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Identification & Distribution

Adult apterae of Megourella purpurea (see first picture below) are dull reddish-violet or pink or occasionally dirty yellow green. They bear longitudinal rows of black spinal, pleural and marginal sclerites bearing slightly capitate hairs (cf. Megoura viciae, which are large apple green to dark bluish green aphids with no black spinal, pleural or marginal sclerites). Their antennae are black apart from right at the base of segment III, and are 1.0-1.3 times the body length with a terminal process that is 3.8-4.4 times as long as the base of segment VI. Antennal segment III bears 1-6 very small secondary rhinaria on the basal half. The apical rostral segment (RIV+V) is about as long as the length of the second hind tarsal segment (HTII). The legs are more or less dark except for the bases of the femora and tibiae. The siphunculi are black and slightly swollen, narrowing from the middle to the apex and with a small flange. The siphunculi are 1.5-1.6 times the length of the yellowish triangular cauda. The body length of adult Megourella purpurea apterae is 2.1-2.9 mm.

Alate Megourella purpurea (see second picture above) have a purple abdomen, and the head, thorax, legs and cauda are darker than in the apterae. The dorsal sclerites are more-or-less fused into cross bands on tergites I-III. Their antennae are longer than the body, and have 26-35 rather large secondary rhinaria on segment III, and 9-16 on segment IV. The siphunculi and cauda are thinner than in the apterous viviparous female. Immature Megourella purpurea (see third picture above) are pale greenish- or pinkish-yellow with longitudinal rows of black spinal, pleural and marginal sclerites.

The micrographs below show an aptera and an alate of Megourella purpurea in isopropyl alcohol.

The clarified slide mounts below are of adult viviparous apterous and alate females.

Micrographs of clarified mounts by permission of Roger Blackman, copyright AWP all rights reserved.

Megourella purpurea lives only on meadow vetchling (Lathyrus pratensis) and does not host alternate. It feeds on the basal parts of the plant usually at or near ground level. Sexual forms have not yet been observed in Britain, but oviparae and apterous males have been reared in October in the Netherlands. The spotted vetch aphid is found in most of Europe apart from the east, and extends into Iran.

 

Biology & Ecology

Interspecific competition / association

Megourella purpurea (see picture below) often shares its host-plant Lathyrus pratensis with other species of aphids.

One of those aphids is the pea aphid (Acyrthosiphon pisum, see first picture below), another is the vetch aphid (Megoura viciae, see second picture below). Whilst the first of these is generally very acceptable to predators, the second is much less so. For example, Tsaganouet al. (2004) showed that Megoura viciae is either toxic or unsuitable for development of the harlequin ladybird (Harmonia axyridis).

van Veen et al. (2009) investigated the interactions between these three species of aphids. They first showed that both Megourella purpurea and Acyrthosiphon pisum tended to share a host plant with the unpalatable species more often than expected by chance. Further evidence suggested this was not due to differential plant suitability or location, but to a positive effect of Megoura viciae on the performance of the other two species.

To test this, field experiments were set up comparing the colony size and persistence of Acyrthosiphon pisum sharing or not sharing an individual plant with colonies of Megoura viciae. Acyrthosiphon pisum colonies tended to be larger and persisted for a longer period of time in the presence of Megoura viciae, an effect that was significant for small colonies exposed to many predators, mainly syrphid larvae. When protected from predation the presence of Megoura viciae had no effect on Acyrthosiphon pisum colonies. It was concluded that the positive effects of Megoura viciae on Acyrthosiphon pisum is thus likely to be natural-enemy mediated rather than plant mediated. It seems probable that the same is true for Megourella purpurea.

Natural enemies

We have only found Megourella purpurea on a couple of occasions, and both times in small numbers so we have few observations on the natural enemies. We did, however, find the mirid bug Plagiognathus arbustorum predating Megourella purpurea on one occasion (see picture below). We have previously recorded Plagiognathus predating Cavariella aphids on hogweed, and others have reported Acyrthosiphon pisum as prey of this mirid.

We watched the predator dispose of number of Megourella immatures over a fairly short observation period (see picture below).

The Plagiognathus above has the tip of its rostrum held against the prey and the stylets inserted through the rostrum into the aphid. The stylets consist of two tubes, one through which (toxic) saliva injected into the prey and the other through which digested juices are sucked up. Despite its voracious consumption of Megourella, it is a generalized (zoophytophagous) predator which also feeds on plant juices, so it probably has little impact on aphid populations compared to (for example) syrphid larvae which feed more or less exclusively on aphids.

 

Other aphids on the same host

Megourella purpurea occurs on 1 species of Lathyrus (Lathyrus pratensis).

Blackman & Eastop list 8 species of aphid as feeding on meadow vetchling (Lathyrus pratensis) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 7 as occurring in Britain (Show British list).

Acknowledgements

We have made provisional identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. We have used the keys and species accounts of Blackman & Eastop (1994) and Blackman & Eastop (2006) supplemented with Blackman (1974), Stroyan (1977), Stroyan (1984), Blackman & Eastop (1984), Heie (1980-1995), Dixon & Thieme (2007) and Blackman (2010). We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure provided by Blackman & Eastop (2006).

Useful weblinks

References

  • Tsaganou, F.C. et al. (2004). Effect of Aphis gossypii Glover, Brevicoryne brassicae (L.), and Megoura viciae Buckton (Hemiptera: Aphidoidea) on the development of the predator Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae). Biological Control 31(2), 138-144. Abstract

  • van Veen, F.J.F. et al. (2009). A positive trait-mediated indirect effect involving the natural enemies of competing herbivores. Oecologia 160, 195–205. Abstract