The biology of Pemphigus spyrothecae galls on poplar leaves
The gall forming aphid Pemphigus spyrothecae is a plant parasite that colonises the leaf petiole of the black poplar Populus nigra and its hybrids and varieties. Models of habitat selection are described and discussed in context with galling aphids. Habitat quality and aphid fitness can be quantified easily. Measures of relative reproductive fitness may be used to determine selection pressures driving habitat selection. The poplar trees differed significantly in budburst phenology, but Pemphigus spyrothecae stem mothers hatched on all trees within two days. Structural differences among and within trees were examined. Within and between host plant variation accounted for a dramatic difference in aphid fecundity. Larger leaves or shoots supported galls containing a higher number of offspring and a lower rate of gall failure. On singly galled shoots, gall position on a shoot did not affect reproductive outcome. Most stem mothers located their galls at the top of the petiole, closest to the base of the leaf, but gall position on the petiole did not affect the number of offspring produced. No relationship between stem mother size and fecundity could be detected. When competitor density increased on the same shoot or petiole, average fitness declined. Gall order on the petiole affected the growth rate of the gall tissue, but not the enclosed colony. However, distal galls (furthest from the leaf) were, on average, heavier and had more offspring than proximal galls. Predators are known to be important in regulating insect populations. Although, predators, on average, approached larger galls more often, this was not statistically significant. Due to the restrictive assumptions of models based on the Ideal Free Distribution, no qualitative fit of the distribution pattern of Pemphigus spyrothecae aphids was attempted. However, the characteristics of this aphid system were found to approximate an unequal competitors model. The availability of closed leaves on shoots at budburst, when stem mothers emerge, appeared to be the most important factor (of those examined) explaining the population distribution seen in spiral galling aphids.