Ecology of a blackberry-leafhopper-parasite system and its relevance to California grape agroecosystems

Natural ecosystems are often sources of pest arthropods and their natural enemies for nearby agroecosystems. This study treats the ecology of the native California blackberry, Rubus ursinus Cham. and Schlecht, and an imported blackberry, Rubus procerus P.J. Mueller, the blackberry leaf-hopper (BLH), Dikrella califomica (Lawson), and its egg parasite, Anagrus epos Girault, in two riparian habitats and a vineyard site. Anagrus is also an effective natural enemy of the grape leafhopper (GLH), Erythroneura elegantula Osborn, which evolved on wild grape in the riparian habitat and became a serious pest after the introduction of commercial varieties.

The parasite overwinters in immature stages in the eggs of BLH in blackberry. In the early spring, growth of the native blackberry and BLH oviposition precede grape growth and GLH oviposition by the length of one parasite generation. Since Anagrus becomes active when BLH oviposition begins, this synchrony allows the parasite to increase its populations in the riparian refuge before entering the vineyard.

The two blackberry species differ in their preferred habitats: the native blackberry is limited to moist shady habitats, while the imported species grows in open sunlit areas. In phenology the imported blackberry appears less synchronized with and less constrained by the California climate. The blackberry leafhopper produces three generations per year and overwinters as a diapausing adult. The leafhopper is well adapted to the phenology of the native blackberry, but less so to that of the imported species, which breaks dormancy in the spring long after BLH terminates diapause. Leafhopper females oviposit primarily in the lower half of the leaf canopy of the native blackberry throughout the season, while in the imported species they shift oviposition from the top of the canopy in early spring to the bottom by late summer.

A time-varying life table analysis shows that BLH immature mortality due to Anagrus, nymphal parasites, and general predators is positively density dependent. Age-specific life table analyses in the laboratory estimate that Anagrus’ intrinsic rate of increase (r_m) is 2.2 times those of BLH and GLH. Although r_m is a questionable index of parasite effectiveness, the difference probably allows Anagrus to respond rapidly to changes in GLH populations. Suggestions for the further augmentation of Anagrus populations in artificial refuges includes the management of blackberry and the monitoring and maintenance of BLH populations.