Pemphigus spyrothecae, or the poplar spiral gall aphid, is a social insect which exhibits apparent altruistic behaviors.

[4] The fundatrix, or parthenogenetic female aphid produced on the primary host plant from an overwintering fertilized egg, is pale green; these individuals allow the second generation alatae to form within the gall.

[7] Pemphigus spyrothecae is in the superfamily Aphidoidea, in the hymopterous division of the order Hemiptera, which consists of insects with sucking parts of the mouth.

A pair of short cornicles protrude from the last abdominal segment, in addition to the cauda, a posterior projection on the tip of the abdomen.

The aphid soldier exhibits its first distinguishing physical traits as a first-instar nymph, the phase following its complete maturation in the egg.

[10] Monomorphic first-instar nymphs of Pemphigus dorocola attack moth larvae, a predator, when it is experimentally introduced to the gall.

[11] After observing a physical resemblance between these aphids and the thick-legged P. spyrothecae larvae, Aoki suggested that these insects also defended the gall.

[17] They manipulated the numbers of soldiers and non-soldiers in sets of galls that remained in contact with poplar trees at the research site.

[19] Although detailed quantitative observations were not included in the Rhoden and Foster study, the attacking behavior was deemed costly to the soldiers.

Since every generation within the gall displays a high degree of relatedness due to cloning, any deviation from a colony's genetic uniformity (excluding mutation) can be traced back to intergall migration.

Given the fact that a gall was defendable and valuable, the authors anticipated that this served as a barrier to the intrusion of other foreign clones via clonal mixing.

Johnson et al. found that while soldiers typically migrate and have the capability to moult and reproduce outside of its own gall, clonal mixing was low overall.

Given this insignificant level of clonal mixing, there is a low chance for barriers to the evolution and propagation of an altruistic soldier caste within the species.

Ultimately, Johnson et al. (2002) became the first researchers to utilize species-specific nuclear DNA markers in order to get a clear sense of the degree of clonal diversity within the P. spyrothecae galls; with the additional usage of trapping data, their prediction of intergall migration was also confirmed.

[13] Since trapping data confirmed that all migrants are soldiers, Johnson et al. infer that these aphids can grow and produce winged emigrants in galls as an adaptive strategy of cloning instead of a chance event.

So, the researchers concluded that the aphids do not adapt to short-term changes in the colony via a reduction of clonal mixing (an increase in soldier reproduction).

[21] The primary explanation for this apparent altruism is explained through kinship; the average degree of mixing in P. spyrothecae was 0.68% which indicates a high relatedness between members of the gall.