Population dynamics of tree-dwelling aphids: the importance of seasonality and time scale

Ecology, Dec, 1997 by Richard Sequeira, Anthony F.G. Dixon

INTRODUCTION

Persistence of natural populations over long periods of time implies some form of regulation (Royama 1977, 1981). In recent years long-term census data on various insect and vertebrate species have been analyzed (e.g., Turchin 1990, Turchin et al. 1991, Royama 1992, Turchin and Taylor 1992, Perry et al. 1993) with the objective of identifying deterministic dynamical behavior. The impetus for these analyses was the counterintuitive conclusion from life table analyses that density-dependent regulation of abundance was infrequent in insects (e.g., Dempster 1983, Stiling 1987, 1988). A recent attempt to reconcile theoretical expectation and empirical results was Turchin's (1990) study showing that the detection of density dependence in abundance data was dependent upon the nature of the density-dependent processes involved. Turchin (1990) and others have shown that, in some cases, the detection of direct (nondelayed) density dependence may be confounded by time-lagged effects and complex dynamical behavior.

Aphid populations are likely to be regulated by density-dependent processes because parthenogenetic reproduction and the overlapping of generations, common to most aphid species during spring and summer, often result in high densities (Dixon 1985). In the past, overlapping of generations and seasonality have proved to be major obstacles in the analysis of aphid population dynamics (Dixon 1990). As a means of avoiding these obstacles, some authors (e.g., Dixon 1990, Turchin and Taylor 1992, Woiwod and Hanski 1992) have resorted to the use of annual abundance.

Turchin and Taylor's (1992) analysis of annual abundance data includes two aphid species, the sycamore aphid, Drepanosiphum platanoidis (Schr.), and the beech aphid, Phyllaphis fagi (L.). Turchin and Taylor categorized the dynamics of the former as a case of 2-yr limit cycles and the latter a case of chaos. Subsequent analyses employing cross-validation selected a simple, stable rather than chaotic model for these data (P. Turchin, personal communication). Woiwod and Hanski (1992) analyzed annual suction trap catch data for 447 species of moths and aphids in the United Kingdom and concluded that some form of density-dependent regulation is typical for these insects. The results of these studies suggest that in aphids, overall abundance in any given year may be influenced by abundance in the previous year, i.e., population regulation on a yearly time scale.

The intriguing patterns of fluctuation in annual aerial abundance have shifted attention away from the dynamics of aphid populations within years. In part, the paucity of census data measured at more frequent intervals, i.e., weekly or monthly, has obscured the role of short-term processes in the population dynamics of aphids. A single estimate of abundance per year subsumes the dramatic within-year dynamics of aphid populations (Dixon 1990). Clearly, in the case of aphids, important population regulatory processes may be operating on shorter time scales. Annual abundance data lack temporal resolution to provide insights into short-term dynamical processes that may contribute significantly to the overall dynamical behavior of aphid populations. Therefore, the nature of density-dependent regulation in aphids cannot be properly addressed without understanding the relationship between the dynamical processes operating on different time scales, i.e., between and within years.

This paper presents the first comprehensive data set on long-term fluctuations in the density of tree-dwelling aphid populations. The analyses presented here demonstrate the nature of population dynamic processes operating on short (within-year) and long (between-year) time scales for the Turkey-oak aphid, Myzocallis boerneri Stroyan (Homoptera: Callaphididae). It is argued that populations of this aphid may be regulated by a see-saw effect between density in spring, autumn, and the following spring, operating over and above a seasonally changing equilibrium.

METHODS

Life cycle of Myzocallis boerneri

In the United Kingdom, M. boerneri occurs mainly on Turkey-oak (Quercus cerris), where it is not known to have specific natural enemies such as parasitoids. The aphid can complete its entire life cycle on Turkey-oak. The cycle begins at the end of April when eggs laid the previous autumn hatch and produce the first spring generation, also referred to as fundatrices. The fundatrix and subsequent generations are winged and asexual. There are several asexual generations throughout spring, summer, and early autumn, each 2-3 wk in duration. The aphid goes through four nymphal instars before becoming an adult. The fourth-instar aphid is similar in size to the adult but lacks wings. Sexual morphs begin to appear by the middle of October, to coincide with the beginning of leaf fall. Males are winged but females are apterous. After mating, eggs are laid on the twigs. The end of November marks the end of leaf fall and the completion of the aphid's 7-mo life cycle.