Pemphigus betae
Pemphigus betae, also known as the sugarbeet root aphid, is a species of gall-forming aphid that forms galls specifically on the commonly found narrowleaf cottonwood, Populus angustifolia.
Sugarbeet root aphids have been found in North America and Europe. They infect sugarbeets, but also other plants like tablebeets and Swiss chard. Their size has been likened to that of a pinhead, and are pale white-yellow in color. Sugarbeet root aphids have soft bodies that are bulbous in shape, with mandibular parts that can pierce and suck and paired abdominal tubes that point backwards, and come in both winged and wingless forms.
They are known for their consequential effects on agriculture due to infestation of plants, and efforts to control the pests have proved to be difficult.
Description
Sugarbeet root aphids have a body length of. They are characterized to be as small as pinheads, and take on a pale white-yellow color. They have globular bodies that are soft, with mandibular parts that allow them to pierce and suck and paired abdominal tubes that point backwards. There are both winged and wingless sugarbeet root aphids.Habitat
The sugarbeet root aphid is found throughout the major sugarbeet growing areas of North America, and has infested areas in Texas, California, Michigan, and Alberta, Canada; it has also been introduced in Europe. The aphid has been recorded at only a few sites in Europe, but the data on other species of aphids suggest that they can increase their range of occurrence, often quickly and in an invasive manner. Infestations are usually more severe under dry soil conditions, either due to dry years in dry land conditions in the upper Midwest of the United States, or to using less water in the irrigated areas of the West and Southwest United States. Infestations are usually the most severe during July and late August.Most species of aphids, at all stages of development, move about over the surface of their host plants and even between adjacent plants. These local movements result in slow diffusive dispersal. In contrast, aphids also show persistent 'straightened out' movements during which their vegetative responses are depressed; these movements are a means of transport over larger distances. Local 'trivial' movements and distant 'migratory' movements can be recognized, and it is possible that they are the extremes of a continuum of movement that disperses all species of aphids.
Dispersal
Dispersal creates a relatively high mortality rate for sugarbeet root aphids. Additionally, alate aphids incur other disadvantages: if aphids fly then they may incur an additional cost in that their potential fecundity is further reduced and there is a further delay in the onset of reproduction. The combined effect is a marked reduction in reproductive potential and rate of increase. Although dispersal results in the colonization of suitable plants, it is not always clear what advantages there are in dispersing except from annual plants that are about to die. As dispersal can be costly in terms of fecundity or survival, or both, then sugarbeet root aphids are likely to delay departure until host quality falls below the average expectation for the habitat.Behavior
Life cycle
In the spring, a female nymph emerges from an overwintering egg and initiates a gall on one of the leaves of the Populus tree. The gall forms around the stem mother, who begins to reproduce parthenogenically while feeding on the leaf's phloem sap. Each stem mother is capable of 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 the Populus tree and asexually produce sexual males and females whose sole purpose is to mate, as they 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
Sugarbeet root aphids 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 closer to the source of nutrients flowing into the leaf-also benefit in the same way. During the aphids' emergence in the spring, large Populus leaves are colonized first. Once a stem mother forms a gall, she is more reluctant to move to a new leaf even if the current one withers, though the ability to colonize a large, healthy leaf close to its stem is crucial in ensuring an aphid's reproductive success.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 available resources, 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. Thus, 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 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 is no difference between the reproductive success of stem mothers occupying leaves singly and stem mothers sharing leaves with other individuals.
As a result of the importance of leaf choice in their reproductive success, sugarbeet root aphid 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 closest 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. A side effect of this intense territorial competition is the creation of a "floater population" of unsettled stem mothers searching for unoccupied leaves.