Observational evidence of herbivore-specific associational effects between neighboring conspecifics in natural, dimorphic populations of Datura wrightii

Associational effects – in which the vulnerability of a plant to herbivores is influenced by its neighbors – have been widely implicated in mediating plant-herbivore interactions. Studies of associational effects typically focus on interspecific interactions or pest-crop dynamics. However, associational effects may also be important for species with intraspecific variation in defensive traits. In this study, we observed hundreds of Datura wrightii – which exhibits dimorphism in its trichome phenotype – from over 30 dimorphic populations across California. Our aim was to determine whether a relationship existed between the trichome phenotype of neighboring conspecifics and the likelihood of being damaged by four species of herbivorous insects. We visited plants at three timepoints to assess how these effects vary both within and between growing seasons. We hypothesized that the pattern of associational effects would provide rare-morphs (i.e. focal plants that are a different morph than their neighbors) with an advantage in the form of reduced herbivory, thereby contributing to the negative frequency-dependent selection previously documented in this system. We found the best predictor of herbivory/herbivore presence on focal plants was the phenotype of the focal plant. However, we also found some important neighborhood effects. The total number of plants near a focal individual predicted the likelihood and/or magnitude of herbivory by Tupiochoris notatus, Lema daturaphila, and Manduca sexta. We also found that velvety focal plants with primarily sticky neighbors are more susceptible to infestation by Tupiochoris notatus and Lema daturaphila. This does not align with the hypothesis that associational effects at the near-neighbor scale contribute to a rare-morph advantage in this system. Overall the results of our study show that the number and trichome-morph composition of neighboring conspecifics impact interactions between D. wrightii and insect herbivores.
Methods
Dataset was collected by observing herbivores/herbivory on Datura wrightii plants in populations across Central and Southern California. Populations were visited in 2017 (Summer, July/aug) and 2018 (Spring, April/May; and Summer, July/Aug).

Herbivory (all .dmg variables) was measured via observer (JKG) estimation from 0% (undamaged) to 100% (complete defoliation). Herbivore presence was recorded as a binary (0 = no occurence; 1 = occurence). Herbivore occurence was not recorded during Spring 2018.

Species of herbivore observed:

ms: Manduca sexta

lema: Lema daturaphila

gh: Grasshoppers (rare generalist)

fb: flea beetles (Epitrix sp.)

mirids: Tupiochoris notatus

tc: Trichobaris compacta (only observed in Summer 2018)

关联效应（associational effects）指植物对植食者的易感性受其邻株影响的现象，该效应已被广泛证实可介导植物-植食者互作。关联效应的研究通常聚焦于种间互作或害虫-作物动态。然而，关联效应对于防御性状存在种内变异的物种而言同样具有重要意义。
本研究对分布于加利福尼亚州的30余个毛状体表型二态性种群的数百株曼陀罗（Datura wrightii）进行了观测，旨在探究邻株同种个体的毛状体表型，与目标植株被4种植食性昆虫取食的概率之间是否存在关联。我们在三个时间点对植株进行采样观测，以评估这些效应在生长季内及生长季间的变化规律。我们提出假说：关联效应的模式将为稀有形态（即与邻株形态不同的目标植株）带来取食压力降低的生存优势，进而助力该系统中此前已被记录的负频率依赖选择（negative frequency-dependent selection）。
研究结果显示，预测目标植株植食程度或植食者存在情况的最佳指标为目标植株自身的毛状体表型。但我们同时也发现了部分重要的邻株效应：目标个体周边的植株总数可预测其被斑翅盲蝽（Tupiochoris notatus）、曼陀罗跳甲（Lema daturaphila）以及烟草天蛾（Manduca sexta）取食的概率和/或严重程度。我们还发现，若具绒毛表型的目标植株周边以粘性表型植株为主，则其更容易被斑翅盲蝽和曼陀罗跳甲侵染。这一结果与“近邻尺度的关联效应可助力该系统中稀有形态生存优势”的假说并不一致。综上，本研究结果表明，邻株同种个体的数量及毛状体形态组成，会影响曼陀罗与昆虫植食者之间的互作关系。

## 研究方法
数据集通过对加利福尼亚州中部及南部种群中的曼陀罗植株的植食者取食情况进行观测收集。采样分别于2017年（夏季，7/8月）及2018年（春季，4/5月；夏季，7/8月）开展。

植食程度（所有以.dmg为后缀的变量）由研究者JKG通过目视评估，取值范围为0%（未受损伤）至100%（完全落叶）。植食者存在情况以二元变量记录（0=未出现；1=出现）。2018年春季未记录植食者出现情况。

观测到的植食者物种如下：
- ms：烟草天蛾（Manduca sexta）
- lema：曼陀罗跳甲（Lema daturaphila）
- gh：蚱蜢（稀见广食性类群）
- fb：跳甲（Epitrix属）
- mirids：斑翅盲蝽（Tupiochoris notatus）
- tc：Trichobaris compacta（仅于2018年夏季被观测到）