The Napa Valley is home to a diverse group of inhabitants, each with their own wants and needs. The economy is largely based upon the success of grape growers and vitners, residents require safe places to live with clean air and water, and wildlife depends upon the preservation of habitat for food and shelter. Providing for the needs of one group without taking too much from another is a delicate balancing act, and being good stewards of the watershed requires thoughtful decision making to maintain harmony. Recent studies of birds and bats are revealing new ways to achieve this balance and improve the overall quality of life in the Napa River watershed.
Managing pests in the Napa River watershed is one challenge in maintaining balance in the watershed. Wine grapes are a very important crop in the Napa Valley, and optimizing this crop's production requires the management of pests and rodents that threaten the fruit and vines. If rodent and insect populations grow too large, the economic damage these pests can cause to vineyards would be devastating to the community. Moles and voles chew on vines and roots and insect infestations can slow growth of the fruit and vines as well as spread bacterial and fungal infections like Pierce’s Disease and Botrytis. In an effort to protect vineyards and crops, people have used different chemicals to target these pests. However, an unfortunate side effect is that these chemicals can make their way into our water and other parts of the ecosystem. They can be washed into nearby streams and waterways, and unintentionally passed on to predators and other animals. This can create a damaging ripple effect through the food chain, killing other species that are actually helpful to maintaining healthy vineyards. Because of this, many farmers and residents look for non-toxic solutions to controlling rodent and insect populations without introducing harmful substances into the watershed.
One natural approach to pest management is the introduction of predatory species. Attracting animals that feed on the species you want to control is part of an integrated pest management (IPM) approach, combining technology, science, and environmental resources to control pest populations. Barn owls, bluebirds, bats are just a few of the species Napa farmers are relying on to help control pest populations. Owls feed on rodents that threaten roots and vines, while bats and bluebirds eat the insects that transmit diseases and harm the vines and fruit. Habitat destruction has reduced the populations of these beneficial species that were once abundant in the Napa Valley. The massive reduction of oak woodlands dramatically limited the availability of adequate nesting locations and shelters, especially in the case of cavity nesters. This category of creature, which includes barn owls, bats, and bluebirds, builds their home in old barns, hollow trees, and branches. In order to recruit these pest management helpers, new homes and habitats must be built.
Efforts to re-oak the Napa Valley will eventually increase habitat for barn owls, bluebirds, and bats once the trees grow and mature, but it will take several decades. In the meantime, farmers and local community members are building and installing bird and bat boxes to provide nesting spaces, attracting these beneficial species to their vineyards. Built to suit the specific needs of the species they hope to attract, these boxes are mounted on tall poles and then installed in vineyards and parks. Bat boxes are also added to existing structures such as barns and outbuildings. These different boxes simulate what each species prefers in the wild- a cavity where a small opening provides limited access to a space large enough to provide shelter for nestlings without being too crowded. Volunteers monitor bird boxes throughout the year and clean them once the young leave the nest. Researchers and local birders keep records of which boxes are inhabited each season as well as the success of each brood (family of yound birds) to learn more about the effectiveness of the boxes and how local barn owl and bluebird populations can be best supported. You can see some of these boxes in person in parks around Napa, including bluebird boxes at O’Brien Park and owl boxes at Trancas Crossing Park.
Rodents such as pocket gophers and voles are of particular concern in vineyards. Pocket gophers will gnaw the base and roots of a vine, destroying a plant in a matter of hours, while voles chew on the bark of the vine as well as irrigation lines, making a vineyard more susceptible to disease. Barn owls are effective at managing pocket gopher and vole populations in vineyards. Recent studies estimate that a Barn owl family consumes over 3,400 rodents a year, providing an ecologically sound alternative to rodenticides (rodent poisons) for vineyards. Rodenticides are effective at killing rodents, but they will also kill other animals, wither directly or from secondary poisoning. Owl boxes provide an inexpensive solution to this problem by providing spaces for female owls to nest and raise their young, introducing natural predators to the vineyards to reduce rodent populations.
