Poplar Spiral Gall Aphid

Appearance

''P. spyrothecae'' is green, red or yellow in color and smooth to the touch. The outer surface of this species develops as the edges of the petiole of ''Populus nigra'' thicken, flatten and twist. In parallel to the petiole's shape, a gall forms into a spiral shape similar to that of the inside of a snail's shell. 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. The galls typically mature in late August to early September and change color from green to red over the course of maturation. At this stage, the alatae exit the gall through many small pores along the spirals' seams. In terms of geographical distribution, ''P. spyrothecae'' are distributed across Europe, North Africa , western Siberia, Pakistan and some locations within Canada.

Behavior

It has been shown that colony defense is more likely in habitats that are difficult to obtain and can hold a large number of individuals. These gall locations are crucial because plants have a short window in which a gall can be produced. Thus, it is important for there to be a defense system that enables the aphids to retain their galls. The need for defense arises when the gall is opened up to allow winged aphid migrants to leave and to release any waste. The process of repairing the holes can take up to 10 days; during this time, the gall is susceptible to intruding predators.

Predators

''P. syprothecae'', like other aphid species that attack predators, maintain a singular thrusting motion as they pierce the predator with their stylets. Repeated stabbing rarely occurs. This species can also utilize its legs—sometimes all six—in order to squeeze the predator. The purpose of this motion is to tear the predator's cuticle and ultimately rupture its internal structures. Although detailed quantitative observations were not included in the Rhoden and Foster study, the attacking behavior was deemed costly to the soldiers. Thus, a cost-benefit analysis most likely plays a role in a soldier's decision to attack the predator.

Defense

''P. spyrothecae'' is capable of fortress defense through manipulation of the composition of aphids in galls. When introduced to a single predator, colonies with soldiers usually killed the predator, incurring the loss of a few soldiers. In colonies with only non-soldiers, predators killed the aphids and ate them.

In a later study, Foster and Rhoden examined the effectiveness of soldiers in fortress defense. They manipulated the numbers of soldiers and non-soldiers in sets of galls that remained in contact with poplar trees at the research site. When there was a combination of soldiers and non-soldiers, the gall was ten times less likely to be attacked by a predator compared to a gall with only non-soldier aphids. These results led Foster and Rhoden to conclude that soldiers effectively defend against predation under natural conditions. Earlier laboratory experiments demonstrated that soldiers could prevent specialist gall predators from attacking the gall. However, in this study, Foster and Rhoden found that such protection could also take place in the field. Five different predators attack the ''P. spyrothecae'' galls: specialist ''A. minki'', generalists ''S. ribesii'' and ''A. nemoralis'' and two other unidentified generalist predators.