Previous herbivory alerts conspecific gravid sawflies to avoid unsuitable host plants

Abstract

<jats:title>Abstract</jats:title><jats:p>The willow sawfly, <jats:italic>Nematus oligospilus</jats:italic> (Förster), is a pest in <jats:italic>Salix</jats:italic> commercial forests and has been reported worldwide. Female adults must recognize a suitable host plant to oviposit, since her offspring lack the ability to move to another host. We evaluated the effect of conspecific herbivory on the oviposition choices of <jats:italic>N. oligospilus</jats:italic> females by providing damaged (DP) and undamaged (UP) plants of <jats:italic>Salix humboldtiana</jats:italic>, a native willow from South America, as oviposition substrates. Local and systemic effects were studied. For the local treatment, a twig from the DP with damaged leaves was contrasted to a twig from a UP in dual choice experiments. For systemic treatment, a twig from the DP with intact leaves was contrasted to a twig from a UP. We estimated the use of olfactory and contact cues by comparing volatile emission of DP and UP, and by analysing the behaviour of the females during host recognition after landing on the leaf surface. In the context of the preference–performance hypothesis (PPH), we also tested if oviposition site selection maximizes offspring fitness by evaluating neonate hatching, larval performance and survival of larvae that were born and bred on either DP or UP. Our results demonstrate that previous conspecific herbivory on <jats:italic>S. humboldtiana</jats:italic> has a dramatic impact on female oviposition choices and offspring performance of the sawfly <jats:italic>N. oligospilus.</jats:italic> Females showed a marked preference for laying eggs on UP of <jats:italic>S. humboldtiana.</jats:italic> This preference was found for both local and systemic treatments. Volatile emission was quantitatively changed after conspecific damage suggesting that it could be related to <jats:italic>N. oligospilus</jats:italic> avoidance. In the dual choice preference experiments, the analysis of the behaviour of the females once landing on the leaf surface suggested the use of contact cues triggering egg laying on leaves from UP and avoidance of leaves from DP. Furthermore, 48 h of previous conspecific feeding was sufficient to dramatically impair neonate hatching, as well as larval development and survival, suggesting a rapid and effective reaction of the induced resistance mechanisms of the tree. In agreement with the PPH, these results support the idea that decisions made by colonizing females may result in optimal outcomes for their offspring in a barely studied insect model, and also opens the opportunity for studying tree-induced defences in the unexplored South American willow <jats:italic>S. humboldtiana.</jats:italic></jats:p>

Sawfly egg deposition extends the insect life cycle and alters hormone and volatile emission profiles

“…Conversely, VOC emission can also be a deterrent cue for gravid females to avoid plants infested by conspecific eggs (Fatouros et al, 2012). N. oligospilus females decreased oviposition preference to plants of the South American willow (S. humboldtiana) damaged by conspecific feeding larvae with increased emission of germacrene D (Valladares et al, 2020). Previous studies have also shown than OIPVs can be perceived by neighboring plants and primed them for incoming herbivore attack, increasing their resistance.…”

“…To estimate 1-day larval growth, the neonate larval size was measured one and 24 h after transfer to NO + F plants (Figure 2). An indirect size measurement was chosen because neonate larvae are small and delicate so that their manipulation for weighing causes their death (Valladares et al, 2020). The same larvae as newly emerged and as 1 day-old were photographed with a digital camera (Canon EOS 700 D Rebel T5i) adjusted to a stereoscopic microscope (Olympus SZ61, Tokio, Japan).…”

“…For example, OIPVs emission presents different volatile blends from those induced by larval feeding (Hilker and Meiners, 2011;Pashalidou et al, 2015b). Although previous studies have investigated herbivore-induced volatiles in Salicaceae (Yoneya and Takabayashi, 2013;McCormick et al, 2019;Valladares et al, 2020), little is known about the specificity of volatile emission according to herbivore stage of development. In addition, there is no information about defense responses of Salicaceae to oviposition.…”

Monoterpene alcohols induced by sweet potato weevil larvae deter conspecific adults from feeding and oviposition

“…Thus, it is conceivable that gravid females presumably have evolved an accurate and efficient indication to identify and qualify the host plants for their progeny. 2,14 It is well known that SPWs rely on volatiles from sweet potato plants for host orientation. 3,15,16 Although volatiles from healthy plants could indicate an available host, they contain no information associated with the potential competitors.…”

“…The population density of SPW larvae exclusively depends on the oviposition location of their mothers. Thus, it is conceivable that gravid females presumably have evolved an accurate and efficient indication to identify and qualify the host plants for their progeny 2,14 …”