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Pemphigus betae is a species of gall-forming aphid that specifically forms galls on the poplar tree Populus angustifolia.

Life cycle
In the spring, a female nymph (called a stem mother ) emerges from an overwintering egg and initiates a gall on one the leaves of the tree. The gall forms around the stem mother, who begins to reproduce parthenogenically while feeding on the leaf's phloem sap, creating up to 300 progeny per gall. The gall occupants develop wings in the middle of the summer and disperse from the gall to deposit their larvae in the ground. These larvae colonize and feed on the roots of nearby Chenopodiaceae plants for the rest of the summer. In the summer, they form alate migrants that fly back to Populus and asexually produce sexual males and females whose sole purpose is to mate, and who even lack mouthparts to feed. The product of sexual reproduction is a single egg in each female which is deposited in the tree's bark and left to overwinter.

Leaf colonization
Pemphigus betae are closely synchronized with their hosts Populus Augustifolia, with the majority of stem mothers colonizing leaves within three days after the leaf buds burst. There is intense competition between Pemphigus stem mothers over leaf choice - galls formed on larger leaves have higher stem mother weight, more aphids overall, and a lower probability of being aborted. Moreover, galls formed closer to the leaf stem -- and thus to the source of nutrients flowing into the leaf -- also benefit the same way. Once a stem mother forms a gall, she is more reluctant to move to a new leaf even if the current one withers, and the ability to colonize a large, healthy leaf close to its stem is crucial in ensuring an aphid's reproductive success. During the aphids' emergence in the spring, large Populus leaves are colonized first.

On average, stem mothers distribute themselves among the leaves of a Populus tree according to the ideal free distribution model. Stem mothers sharing a leaf have to split the resources available, and this sharing comes at a cost to the stem mother's reproductive success. Two stem mothers sharing a leaf spend a significant amount of their time engaging in territorial behavior instead of feeding or gall forming, and so stem mothers sharing a leaf produce less offspring than single stem mothers on leaves of the same size. When one stem mother is removed from a shared leaf, the reproductive success of the remaining occupant(s) rise accordingly. Some stem mothers choose to settle alone on smaller leaves instead of sharing a larger leaf with another individual. This creates a scenario where, on average, there's no difference between the reproductive success of stem mothers occupying leaves by themselves and stem mothers sharing leaves with other individuals.

As a result of the importance of leaf choice in their reproductive success, Pemphigus betae stem mothers are highly territorial and will compete with each other for the chance to form galls at the bases of the largest leaves. This competition usually takes the form of kicking and shoving contests -- two stem mothers will align rear-to-rear and push against the other forcefully using their hind legs. The winner of these contests then settles close to the leaf base, and the loser settles more distally. If the basal stem mother dies or is removed, the distal stem mother often moves down to the base of the gall to replace her. These territorial contests are unusually long, and can span the course of several days. One side effect of this intense territorial competition is the creation of a "floater population" of unsettled stem mothers searching for unoccupied leaves.

Gall formation
P. betae stem mothers induce gall formation in the leaf by probing the leaf tissue with their stylets. This leads to the formation of a small depression on the leaf, which eventually closes up over the stem mother and forms a gall. The extent of the probing activity dictates gall size, and removing the stem mother early on in the process leads to the formation of an unclosed, rudimentary gall. The extent of probing activity is correlated with the aphid's reproductive success. The probing activity may also serve a secondary purpose in helping the aphid determine leaf size. Leaf size is an important factor in determining what leaf a stem mother chooses to develop a gall on, but at the time of leaf colonization, leaves that are being colonized are only a quarter to a tenth of their size at maturity. It has been suggested that, during the probing of the leaf by the stem mother that leads to gall formation, the stem mother gauges the chemical environment of the leaf and uses that as a predictor of final leaf size and suitability as a gall site.

Interaction with the leaf
The benefits of settling basally are significant, with basal stem mothers producing 49-65% more offspring than their distally settled counterparts. The benefits of settling basally relate to the aphid's ability to manipulate the plant's food resources. The galls formed by P. betae act as physiologic sinks, diverting and intercepting the plant's normal transport of resources and nutrients. 14C labeling experiments have shown that P. betae galls intercept resources being transported from the midvein to the distal parts of the leaf. In addition, these galls are able to divert 14C from neighboring leaves. One study showed that, on average, 29% of the 14C accumulating inside a basal gall was supplied by a neighboring leaf and not the leaf the gall itself was on. In contrast, neighboring leaves only supplied 7% of a distal gall's 14C, illustrating the importance of settling basally.

Fruiting bodies also act as resource sinks, and the same study found that the removing female catkins near an aphid's gall led to a 31% increase in gall success.

Tree Colonization
After emerging in the autumn, the migrant forms of P. betae seek out Populus trees to colonize. These migrant forms are short-lived, and usually die within 12 to 48 hours. In selecting trees to colonize, the autumn migrants of P. betae prefer to colonize larger trees over smaller ones, and are likely to use simple cues such as tree crown size or tree resistance to colonization to decide one which trees to colonize. Despite the importance of leaf size to stem mothers, autumn migrants do not appear to take leaf size into consideration when choosing a tree to colonize.