THE ASSOCIATION OF THE ROOF RAT (Rattus rattus) WITH THE HIMALAYAN BLACKBERRY (Rubus discolor) AND ALGERIAN IVY (Hedera canariensis) IN CALIFORNIA

The roof rat (Rattus rattus) u t i l i z e s Algerian ivy and the Himalayan blackberry for food and cover, often l i v i n g independent of man. Algerian ivy is the most popular ornamental and ground cover plant in California and is used extensively for landscaping, particularly in southern California. The Himalayan blackberry inhabited by feral roof rats, grows abundantly in northern C a l i f o r n i a along i n l a n d creeks and in pasture land of the Sacramento Valley and in the foothills of the Sierra Nevada. Rats construct platform nests on or within the dense layer of canes that accumulate w i t h i n the thickets. Information on d i s t r i b u t i o n and recommendations for the control of the roof rat w i t h i n the blackberry habitat are presented. A potential human plague threat exists where rodent cohabitants of the berry thickets may become involved in epizootics. The roof rat (Rattus rattus) is p r i n c i p a l l y an outdoor species in C a l i f o r n i a as it is in most parts of the world and i t s role here is p r i m a r i l y that of a pest. It is commonly found l i v i n g in association with Algerian ivy (Hedera canariensis) in southern California, and with the Himalayan blackberry (Rubus discolor) which grows w i l d in northern C a l i f o r n i a . In both situations the roof rat is a b l e to obtain the necessities of l i f e without supplementary food or cover. Both plant species are important not only because they supply food and cover for rats but because of their overabundance and widespread d i s t r i b u t i o n . It should not be surprising to find the roof rat l i v i n g independent of man inasmuch as t h i s species probably experienced an e a r l i e r period of development in an Asian habitat that possesses some of the characteristics of both the Himalayan blackberry and Algerian ivy. ROOF RAT COLLECTIONS IN CALIFORNIA The currently known distribution of the roof rat in California is shown in Figure 1. Collections from coastal and i n l a n d southern C a l i f o r n i a are p r i m a r i l y from residential areas heavy w i t h Algerian ivy and other ornamental plantings. However, in northern California, i n c l u d i n g the Central V a l l e y , roof rats are p r i m a r i l y associated w i t h the Himalayan blackberry. Thus the d i s t r i b u t i o n of t h i s plant and i t s tendency to form large, dense thickets become of primary importance in determining roof rat occurrence. ALGERIAN IVY--ROOF RAT ASSOCIATION The roof rat--Algerian ivy association presents a major problem in many areas. In Los Angeles County over 75% of the rat complaints (including Norway rats) received by the County Health Department concern roof rats l i v i n g in Algerian i v y (J. C. Ruddock, personal communication). In the San Francisco Bay area, ivy is responsible for a substantial number of residential roof rat infestations, but the problem becomes progressively less significant northward where colder winters tend to restrict i t s growth. Infestation rates of 25-50% occur in yards of some newer tract neighborhoods that are set amidst former orchards and 20-30% in older residential areas w i t h heavy shrubbery. After about 10 years, trees and shrubbery have u s u a l l y become s u f f i c i e n t l y dense for roof rat h a b i tation and often by the 15th year a chronic problem has developed (Brooks, 1966). Rampant w i l d growth of Algerian ivy has not become a problem as it is dependent upon a constant water supply for i ts survival and grows p r i m a r i l y in managed situations. A future problem could, however, develop around permanent water sources. Vertical growths of ivy along fences or w a l l s are excellent avenues for movement in a neighborhood or premises. Our records indicate heavy use of vertical growths of ivy for nesting and feeding with additional use of adjacent f l a t or s l o p i n g surfaces for feeding, particularly under shrubbery or tree branches that offer some concealment. Although Algerian ivy is capable of supporting roof rats by itself, a variety of man-made harborages are sometimes more readily accepted. Some favorite s i t e s are firewood p i l e s , pyracantha shrubs, date palm trees, honeysuckle vines and many other dense growth ornamentals.