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Turkey oak aphidIdentification & Distribution
Identification & Distribution:Winged adults of Myzocallis boerneri are yellowish, with the head and thorax sometimes partly dusky. Their dorsal abdomen has small paired spinal and marginal specks of brown pigment. The antennae are pale but ringed with brown-black. The length of the last segment of the rostrum is usually less than 1.2 times the length of the second segment of the hind tarsus (distinguishes from Myzocallis schreiberi ). The body length of Myzocallis boerneri apterae is 1.3-2.2 mm.
Immature Myzocallis boerneri are pale yellow with paired, dusky, rather indistinct spinal and marginal spots.
The Turkey oak aphid lives on the undersides of leaves of several oak (Quercus) species, especially the Turkey oak (Quercus cerris), but also holm oak (Quercus ilex) and sessile oak (Quercus petraea). Myzocallis boerneri is widely distributed in Europe, the Middle East, and has been introduced to New Zealand, California and Argentina.
Biology & Ecology:
In Britain Myzocallis boerneri is (relatively) host specific on turkey oak (see picture below) and without specialized insect natural enemies.
There is then a distinct seasonal pattern of abundance with a period of spring increase, summer decrease, early autumn increase and late autumn decline. Their numbers are regulated by density dependent processes which have been investigated in depth by Dixon and his co-workers. The seasonal fluctuation in abundance is known to follow changes in host plant quality (Dixon, 1970). By virtue of short generation time and programmed anticipation of seasonal trends these aphids track changes in habitat quality very closely. Migration is the most important factor determining the summer decline in abundance (Kindlmann & Dixon, 1996 ). Intraspecific competition for resources in summer results in immediate size related reduction in recruitment and increase in the tendency to migrate (Dixon et al., 1996 ).
Sequeira & Dixon (1996) investigated the seasonal changes in life history traits under field conditions and the effects of high density on body size and reproductive potential of Myzocallis boerneri. Aphid body size is at its maximum in early spring, soon after bud burst, and declines progressively as the turkey oak foliage matures. The fat content of individuals, as a proportion of total weight, remains constant through the season. By comparison a small linear decline in the size of the soma results in an exponential decline in the size of the gonads. At small body size, embryo size is conserved presumably so that the survival of nymphs born in the summer is maximised.
Sequeira & Dixon (1997) analyzed changes in the weekly abundance in two natural populations (i.e. on 2 trees) of Myzocallis boerneri, from 1975 to 1992, using autoregression techniques to determine the nature of dynamic processes. Analysis of time-series of weekly and monthly data indicated statistically-detectable seasonality in density changes. The monthly data, with seasonal effects removed, showed that density fluctuates around a seasonally-changing equilibrium value. A statistical test applied to the seasonally-adjusted monthly data revealed density-dependence. Further analysis indicate time-lagged density-dependent processes operating between seasons, and possibly on a shorter time scale of about a month. There was evidence for the existence of a 'see-saw' relationship between density in spring, autumn, and the following spring. The pattern of changes in abundance between years is most likely the consequence of the see-saw effect operating between seasons. Sequeira & Dixon argued that aphid population density was regulated by means of density-dependent processes (mainly intraspecific competition) acting within years, which was reflected in the year-to-year changes in overall abundance.
Jarosik & Dixon (1999) focused on regulation and density-independent processes for the same data (from 2 trees). On one tree the aphids exhibited a distinct seasonal pattern with a spring increase, summer decrease, early autumn increase, and late autumn decline. Significant under-compensating density-dependence occurred during all periods of the seasonal population development, and their strength varied little during the course of the season. On the other tree the aphids remained at low densities after the decrease in summer. Significant density dependence compensated exactly for spring increase, but appeared only after the decrease in summer when the population remained at very low densities for the rest of the season. Density-independent weather-variables affected the population dynamics very little. Their influence was marginally significant only at very low densities, when the aphids were regulated exactly by compensating density-dependent factors. The results suggest a curvilinear density-dependence, with strong regulation at low densities, and weak at high densities. In other words this aphid was most regulated not at the peak but at the trough densities.
Although many natural enemies, such as coccinellid and syrphid larvae (see pictures below) can be found attacking Myzocallis boerneri, they are not generally thought to play a major role in its population regulation. There does not appear to be a host-specific parasitoid, at least not in Britain.