Learn more at ucanr.edu.