The activities of generalist parasitoids can be segregated between crop and adjacent non-crop habitats

Abstract

Non-crop habitat adjacent to crops may be important for enhancing the activity of natural enemies in crops. However, it is not always clear whether natural enemies that are active in non-crop habitats actually contribute to pest suppression in adjacent crop habitats. We hypothesised that parasitic wasps that utilise the same hosts can be segregated between crop and non-crop habitats in an agro-ecosystem. We tested this hypothesis using the light brown apple moth, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), in vineyards and adjacent native vegetation. We experimentally measured the parasitism rate of larval E. postvittana at six and eight sites in both vineyards and the adjacent native vegetation in two consecutive years. Wild larval Tortricidae were also collected at each experimental site to assess their diversity and related parasitoids. Parasitised hosts were then identified using a PCR-based protocol to examine the parasitoids' host ranges. The parasitoid Therophilus unimaculatus (Turner) (Hymenoptera: Braconidae) was most active in non-crop native vegetation, whereas Dolichogenidea tasmanica (Cameron) (Hymenoptera: Braconidae) parasitised the most larvae in vineyards. Parasitism of E. postvittana by D. tasmanica was higher on grape than on plantain, which indicates that host plants influence activities in different habitat. Both species shared the same range of tortricid hosts. Overall, our results indicate the two key parasitoids that attack E. postvittana differ in their pattern of habitat use. The native vegetation adjacent to crops may not enhance the activity of some natural enemies for pest control in an agricultural ecosystem.

A dearth of data: fitting parasitoids into ecological networks

“…Firstly, barcoding can be used to identify parasitoids that have been reared from hosts, or the hosts themselves, when morphological identification is difficult or cryptic speciation is expected [7,12,43]. This approach has been used within an agricultural context to evaluate the importance of non-crop habitats for pest control [44] and cross-habitat indirect effects [7]. The molecular techniques involved are identical to those described previously for identifying individual parasitoid specimens and are similarly limited by rearing methods.…”

Direct and indirect influences of intercrops on the coconut defoliator Opisina arenosella

“…Further, the presence of apparent competition in agro-ecosystems does not necessarily reduce pest damage (Jaworski et al 2015 ). The potential for intercrop herbivores to sustain parasitoids of O. arenosella during non-infestation periods is, however, clear and this is likely to promote pest suppression by decoupling parasitoid populations from the constraining seasonality of O. arenosella availability (Settle et al 1996 ; Holt and Hochberg 2001 ; Clementine et al 2005 ; Feng et al 2017 ). In other agro-ecosystems, specialist insect herbivores have been shown to exhibit lower population densities in diverse habitats containing host and non-host plants compared with simple habitats containing host plants only (Kareiva 1983 ; Risch et al 1983 ; Stanton 1983 ; Andow 1988 ).…”

“…An understanding of why given parasitoid species attack some intercrop herbivores, but not others could be gained by metabolomic analysis (Snart et al 2015 ) to identify biochemical differences between herbivore species, or tri-trophic effects of the host plants fed upon (Bukovinszky et al 2008 ; Schuman et al 2016 ), that might prevent extreme polyphagy and thus the wider sharing of natural enemies. Further, direct observations that parasitoids developing on intercrop herbivores subsequently attack O. arenosella would provide key evidence for whether coconut and its intercrops form single or segregated habitats (Feng et al 2017 ).…”

“…One possibility is that the pest population is maintained by utilizing non-palm plant species. In other cropping systems, alternative host plants can support pests during periods when primary hosts are seasonally unavailable, and subsequently these pests migrate back to the primary host plants (Clementine et al 2005 ; Goodell 2009 ; Saeed et al 2015 ) and the availability, density and type of alternative host plants can be important factors influencing the damage caused by insect pests (Power 1987 ; Settle et al 1996 ; Atakan and Uygur 2005 ; van Veen et al 2006a ; Zhang et al 2017 ; but see Feng et al 2017 ). Such alternative host plants are potentially present in the coconut agro-ecosystem because a wide variety of intercrop species are commonly grown within plantations and at all times of the year.…”

Conspecific and heterospecific interactions modify the functional response of Harmonia axyridis and Propylea japonica to Aphis citricola

“…Consequently, the impact of predators on prey populations can fluctuate with a change of predator diversity (Duffy et al, 2005;Ives et al, 2005). Therefore, it is essential to evaluate the effects of multiple predators on trophic interactions in agroecosystems (Simberloff & Von Holle, 1999;McCoy et al, 2012;Feng et al, 2017). Previous studies have mostly focused on single predator-prey interactions in which the community and population consequences of predation are somehow neglected (Fielding et al, 2003;Renai & Gherardi, 2004;Stoffels et al, 2011).…”