Description of the Pest
The roof rat, sometimes called the black rat, is a common vertebrate pest in citrus and nut orchards. In citrus, it builds leaf and twig nests in trees or it can nest in debris piles, thick mulch on the ground, or in shallow burrows under the tree, especially in hot weather. In nut orchards, roof rats often nest around the base of trees. In livestock feed yards and barns, roof rats often burrow under feed bunks or into hay bales. This agile, sleek rat has a pointed muzzle and tail that is longer than the body and head combined.
Be sure to identify the species of rat present to avoid killing nontarget or protected species.
Be aware that endangered native kangaroo rats (Dipodomys spp.) and the riparian woodrat (Neotoma fuscipes riparia) somewhat resemble pest rats, but are protected by law.
| Roof rat | Kangaroo rat | Riparian woodrat | |
| Tail |
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| Time when active | Night | Night | Night |
| How they move | Move on all four legs | Hold their front legs off the ground and travel by hopping on their hind legs | Move on all four legs |
Unlike the hairless, scale-covered tail of roof rats, the tails of kangaroo rats and the riparian woodrat are covered with fur. The riparian woodrat is active mostly during the day, and its tail is somewhat shorter than the combined length of its body and head. A kangaroo rat's tail is noticeably longer than its body and head combined. Kangaroo rats are nocturnal, but unlike Norway rats and roof rats, which move on all four legs, kangaroo rats hold their front legs off the ground and travel by hopping on their hind legs.
Damage
Rats gnaw on electrical wires, wooden structures, and fruit and nuts on trees. They can also girdle limbs or stems, leading to the mortality of part or all of a tree. Roof rats often feed on orange, avocado, and other fruits, sometimes leaving hollow fruit skins hanging on the tree. After harvest, they damage fruit and nuts in bins by chewing them and leaving excrement. Rats are active throughout the year, and mostly at night.
Management
Cultural Control
Reduce shelter and nesting sites of rats. Eliminate debris and woodpiles. Store materials neatly off the ground. Thin and separate non-crop vegetation around orchards where feasible and remove dead wood from fruit trees, especially citrus and avocado.
Monitoring and Treatment Decisions
Baiting
The use of elevated bait stations containing 0.005% diphacinone*-treated oats (sold at some county agricultural commissioner's offices) is highly effective at controlling roof rats in orchards. Secure bait in a bait station before placing in trees on limbs 6 feet or more above the ground. Placing the bait in a secure bait station will prevent bait from dropping to the ground and creating a hazard. Bait stations should also prevent nontarget animals from feeding on the bait. Bait can only be applied during the non-bearing season, so growers must take a proactive approach to managing problematic rat populations. Specific details on a roof rat baiting program can be found in Managing Roof Rats and Deer Mice in Nut and Fruit Orchards, UC ANR Publication 8513.
Trapping
Rat-sized snap or wooden box traps placed in trees are also effective, although a more time-consuming control option. Do not use glue board traps outdoors, as birds, lizards, and other nontarget wildlife may be trapped. Rats are wary, tending to avoid baits and traps for at least a few days after their initial placement. Fasten traps to limbs and bait them with sweet fruit or nut meats, but do not set the traps until after bait is readily eaten.
Be aware that certain types of rat baits for use inside buildings are not labeled for use outdoors in orchards; these are hazardous to wildlife and should not be used.
*User must be a certified applicator or be under the supervision of someone who is. Some products also require a permit from the county agricultural commissioner for purchase or use.