Abstract
Some experiments indicate that ladybirds can significantly suppress aphid abundance. For example, exclusion of predators by caging aphid-infested plants repeatedly results in higher aphid populations and faster aphid population growth rates. However, aphidophagous ladybirds have never proved effective in controlling aphid populations in the field, which is consistent with the theoretical prediction that long-lived predators cannot be effective in controlling a short-lived prey (the generation time ratio hypothesis, GTR). To resolve this paradox, field experiments, involving two species of ladybirds, Coccinella septempunctata bruckii and Harmonia axyridis were used to determine their efficiency in suppressing populations of the aphid, Aphis gossypii, on small shrubs of Hibiscus syriacus under natural conditions. Instead of by caging, the effect of each ladybird species on aphid population dynamics was determined by removing all the eggs of C. septempunctata from 8 shrubs, those of H. axyridis from a further 8 shrubs, all those of both species from an additional 12 shrubs and leaving the eggs on 6 control shrubs. These predators did not have a negative effect on the peak numbers of the aphids. Thus one should be cautious when interpreting the results of cage experiments, used to assess the efficiency of predators in reducing the abundance of their prey.
Highlights
The outstanding success of the ladybird beetle, Rodolia cardinalis Mulsant in controlling the cottony-cushion scale, Icerya purchasi Maskell, an important pest of citrus in California late in the nineteenth century, resulted in the widespread use of ladybirds as biocontrol agents (Dixon and Kindlmann, 1998)
12.9 egg batches of H. axyridis and 13.9 of C. septempunctata bruckii were laid on each shrub in 2000 and 2001
The results indicate that the presence of predators had no significant effect on the peak numbers of aphids (Two-Way ANOVA gives F = 95.8, P = 0.002 for the effect of year and F = 4.95, P = 0.11 for the effect of predator removal), despite the large numbers of ladybird larvae that hatched on shrubs, from which predators were not removed (Table 1)
Summary
The outstanding success of the ladybird beetle, Rodolia cardinalis Mulsant in controlling the cottony-cushion scale, Icerya purchasi Maskell, an important pest of citrus in California late in the nineteenth century, resulted in the widespread use of ladybirds as biocontrol agents (Dixon and Kindlmann, 1998). It is surprising that many quantitative assessments indicate that coccinellids can substantially suppress aphid density below the predator-free value (e.g., Chambers et al, 1983; Michels et al, 2001; Symondson et al, 2002; Basky, 2003; Cardinale et al, 2003; Snyder et al, 2008, 2004; Klausnitzer and Klausnitzer, 1979; Thies et al, 2011). This apparent contradiction needs to be resolved
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