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3 Brown, P.M.J. & Roy, H.E. (2017) Native ladybird decline caused by the invasive harlequin ladybird Harmonia axyridis: evidence from a long-term field study. Thus, I decline in percentage abundance does not necessarily indicate a negative effect of the invader on the abundance of the native species i , as is often claimed (Alyokhin & Sewell, 2004; Grez I et al i ., 2016; Brown & Roy, 2017, etc.). It is seductive to say that an invader in reducing the abundance of native predators lowers the biocontrol potential of the whole guild (Soares I et al i ., 2008; Brown I et al i ., 2011; Roy I et al i ., 2012). [Extracted from the article]

Modelling of empirical data on the abundance of host-specific, tree-dwelling aphids indicates that their abundance is determined by the effect of the abundance of their host plants on the realised annual population growth rate ( I R i ) of the aphids (Dixon & Kindlmann, [7]). Viewed in this way it is not just the abundance of the host plant of aphids that determines the abundance of aphids (Dixon & Kindlmann, [7]) but also their potential annual population growth rate ( I r i SB p sb ) and subsequently their realised population growth rate ( I R i ). The most striking and consistent feature of rare aphid species is that the period suitable for their development and reproduction is shorter than for the common species and, consequently, the potential yearly rate of increase of these rare species is lower. [Extracted from the article]

Abstract. 1. The incidence and timing of emigration, cannibalism, and intraguild predation of larvae of three aphidophagous ladybirds (Coleoptera: Coccinellidae), Harmonia axyridis Pallas, Coccinella septempunctata brucki Mulsant, and Propylea japonica Mulsant, relative to the presence of prey was determined in the laboratory in single- and mixed-species populations. 2. In single-species populations, 80% of the larvae of C. s. brucki emigrated prior to the extinction of the aphid population and no larvae were lost due to cannibalism; however > 80% of the larvae of the other two species were still present when the aphid became extinct and the losses due to cannibalism for H. axyridis and P. japonica were 25% and 14% respectively. Finally, 28% of the P. japonica larvae completed their development, whereas no larvae of the other two species became adult. 3. In mixed-species populations, mortality of P. japonica attributable to cannibalism or intraguild predation increased greatly to 60%, whereas that of the other two species remained about the same. Consequently, survival of H. axyridis larvae improved and survival of P. japonica worsened; however the survival of C. s. brucki larvae was not affected by the other two species. Early emigration by C. s. brucki larvae may have enabled them to escape intraguild predation by H. axyridis in this system. [ABSTRACT FROM AUTHOR]

Abstract. 1. The population growth of three aphid species, Metopolophium dirhodum (Walker), Rhopalosiphum padi (L.), and Sitobion avenae (F.), on winter wheat, was analysed by regression. The calculations were based on censuses of aphids made in 268 plots at 3- or 7-day intervals for 10 years on leaves and 6 years on ears. The calculations were made separately for each plot each year, then repeated on the pooled data from all plots monitored in a year. 2. At the level of individual plots, no population growth was detected at very low densities. At high densities, the populations grew exponentially and the growth rates did not decrease with increasing aphid density. 3. Significant growth was always detected in the pooled data. These growth rates decreased significantly at the highest densities. Field estimates of the intrinsic rate of increase derived from these data ranged from 0.010 to 0.026 for M. dirhodum , 0.0071–0.011 for R. padi , and 0.00078–0.0061 and 0.0015–0.13 for S. avenae , on leaves and ears respectively . 4. The apparent lack of growth in the individual plots at low densities is attributable to small sample size. It is concluded that the natural enemy ravine in the population dynamics of cereal aphids, identified by Southwood and Comins (1976), is a consequence of low population densities at which population increase is undetectable unless very large samples are taken. [ABSTRACT FROM AUTHOR]

1. The birch (Betula)-feeding aphid, Monaphis antennata, is always found at low densities on individual hosts and has low local abundance, but another birch-feeding aphid, Euceraphis betulae, is often found at high densities on individual hosts and has high local abundance. 2. This work attempts to establish whether the interaction between M. antennata and its host or the behaviour of individuals limits its densities. 3. Both species were reared on saplings, and population sizes were monitored for 6 weeks. Two levels of host quality were used and feeding space was kept constant throughout the experiment. Adults were prevented from leaving the saplings by clipping their wings, and predators were excluded. 4. On plants of similar host quality, both species achieved similar population sizes. 5. It is concluded that resource availability or the interactions between individuals are unlikely to be important causes of rarity. [ABSTRACT FROM AUTHOR]