The Gall Maker Neopelma baccharidis Burck. (Homoptera:Psyllidae) on Baccharis dracunculifolia DC. (Asteraceae): Individual, Local, and Regional Patterns

Baccharis dracunculifolia, the host plant of the gall maker Neopelma baccharidis Burck., is locally distributed in delimited patches. The effects of the host plant individual (plant height, crown diameter, and crown volume), local (host plant patch), and regional (locality) characteristics on the number of galls were analyzed, in Viçosa (three patches) and in Rio Doce State Park, Marliéria (two patches), Minas Gerais State (20°45’S, 42°50’W; and 19°50’S, 42°30’W, respectively). The number of galls/individual plant increased homogenousliy with plant dimensions on local and regional scale. Gall load was also homogenous, Collevatti & Sperber 46 Herbivore occurrence is generally affected by the spatial distribution of its host plants (Strong et al. 1984). Large patches of host plants may present higher densities of specialized herbivores, because of higher probability of locating and remaining on these resources (Root 1973). Higher immigration/ emigration ratios (MacArthur & Wilson 1967, Raupp & Denno 1979, Kareiva 1985), or higher local population carrying capacity can account for these trends. If there is aggregated and unpredictable mortality, the herbivore may ”spread the risk” among host plants (Root & Kareiva 1984), leading to the opposite pattern (Courtney & Courtney 1982). The size of the individual plant may work in the same way as patch size, although not necessarily with the same trends. Larger plants may be more often found by herbivores (Feeny 1976, Lawton 1983), or may support greater populations of its consumers, mitigating the local extinction rate. Lara & Fernandes (1994) verified that the number of Neopelma baccharidis Burck. (Homoptera: Psyilidae) galls on Baccharis dracunculifblia (Asteraceae) increased with plant height and that galled branches were smaller and had less leaves than non-galled branches. Recently, the ecological literature focused on development of metapopulation models, where the dynamics of local and regional populational processes interact (Gilpin & Hanski 1991). In these models the local population dynamics can be simplified to local presence or absence of the organism, regulated by colonization and local extinction (Verboom & Lankester 1991). To evaluate such models it is important to establish the importance of local and regional processes to population dynamics. B. dracunculi folia is a dioecious perennial shrub, 4 m height, that occurs in southeastern and central Brazil, and possibly in other contiguous regions of South America, where it invades pastures (Lorenzi 1982), resprouting when cut down. Individual plants are locally distributed in well delimited patches. The jumping plant lice N. baccharidis induces the most common gall of B. dracunculifolia (Lara & Fernandes 1994). The hole leaf is swollen, with its borders joined but not melted, forming an inner chamber, covered with the gall maker’s produced silk, and inhabited by one to 10 N. baccharidis nymphs. Galls are, also, occupied by other arthropods, such as Diptera, Hemiptera, Homoptera (Aphididae), Neuroptera (Chrysopidae), Thysanoptera and Acarina immatures (RGC personal observation). These locatari (Mani 1964) may utilize the chamber for protection, consuming the gall maker’s silk and plant products, but can also predate on the gall maker, and/or gall tissues. With the gall maker’s maturity the gall opens and the adult gall maker exits the gall. Thereafter, the gall dries up and falls from the stem. This work tested if there is spatial heterogeneity of gall maker attack in two spatial but the proportion of galled plants differed on a local scale. The differences were related to local plant size differences, and not to host plant patch size. Plants without galls presented different frequency distributions of plant dimensions than galled plants, with greater number of little plants, which indicated a minimum plant size to be gall maker prone. Local factors prevailed over regional factors in determining the number of N. baccharidis galls on 5. dracunculifolia, affecting the size of individual plants and suitability to the gall maker, without modifying the relationship between gall load and plant dimensions.