Natural enemy composition rather than richness determines pest suppression

Abstract

Natural enemy (NE) biodiversity is thought to play an important role in agricultural pest suppression. However, the relative importance of the number of NE species (species richness), versus the particular combinations of species (species composition), in determining aphid suppression and ultimately crop yields, remains poorly understood. We tested the effects of NE richness and composition on pea aphids Acyrthosiphon pisum (Harris) and broad bean plants Vicia faba (Linn.). We used the larvae of two predator species, the ladybird Adalia bipunctata (Linn.) and the green lacewing Chrysopa carnea (Stephens), and the parasitic wasp Aphidius ervi (Haliday) as enemies. NEs generally reduced aphid density but did not increase final plant biomass, despite a significant negative correlation between aphid density and plant biomass. Among NE treatments, species richness had an inconsistent effect on aphid density. The composition of NEs within richness levels also affected final aphid density: the ladybird was a key species among the treatments in controlling aphid density and was especially effective in combination with the parasitoid. This ladybird/parasitoid combination also appeared to drive the higher level of suppression observed at the two, relative to three, species richness levels. Although these three species of aphid NEs are commonly used in aphid control, this is the first study, to our knowledge, that simultaneously examined these three species and highlighted the composition effect between the A. bipunctata and A. ervi. In conclusion, increasing NE species richness had an inconsistent effect on aphid density. Meanwhile, the presence of a key species (the ladybird) and its combination with a parasitoid was an important determinant of aphid biological control.

Functional diversity of predators and parasitoids does not explain aphid biocontrol efficiency

“…The traits of species (i.e., functional identity) ultimately underpins differences in the performance of natural enemies. A. bipunctata achieved the highest level of aphid suppression in our experiment, suggesting that its traits are particularly well suited to efficient aphid biocontrol, consistent with previous studies (Alhadidi et al 2018;Griffiths et al 2008;Wyss et al 1999). Other traits, such as mobility, metabolic rate or degree of dietary specialisation, might help to explain species differences (especially the strong effect of A. bipunctata) and better define the functional role of species in biocontrol systems.…”

“…We hypothesised that (H3) functional diversity would lead to a relaxation of competition and the emergence of positive non-additive effects despite the occurrence of IGP upon the mummified aphids. Finally, based on previous work (Alhadidi et al 2018;Griffiths et al 2008;Wyss et al 1999), we predicted that (H4) the coccinellid A. bipunctata will be the most efficient single NE species and (H5) its combination with the parasitoid A. ervi would prove highly complementary and outperform other combinations.…”

“…In contrast, parasitoids develop with a single host individual, and, for just a limited period of their lifespan (a few days after the adult emergence) (Bonet 2009), female parasitoids attack and oviposit on a restricted number of species. There is some evidence of complementarity between generalist predators and specialist parasitoids in the biological control of aphids (Alhadidi et al 2018;Gontijo et al 2015;Snyder et al 2004). However, many studies that have included both predator and parasitoid guilds have reported IGP upon mummified aphids (Bilu and Coll 2007;Snyder and Ives 2003;Wheeler 1977), which has been confirmed by molecular diagnostics on predator guts (Traugott et al 2012).…”

Habitat edge responses of generalist predators are predicted by prey and structural resources

“…Thus, habitat fragmentation and the creation of edges will lead to changes in community composition of predators. Such alterations to predator composition are important because predator composition, rather than predator richness, has recently been suggested as best explaining prey suppression (Alhadidi et al 2018). Predator identity often plays a large role in prey suppression, and the loss of different predator species from the same functional guild can impact ecosystem processes, even when other members of the guild are present in the community (Duffy et al 2007, Griffin et al 2013).…”

Faunistic analysis and seasonal fluctuation of ladybeetles in an agro-ecological system installed for organic vegetable production

“…However, although communities of pest insects in agro-ecosystems can partially be balanced by increased populations of their natural enemies due to vegetal diversity, the effects shall not be generalized (Alhadidi et al 2018;Altieri and Nicholls 2018). Ecological studies featuring populations of natural enemies in these systems can help better understanding such effects and, consequently, help planning management practices focused on vegetal diversity, whose premise follows agro-ecological principles, to control pests (Gliessman 2001;Altieri et al 2003;Aguiar-Menezes 2017).…”