1 3 Oecologia DOI 10.1007/s00442-014-3024-5 COMMUNITY ECOLOGY - ORIGINAL RESEARCH Bottom-up impact on the cecidomyiid leaf galler and its parasitism in a tropical rainforest Geoffrey M. Malinga · Anu Valtonen · Philip Nyeko · Eero J. Vesterinen · Heikki Roininen Received: 2 April 2013 / Accepted: 9 July 2014 © Springer-Verlag Berlin Heidelberg 2014 Total leaf area was positively associated with galler density, but within galled replicates, the galled leaves were larger than the ungalled leaves. Although highest levels of ferti- lization and density of host trees caused significant change in the densities of parasitoids, the rate of parasitism did not change. However, tree-density manipulations increased the rate of inquilinism, but on a very low level. Our results demonstrate a trophic cascade in the tropical galler and its parasitoids as a response to bottom-up effects. Keywords Plant quality · Plant vigour · Resource concentration · Trophic cascade · Tropical gallers Introduction The relative importance of host-plant resources (bottom-up forces) and natural enemies (top-down forces) in control- ling the population of galling insects and their parasitoids is poorly known for tropical gallers. It has been considered that both bottom-up and top-down factors act simultane- ously in the regulation of populations (Hunter and Price 1992). However, some studies have provided strong sup- port for bottom-up factors over top-down forces (Roininen et al. 1996; Price and Hunter 2005; Cornelissen and Stiling 2006). Host-plant resource quality and quantity (host den- sity) are among the most important bottom-up factors (Stil- ing and Moon 2005), and both might influence the herbivore population by altering feeding preference or performance. Consequently, two major hypotheses have been advanced to explain the importance of bottom-up factors: the plant vigour hypothesis (PVH) (Price 1991), and the resource concentration hypothesis (Root 1973). Accord- ing to the PVH, vigorous plants support higher densities of insect herbivores (those whose larval development is Abstract The relative importance of host-plant resources, natural enemies or their interactions in controlling the population of galling insects and their parasitism is poorly known for tropical gallers. In this study, we assessed the impacts of plant quality and density of host trees in regu- lating the densities of a galler species, the cecidomyiid leaf galler (Cecidomyiini sp. 1EJV) and its parasitoids and inquilines on Neoboutonia macrocalyx trees in Uganda. We manipulated the nutritional quality (or vigour) and the resource concentration with four levels each of fertiliza- tion and the group size of host tree. We then recorded the effects of these treatments on the growth rate and total leaf area of host plants, the density of gallers and their mortal- ity by parasitoids and inquilines. Higher levels of fertiliza- tion and host density resulted in significantly higher total leaf area than did ambient nutrient levels, and lowest tree densities, respectively. Fertilization also caused significant change in the growth rate of leaf area. Both higher ferti- lization and host density caused higher density of gallers. Communicated by George Heimpel. Electronic supplementary material The online version of this article (doi:10.1007/s00442-014-3024-5) contains supplementary material, which is available to authorized users. G. M. Malinga (*) · A. Valtonen · H. Roininen Department of Biology, University of Eastern Finland, P.O. Box 111, 80101 Joensuu, Finland e-mail: malingageoffrey@yahoo.com P. Nyeko Department of Forestry, Biodiversity and Tourism, Makerere University, P.O. Box 7062, Kampala, Uganda E. J. Vesterinen Section of Ecology, Department of Biology, University of Turku, 20014 Turku, Finland