Argentine stem weevil plant-mediated behavioural responses towards its biological control agent

New Zealand pastures largely comprising Lolium ryegrass species (Poales: Poaceae) are worth $19.6B and are subject to major pest impacts. A very severe pest is the Argentine stem weevil Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae). This has been previously suppressed by the importation biological control agent, Microctonus hyperodae Loan (Hymenoptera: Braconidae). However, this suppression has recently declined and is subject to investigation. It has been hypothesised that grass type influences the parasitism avoidance behaviour by the weevil and thus parasitism rates. This study explored the hypothesis using three common pasture grasses: a diploid Lolium perenne x Lolium multiflorum hybrid ryegrass (cv. Manawa), a tetraploid Italian ryegrass L. multiflorum Lam. (cv. Tama), and a diploid perennial ryegrass L. perenne L. (cv. Samson). The described laboratory-based microcosm methodology determined the extent of weevil avoidance behaviour on each of these three grasses when subjected to the parasitoid. Such reaction was gauged by the extent of reduced weevil on-plant presence and feeding compared to the control populations. Please note some behavioural, plant section by height and feeding scar raw data variables were not analysed or were not published. In the absence of the parasitoid, the hybrid cv. Manawa ryegrass is as highly favoured by the weevil as the tetraploid cv. Tama. On diploid cv. Samson, feeding is considerably less. In the presence of the parasitoid, weevils on the tetraploid cv. Tama plants showed little avoidance activity in response to the parasitoid and it can be argued that the benefits of staying on this plant outweighed the possibility of parasitism. Conversely and surprisingly, in the parasitoid’s presence, weevils on diploid cv. Manawa showed very strong avoidance behaviour leading to levels of exposure similar to those found on the less-preferred diploid cv. Samson. These findings reflect how weevil parasitism rates have declined in most Lolium grasses, particularly diploids, since the 1990s, but not in the tetraploid L. multiflorum. This contribution supports the hypothesis that the decline in weevil parasitism rates has been the result of rapid evolution arising from parasitoid-induced selection pressure and the countervailing effect of the nutritional quality of the host plants.以黑麦草属（Lolium，隶属于禾本目Poales、禾本科Poaceae）植物为主要组成部分的新西兰牧场，年产值达196亿新西兰元，且遭受着严重的虫害侵扰。其中危害最为突出的害虫为阿根廷茎象鼻虫（Listronotus bonariensis (Kuschel)，鞘翅目Coleoptera：象甲科Curculionidae）。此前该虫害可通过引进式生物防治天敌（importation biological control agent）——广肩小蜂科（Braconidae）的Microctonus hyperodae Loan（膜翅目Hymenoptera）进行有效抑制，但近年来该抑制效果已出现衰退，相关机制正处于研究之中。 已有假说提出，牧草种类会影响象鼻虫的寄生躲避行为，进而改变寄生率。本研究针对该假说展开验证，选用了三种常见牧场牧草：二倍体（diploid）黑麦草与多花黑麦草的杂交品种Manawa、四倍体（tetraploid）意大利黑麦草（L. multiflorum Lam.，品种Tama）以及二倍体多年生黑麦草（L. perenne L.，品种Samson）。 本研究采用所述实验室微宇宙实验法（microcosm methodology），测定了象鼻虫在暴露于该寄生蜂时，于三种牧草上的躲避行为强度。该躲避反应的强弱，通过与对照组种群相比，象鼻虫在植株上的停留占比与取食行为的减少幅度来量化。请注意，部分行为学、植株高度分层及取食痕迹的原始数据变量未进行分析，或未予发表。 在无寄生蜂存在的情况下，杂交品种Manawa黑麦草与四倍体品种Tama黑麦草均受到象鼻虫的高度偏好；而在二倍体品种Samson上，象鼻虫的取食行为显著减少。当存在寄生蜂时，栖息于四倍体品种Tama黑麦草上的象鼻虫几乎未表现出针对寄生蜂的躲避行为，由此可推测，停留在该植株上所能获得的益处超过了被寄生的风险。与之相反且出人意料的是，当存在寄生蜂时，栖息于二倍体品种Manawa黑麦草上的象鼻虫展现出极强的躲避行为，最终其被寄生的暴露概率与偏好性较低的二倍体品种Samson上的水平相当。 上述研究结果可解释，自20世纪90年代以来，多数黑麦草属植物（尤其是二倍体品种）上的象鼻虫寄生率为何出现下降，但四倍体多花黑麦草并未出现该现象。本研究结果支持以下假说：象鼻虫寄生率的下降，是由寄生蜂施加的选择压力所引发的快速进化，与寄主植物营养品质的抵消效应共同作用的结果。