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Woolly oak aphidsOn this page: Stegophylla essigi
All viviparae are apterous. The eyes are small, and have an ocular tubercle with triommatidium below, and a more-or-less distinct upper half without facets. Antennae are 5-6 segmented, with segment II longer than I, and often subequal to III. The first tarsal segments are always with 2 hairs. Empodial hairs are very narrowly linear and blunt. Wax plates in viviparae are not bordered, but groups of pores are more or less bordered. Oviparae have sharply bordered subsiphuncular wax plates with facets. Siphunculi are present as elevated pores, with 5-8 hairs around them (cf. the mainly Japanese oak aphids, Diphyllaphis, which at most have 1 or 2 hairs near the siphunculi). The cauda is hardly visible, with hairs that are very much shorter than other dorsal hairs.
There are seven species in genus Stegophylla, all monoecious on North American species of oak. They are most likely all holocyclic, although anholocyclic populations are present. Species are present in both Eastern and Western USA, as well as in Mexico and Panama.
Stegophylla essigi ( Californian woolly oak aphid) California & Oregon, USA
Feeding by fundatrices of Stegophylla essigi in spring causes the edges of young leaves of its host, coast live oak (Quercus agrifolia), to curl upward (see first picture below of young galls). Over time the resultant pseudogalls become thickened and red. Fundatrices inside the gall are dark green, covered in curly white wool. They are very much like apterae of later generations, but their antennae nearly always have just 5 segments. Subsequent apterous viviparae of Stegophylla essigi (see second picture below) are broadly oval, pale grey-green to olive, covered in white wax wool. Their antennae have 5 or 6 segments. Antennal segment II is at most 2.5 times as long as the width of the mid-portion of that segment (cf. Stegophylla quercifoliae, where antennal segment II is 3.0-3.5 times as long as its midlength width) The vertex has wax glands like those on the dorsal sclerites (see below). The rostrum, when extended, reaches past the middle coxae, and the apical rostral segment (RIV+V) is 1.0-1.1 times the length of the second hind tarsal segment (HTII) (cf. Stegophylla quercicola, which has RIV+V 0.8-0.9 times as long as HTII).
First image above copyright C.A. Clark under a Creative Commons licence.
There are 6 rows of dusky sclerites down the abdominal dorsum (see clarified mount in third picture above), each sclerite bearing 1 hair, and a variable number (3-14) of roundish, evenly stippled wax-glands. These sclerites are often absent or rudimentary on the anterior tergites. On tergite VII the sclerites are partially fused to give 4 sclerites, on tergite VIII they are fused to fused to an uninterrupted band. The dorsal hairs are thin and rather long. The legs are pale, with very thick femora and stout tibiae. The siphunculi are pore-like, each set in the center of a convex sclerite of about 0.11 mm diameter with 5-7 hairs. The body length of adult Stegophylla essigi apterae is 1.2-1.8 mm.
Stegophylla essigi feeds on various native oak species in California, including California live oak (Quercus agrifolia), blue oak (Quercus douglasii), coastal sage scrub oak (Quercus dumosa), and interior live oak (Quercus wislizeni). The species is not rare, but in an area where it occurs, often only a very few trees out of many have colonies. Colonies may be found on both the upper and undersides of the leaves in summer. It is holocyclic, with oviparae and both apterous and alate males produced in November. Hille Ris Lambers (1966) found that populations may also overwinter parthenogenetically on evergreen oaks, especially between leaves spun together by caterpillars. It is so far recorded from California and Oregon in the USA.