Another species that helps control pests in the vineyards are Western bluebirds. Like owls, bluebirds are also cavity nesters. Bluebirds are insectivore, which means they only eat insects and not any grapes. Recent studies on bluebirds in Napa Valley shows that they prefer to eat mosquitos, as well as insects harmful to grape vines such as leafhoppers, sharpshooters, and moth larvae. Leafhoppers and sharpshooters spread Pierce’s Disease in vineyards, which makes the vines unable to distribute water throughout their limbs, vines, and leaves, causing them to wither. Damage caused to leaves, fruit, and buds by moth larvae makes vines more susceptible to Botrytis fungal infections, which can require the removal of entire vinyards. Because many of the natural nesting cavities preferred by bluebirds have been destroyed, attracting bluebirds to vineyards requires the installation of small bird boxes designed to keep predators out and spaced away from other bluebirds to minimize competition for resources. Once established in a box, bluebird mating-couples will produce a clutch of 4-6 eggs twice a year. A bluebird family will eat 124g of insects a day- that is the equivalent of 230 grasshoppers or a brown paper grocery bag full of insects every 24 hours! It's no wonder farmers want to attract these voracious eaters to their vineyards- bird boxes can be a more cost effective and environmentally friendly option to insecticides. As vineyards move toward more sustainable practices, bird boxes are a win win, since the birds have new homes and there are fewer harmful bugs in the vineyards.
Another thing with wings that can help manage pests in vineyards is the bat. The only mammals that can fly, bats are an unsung hero of vineyard pest management. Northern California is home to 17 species of bats, 4 of which are commonly found in the Napa Valley. Mexican Free-Tailed bats are the most beneficial visitor to vineyards, living in large colonies of up to 1,000 bats. Zipping through the dusk and dawn skies at speeds of 60-100 mph snatching up insects, these bats are the fastest mammals on earth. Echolocation (using sound waves to locate objects) helps the bats track down and capture insects in the dark night sky. Bats eat grape pests such as the grape leaffolder, grape bud beetle, light brown apple moth, glassy-winged sharpshooter, western grapeleaf skeletonizer, European grape moth, and orange tortrix. Bats will consume over half their body weight every day when feeding; pregnant bats will consume their entire body weight! Estimates show that bats save the US agricultural industry up to $53 billion a year on pest control services.
To attract bats, which are also cavity dwellers, farmers can install specialized boxes for the bats to roost in during the day, and for mothers to birth and raise their young. Although you can mount bat boxes on poles or posts like bird boxes, bats prefer their boxes mounted on structures, preferably east facing. This helps the bats regulate their temperature in the hot summer days, warming with the morning sun and staying cooler in the afternoon. Bats also prefer a different structure to their homes. Unlike bird boxes, which have a small hole in the side to enter and a large open cavity inside to nest, bats prefer a box with narrowly spaced slats to mimic the bark gaps in trees and narrow rock crevices they would choose for roosting(sleeping and resting) in the wild. Bat boxes are open on the bottom to allow bats to drop out to fly away.
As we gain a better understanding of the life cycles of bats, owls, and bluebirds in our region, we can make smarter choices that attract these predators during the critical life stages of the pests that threaten to cause harm. By integrating the cycles and habitats of wildlife into farming and pest management plans, farmers are saving money on chemicals, integrating more sustainable farming practices, and helping to keep harmful chemicals out of our watershed. Whoooo would have thought minimizing pollution could be as simple as putting up a box?
Check us out! Books and resources available from the Napa County Library:
1). Barn Owls: Nocturnal Hunters by Rebecca Rissman
2). Owling: Enter the World of the Mysterious Birds of the Night by Mark Wilson
3). The Bat Scientists by Mary Kay Carson
4). The Secret Life of the Little Brown Bat by Laurence Pringle
5). Handmade Bird, Bat, and Bee Houses: 25 Homes, Feeders, and more to Attract Wildlife into your Garden by Michele McKee-Orsini
6). Songbird, Bat and Owl Boxes: Vineyard Management with an Eye Toward Wildlife by Emily Heaton
Go deeper! Explore these great resources online:
Bat Appreciation Day: Napa Valley
Bat Conservation International: All About Bats
Barn Owls: The Secret Saviors of Napa Valley's Vineyards (Video)
Barn Owl Maintenance Program
Wild Farm Alliance: Tres Sabores Success Story (Video & Interactive Map)
eBird: All About Bluebirds
Bohmeian Article: Put a Bird on It! Owl & Bluebird IPM
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