Galerucella calmariensis L. has been widely released in North America to control Lythrum salicaria L., an invasive Eurasian wetland perennial. Previous studies suggest that generalist predators such as Coleomegilla maculata (DeGeer) may affect establishment and spread of G. calmariensis. The objectives of this study were to determine the preferred feeding sites of neonate G. calmariensis, to quantify neonate survivorship on L. salicaria leaves and shoot tips, to assess the relationship between larval density and residence time in shoot tips, and to determine if larval density influences survivorship of larvae on L. salicaria in the presence/absence of predators. Neonate G. calmariensis exhibited a significant propensity to ascend L. salicaria stems and conceal themselves inside shoot tips, where 77% were located after 1 h. Neonate survival in the presence of C. maculata was higher in shoot tips (70%) than on leaves (7.1%). At low densities (one to two larvae per shoot tip), G. calmariensis fed inside shoot tips for a prolonged period of time with only 25–30% exiting the shoot tip by 126 h. At higher densities, larvae left shoot tips more rapidly with 50% (seven larvae per shoot tip) and 80% (16 larvae per shoot tip) exiting by 126 h. The number of larvae that fed externally on foliage, i.e., were exposed to potential predation, similarly increased with increasing larval density. In the presence of adult C. maculata, G. calmariensis survival was inversely density dependent. At low larval densities, larvae remained concealed in shoot tips longer and were protected from predation. However, at high densities, larvae left shoot tips resulting in greater mortality by predators. This study indicates that density-dependent predation of G. calmariensis may occur as sheltered feeding sites within shoot tips become a limiting resource. This would occur under high G. calmariensis density and require the concurrent presence of effective predators. Further studies are necessary to determine if these conditions occur in nature and are of sufficient magnitude to alter G. calmariensis establishment or spread as a biological control agent of L. salicaria.
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