The significance of a facultative bacterium to natural populations of the pea aphid Acyrthosiphon pisum

1. Laboratory studies have implicated various accessory bacteria of aphids as important determinants of aphid performance, especially on certain plant species and under certain thermal regimes. One of these accessory bacteria is PABS (also known as T-type), which is distributed widely but is not universal in natural populations of the pea aphid Acyrthosiphon pisum in the U.K. 2. To explore the impact of PABS on the performance of A. pisum , the nymphal development time and fecundity of aphids collected directly from natural populations and caged on the host plant Vicia faba in the field were quantified. Over 4 consecutive months June-September 1999, the performance of PABS-positive and PABS-negative aphids did not differ significantly. 3. Deterministic modelling of the performance data showed that the variation in simulated population increase of PABS-positive and PABS-negative aphids would overlap substantially. 4. Analysis of aphids colonising five host plants (Lathyrus odoratus, Medicago sativa, Pisum sativum, Trifolium pratense, Vicia faba) between April and September 2000 and 2001, identified no robust differences between the distribution of PABS-positive and PABS-negative aphids on different plants and with season or temperature. 5. It is concluded that PABS is not an important factor shaping the performance or plant range of A. pisum under the field conditions tested. Reasons for the discrepancies between this study and laboratory-based studies are considered.