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 <em>Listronotus bonariensis</em> (Kuschel) (Coleoptera: Curculionidae). This has been previously suppressed by the importation biological control agent, <em>Microctonus</em> <em>hyperodae</em> 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 <em>Lolium perenne</em> x <em>Lolium multiflorum</em> hybrid ryegrass (cv. Manawa), a tetraploid Italian ryegrass <em>L. multiflorum</em> Lam. (cv. Tama), and a diploid perennial ryegrass <em>L. perenne</em> 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 <em>L. multiflorum</em>. 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）牧草为核心组成的新西兰牧场（禾本目：Poales，禾本科：Poaceae）市值高达196亿美元，且遭受严重的虫害侵扰。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）。此前该害虫可通过引进的生防天敌——小茧蜂（Microctonus hyperodae）Loan（膜翅目：Hymenoptera，茧蜂科：Braconidae）得到有效抑制，但近年来该抑制效果出现衰退，相关机制正处于研究阶段。有假说提出，牧草类型会影响象鼻虫的寄生规避行为，进而改变寄生率。本研究选用三种常见牧场牧草验证该假说：二倍体黑麦草（Lolium perenne）×多花黑麦草（Lolium multiflorum）杂交栽培品种‘马瓦’（cv. Manawa）、四倍体意大利黑麦草（L. multiflorum）Lam.栽培品种‘多摩’（cv. Tama），以及二倍体多年生黑麦草（L. perenne）L.栽培品种‘萨姆森’（cv. Samson）。本研究采用基于实验室的微宇宙实验方法，测定象鼻虫在暴露于寄生蜂时，对上述三种牧草的规避行为程度，该行为反应通过与对照组相比，象鼻虫在植株上的停留量与取食程度的降低幅度进行量化。需说明的是，部分行为学、植株高度分层及取食痕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，杂交栽培品种‘马瓦’黑麦草与四倍体栽培品种‘多摩’均受到象鼻虫的高度偏好，而二倍体栽培品种‘萨姆森’上的取食量则显著更低。当存在寄生蜂时，四倍体栽培品种‘多摩’上的象鼻虫几乎未表现出针对寄生蜂的规避行为，可认为停留在该植株上的收益超过了遭遇寄生的风险。与之相反且出乎意料的是，在寄生蜂存在时，二倍体杂交栽培品种‘马瓦’上的象鼻虫展现出极强的规避行为，其暴露风险水平与取食偏好更低的二倍体栽培品种‘萨姆森’相近。上述发现反映了自20世纪90年代以来，多数黑麦草属牧草（尤其是二倍体品种）上的象鼻虫寄生率出现下降，但四倍体多花黑麦草并未出现该现象。本研究结果支持如下假说：象鼻虫寄生率的衰退源于寄生蜂诱导的选择压力引发的快速演化，以及寄主植物营养品质带来的抵消效应。