- Author: Sonia Rios
The roof rat (Rattus rattus, also known as a citrus rat, fruit rat, black rat, or gray rat) is an introduced species of rat native to southern Asia. It was brought to America on the first ships to reach the New World. This is the same species that carried the bubonic plague around the world and is the reservoir host for murine typhus, which is a disease that is transmitted by fleas. This primarily nocturnal vertebrate is a pest in citrus, nut orchards and other tree crops. In citrus, it builds leaf and twig nests in trees or it can nest in debris piles, thick mulch pile on the ground, or in shallow burrows under the tree. In livestock feed yards and barns, roof rats often burrow and hide under feed bunks or in the hay bales. Adult roof rats range from 12-14 inches long (30-36cm) and weigh 5-10oz. (150-250g) (UC IPM 2017). The large, sleek rat has a pointed muzzle and hairless scale-covered tail that can be longer than the body and head combined.
A rats gnawing can cause some serious damage to just about anything, electrical wires, wooden structures, and they tend to not be picky about which agriculture crop to invade. Roof rats often feed on citrus, avocados, and other fruits, sometimes leaving hollow fruit skins hanging on the tree. In tree crops, they can girdle limbs or stems, leading to mortality to part or all of a tree. After harvest, they damage fruit and nuts in bins by chewing them and leaving excrement. This can cause major esthetics damage to fruit and become a food safety issue. Since rats are active throughout the year, and mostly at night, this can be a challenge to growers and can become infestation because of their quick gestational period of 3 to 4 weeks.
Because roof rats are such good climbers, swimmers, and hitchhikers it is hard to completely exclude them from your grove or orchard. Fruit trees should be isolated, not touching fences, overhead wires, or the scaffolds or branches of other trees. Roof rats will run along fence stringer boards or support poles, phone and cable TV wires, and tree branches to reach your fruit tree. Lower branches of the tree should never touch the ground. Reducing shelter and nesting opportunity sites of rats is crucial. Eliminate debris and woodpiles and store materials neatly off the ground. Thin and separate non-crop vegetation around orchards, such as weeds and remove dead wood from fruit trees, especially in citrus and avocado (UC IPM 2017). A low-hanging skirt of drooping branches give the rats additional access routes and provides them with protective cover while feeding. It's best to prune tree skirts so that the ground under them is open and visible. This lack of cover makes the rats uncomfortable and more susceptible to predators such as snakes and birds of prey.
Sanitation is also an important component to an IPM program. Use or remove all fallen fruit, do not leave any fruit behind, as the roof rat is an opportunist and will take advantage of the mess left behind.
Monitoring and Treatment Decisions
According to the UC IPM guidelines, 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 for non-target species. Bait can only be applied during the non-bearing season, so growers must take a proactive approach to managing problematic rat populations (UC IPM 2017).
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 non-target 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 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 (such as sticky traps) are not labeled for use outdoors in orchards; these are hazardous to wildlife and should not be used.
Preventative care, sanitation and scouting for rat's nests or damage is the easiest way to stop a problem before it becomes a problem. For more information regarding the roof rat, please visit the UC IPM website: http://ipm.ucanr.edu/PMG/PESTNOTES/pn74106.html.
Baiting *Be sure to identify the species of rat present to avoid killing non-target or protected species.*
- Author: Ben Faber
Date growers in the California deserts have many insects to worry about such as carob moth, hibiscus mealybug, and giant palm borer. Now the industry is under threat from another potential pest, the highly damaging and invasive South American palm weevil (SAPW) (Rhynchophorus palmarum). It was first identified by county and state agriculture officials in 2011 in San Ysidro in San Diego. They made the discovery while looking for a closely related palm weevil, R. vulneratus (originally mis-identified as the notorious red palm weevil, R. ferrugineus), which was found in Laguna Beach and declared eradicated in Jan. 2015.SAPW has been reported on at least 35 plant species in 12 families and is especially economically important on plantation crops such as oil and ornamental palms of which date palm, Phoenix dactylifera, is a recorded host (CABI 2016; Dean 1979; Esser and Meredith 1987). SAPW has killed hundreds of Canary Island date palms (P. canariensis) in Tijuana and parts of San Diego County. These large urban infestations pose a significant risk to the multi-million dollar date palm industries (edible fruit and ornamentals) in the Coachella Valley. Losses of ornamental Canary Island date palms in San Diego County, are probably significant and likely now reaching millions of dollars in killed palms, reduced aesthetics, and increase removal costs.
SAPW has a long rostrum (this is the beetle's snout) and is large often up to 1 ½ inches to 2 inches in length (CDFA 2018). SAPW is now California's biggest weevil species! Inside the palm crown, weevil larvae feed on the meristematic tissue and it is this feeding that kills the palm crown which results in palm death. Larvae pupate inside 3-inch cocoons made of palm fibers. The pupal stage typically lasts two to three weeks. Adult weevils emerge from these protective cocoons, mate, and they are capable of flying significant distances, perhaps as far as 15 miles in a single day, to find new palm hosts. Female weevils use their snout to chew holes in the apical regions of the palm and they lay eggs in these holes. Larvae that hatch from eggs burrow into the palm crown and feed turning the meristem tissue in a fermenting “mash”. Feeding wounds that result in fermenting damage in association with aggregation pheromone released by male weevils create a highly attractive airborne cocktail of odors that weevils fly too. Adult weevils can live for at least 40 days, often longer (CDFA, 2018).
A single infected palm can result in the production of hundreds of weevils and detection of weevil infested palms at the early stages of attack can be difficult to identify because larvae live inside their host trees. The first obvious symptom of attack is a crown that is starting to collapse. Unless palms are treated within systemic insecticides at the early stages of attack, infested palms will ultimately die in as little as 2-3 months once visual symptoms become apparent.
In addition to direct physical damage SAPW inflict via feeding, it is a primary vector of the nematode that causes red ring disease (RRD), a fatal wilt disease of palms. Fortunately, RRD has not yet been detected in SAPWs or palms attacked in San Diego (Hoddle et. al. 2016). Removal of infected trees is necessary not only to remove breeding weevil populations from the environment, but also to minimize risk of harm to people, pets, and property from crown and frond drop.
More information on the SAPW invasion and to report suspect palms please visit this website: http://cisr.ucr.edu/palmarum.html