Wheat stem sawfly‐infested plants benefit from parasitism of the herbivorous larvae

Abstract

1 Parasitoids Bracon cephi (Gahan) and Bracon lissogaster Muesebeck and their herbivorous host the wheat stem sawfly Cephus cinctus Norton, a pest of wheat Triticum aestivum , were investigated for yield in T. aestivum grown in the field. 2 Wheat stem sawfly-infested stems had a higher yield potential than uninfested stems. However, final reproductive output was not significantly different between ears on infested stems that supported complete larval development compared with ears on uninfested stems. 3 Stems containing parasitized larvae and stems containing larvae that died before completing their development had a higher mean number of seeds and seed weight, when accounting for number of fertile spikelets of each ear, than either infested with live larvae and uninfested stems. 4 The results obtained suggest that larval feeding prevented infested stems from attaining their yield potential, and that the negative impact of the pest on wheat yield was reduced when late instar sawfly larvae were parasitized. Even though some feeding occurs before parasitism, this early damage has a comparatively low impact on yield. 5 This is the first study to show a yield benefit and enhanced plant fitness due to the wheat stem sawfly parasitoids B. cephi and B. lissogaster . This results from the maintenance of increased seed number and seed weight in the higher yielding stems that are preferentially infested by this pest.

First record of Bracon lissogaster (Hymenoptera: Braconidae) in Canada – a potentially important parasitoid of Cephus cinctus (Hymenoptera: Cephidae) in the prairies

“…It emerged from a stub of red spring wheat that was cut by a C. cinctus larva. Therefore, we have confirmed that B. lissogaster is present in Canada and attacks C. cinctus as reported in the USA (Morrill et al 1998;Runyon, Hurley, Morrill, and Weaver 2001;Buteler, Weaver, and Miller 2008). The low numbers of B. lissogaster around the Lethbridge area likely represent a recent range expansion for this species.…”

Farming system and wheat cultivar affect infestation of, and parasitism on, Cephus cinctus in the Northern Great Plains

“…Also, in organic cropping systems, greater abundance and diversity of weeds can potentially provide more food and shelter for parasitoids of C. cinctus than fields conventionally managed where weeds are sprayed with herbicides. As a result of increased parasitoid pressure, C. cinctus survival and abundance can be reduced . While tillage kills C. cinctus larvae overwintering within wheat stems by burying the wheat stubble, it may also disproportionately kill parasitoids of C. cinctus if they overwinter belowground …”

“…Bracon cephi (Gahan) and Bracon lissogaster Muesebeck (Hymenoptera: Braconidae) are the only parasitoids that commonly attack C. cinctus in Triticum aestivum , reducing its survival and abundance . Both parasitoid species are specialists ( C. cinctus is the only host), idiobiont (prevent further development and kill the host), and ectoparasitoids (live externally on the host), with two generations per year .…”

“…As a result of increased parasitoid pressure, C. cinctus survival and abundance can be reduced. 17,22,25,26 While tillage kills C. cinctus larvae overwintering within wheat stems by burying the wheat stubble, 27 it may also disproportionately kill parasitoids of C. cinctus if they overwinter belowground. 11,18,22 Bracon cephi (Gahan) and Bracon lissogaster Muesebeck (Hymenoptera: Braconidae) are the only parasitoids that commonly attack C. cinctus in Triticum aestivum, reducing its survival and abundance.…”

Increased area of a highly suitable host crop increases herbivore pressure in intensified agricultural landscapes

“…In wheat crops, these two species represent the only important insect natural enemies of C. cinctus, with B. cephi generally dominating (Runyon et al, 2002). These beneficial enemies can reach high densities and reduce economic damage by C. cinctus (Morrill et al, 1998;Buteler et al, 2008), but populations are highly variable across wheat fields and regions (Shanower and Waters, 2006) and causes of their population fluctuations are not well understood. The only documented host of the braconid parasitoids is C. cinctus, which is attacked during the larval stage regardless of whether host plants are crop or non-crop grasses.…”