Holly leaf‐miners on two continents: what makes an outbreak species?

Abstract

Summary1. Some herbivore species periodically undergo damaging, high‐density outbreak phases followed by less damaging low‐density phases. Others maintain steady, low to moderate density levels that do little damage to their hosts.2. Two closely related holly leaf‐miner species were compared that share many ecological traits and have very similar life cycles, but only one of which exhibits outbreaks. Phytomyza ilicicola in the eastern U.S.A. varied widely in mortality and infestation levels, reaching local densities of over 10 mines per leaf. In contrast, Phytomyza ilicis in the U.K. showed low infestation and high mortality at all sites. Using data from the literature and from field studies, the factors responsible for these contrasting dynamics were sought.3. Phytomyza ilicicola oviposits into the leaf lamina, and experiences weak larval competition only at high densities. Phytomyza ilicis oviposits into the leaf midrib, which leads to high mortality of young larvae before mine formation. Multiply mined leaves were therefore very common in P. ilicicola but rare in P. ilicis.4. Differences in the parasitoid complexes of the two systems accounted for further differences in survival to adulthood. The main (larval) parasitoid, which was found to impose high, density‐dependent mortality on P. ilicis, is missing on P. ilicicola. It is replaced by an egg–pupal parasitoid, which varies in its impact at differe∼t sites. Multiple emergence of adults from multiply mined leaves is therefore widespread in P. ilicicola but does not occur in P. ilicis.5. The differences in oviposition behaviour and in the parasitoid complexes are likely to allow P. ilicicola to outbreak when habitat conditions are favourable, while P. ilicis is always tightly regulated.

Diverse population trajectories among coexisting species of subarctic forest moths

“…Density fluctuations have been associated with monophagy (MacArthur 1955; Redfearn and Pimm 1988), high fecundity (Spitzer et al 1984), feeding in early spring, gregarious feeding habit, eggs placed in clusters, larval defences other than crypsis (Hunter 1991(Hunter , 1995a, wing reduction in females (Hunter 1995b), feeding on annual rather than perennial plants (van Emden and Way 1972), female selectivity in respect of oviposition substrate (Price 1990(Price , 1994(Price , 2003Price et al 1990;Tammaru et al 1995) or oviposition behaviour (Eber et al 2001), presence/absence of imaginal feeding (Tammaru and Haukioja 1996) and the strength of bottom-up regulation (Eber et al 2001). Furthermore, moth size may contribute to population variability either directly (Gaston 1988) or via some of the characteristics listed above (Wasserman and Mitter 1978;Niemelä et al 1981;Gaston 1988;Inkinen 1994;Lindström et al 1994).…”

Time tells: long‐term patterns in the population dynamics of the yew gall midge,Taxomyia taxi(Cecidomyiidae), over 35 years

“…Long-run studies of insect populations have been one of the most valuable and fruitful sources of data for the testing and development of theory in population dynamics (Varley & Gradwell, 1968;Crawley & Long, 1995;Hunter et al, 1997;Ylioja et al, 1999;Eber et al, 2001;Walker & Jones, 2001). Gall-causing insects have a particular advantage in this respect, in that nearly all mortality in the larval and pupal stages can be accounted for in terms of both cause and timing.…”

The geographical range structure of the Holly Leaf‐miner. III. Cold hardiness physiology

“…This means that single collections of leaves can reveal much about the population dynamics of the species at a site (which has made it a popular system for teaching about population dynamics; Metcalfe, Marcal & Gaston 2000; http://www3.open.ac.uk/courses/bin/p12.dll?CO1S328_biology). Fourth, numerous studies have been conducted on the local population dynamics of this and other closely related leaf‐miner species (McGeoch & Gaston 2000; Brewer 2000; Bonsall & Eber 2001; Eber 2001; Eber et al . 2001).…”

“…Considering likely external constraints influencing the structure of the geographical range of the Holly Leaf‐miner, an explanation has not thus far been provided for one of the most significant forms of mortality. Throughout the majority of the range, the highest proportion of individuals at a site succumb to larval mortality that cannot be directly attributed to parasitoids (Eber et al . 2001; Brewer & Gaston 2003) and has been thought likely to result, at least in part, from adverse climatic conditions, particularly temperature (Brewer & Gaston 2003).…”

“…The causes of the additional mortalities are unknown and are classified as miscellaneous larval and pupal mortalities (Heads & Lawton 1983). Miscellaneous pupal mortalities account for a rather small proportion of deaths in most areas (Eber et al . 2001) and will not be considered further.…”