Food quality effects on instar-specific life histories of a holometabolous insect

It is a long-standing challenge to understand how changes in food resources impact consumer life history traits and, in turn, impact how organisms interact with their environment. To characterize food quality effects on life history, most studies follow organisms throughout their life cycle and quantify major life events, such as age at maturity or fecundity. From these studies, we know that food quality generally impacts body size, juvenile development, and life span. Importantly, throughout juvenile development, many organisms develop through several stages of growth that can have different interactions with their environment. For example, parasitoids typically attack larger instars, whereas larval insect predators typically attack smaller instars. Interestingly, most studies lump all juvenile stages together, which ignores these ecological changes over juvenile development.  
We combine a cross-sectional experimental approach with a stage-structured population model to estimate instar-specific vital rates in the bean weevil, Callosobruchus maculatus across a food quality gradient. We characterize food quality effects on the bean weevil’s life history traits throughout its juvenile ontogeny to test how food quality impacts instar-specific vital rates.
Vital rates differed across food quality treatments within each instar; however, their effect differed with instar. Weevils consuming low quality food spent 38%, 37% and 18% more time, and were 1%, 8% and 60% smaller than weevils consuming high quality food in the second, third and fourth instars, respectively. Overall, our results show that consuming poor food quality means slower growth, but that food quality effects on vital rates, growth and development are not equal across instars. Differences in life history traits over juvenile ontogeny in response to food quality may impact how organisms interact with their environment, including how susceptible they are to predation, parasitism, and their competitive ability.

Methods
Four-day old eggs collected from stock Callosobruchus maculatus females were transferred to artificial seed pulses (pellets) for the three pellet quality treatments (100%, 95%, and 90% black-eye pea flour). Beginning on day 5 of the experiment, (i.e. 5-day old weevils), a single pellet was removed from the environmental growth chamber each day for 45 days (i.e. 50 days of weevil development from hatching) from each treatment in each replicate. Sampled pellets were placed in a -10°C freezer to stop growth and development of the larva, giving us cross-sectional data on development and survival every 24 h. We retrieved the larvae from each frozen pellet using a scalpel under a dissecting microscope. The experiment was replicated 40 times, such that the total number of sampled larvae (40 replicates X 46 days X 3 treatments) was: 5,520.

For each dissected pellet, we noted survivorship (i.e. whether an individual survived until its freeze date), identified larval instar (L1, L2, L3 and L4) by counting the number of head capsule molts recovered during dissection, measured larval head capsule width at its largest width, and quantified larval dry mass. Samples were dried in a drying oven at 76°C for 72 hours. Dry mass was measured three times and the average of these three measurements was used for all analysis.

解析食物资源变化如何影响消费者的生活史性状，进而影响生物与环境的互作模式，是学界长期以来的一项核心挑战性难题。为阐明食物质量对生物生活史的调控效应，多数研究将追踪生物的完整生命周期，并量化其关键生活史事件，如成熟年龄或繁殖力。通过此类研究，学界已明确食物质量通常会影响生物的体型、幼体发育进程与寿命。尤为关键的是，多数生物的幼体发育会经历多个生长阶段，不同阶段的生物与环境的互作模式存在差异。例如，寄生性天敌通常偏好攻击体型更大的幼虫龄期，而捕食性昆虫幼虫则多以体型更小的龄期为捕食对象。但有趣的是，多数研究将所有幼体阶段合并分析，这忽略了幼体发育过程中发生的生态特征变化。

本研究结合横断式实验方法与阶段结构化种群模型，在食物质量梯度下估算四纹豆象（Callosobruchus maculatus）的龄期特异性生命率。本研究旨在阐明食物质量对四纹豆象幼体个体发育全程中生活史性状的调控效应，以此验证食物质量如何影响其龄期特异性生命率。

各龄期的生命率均随食物质量处理水平发生显著变化，但这种影响效应因龄期而异。与取食高质量食物的四纹豆象相比，取食低质量食物的个体在第二、第三和第四龄期的发育时长分别增加38%、37%和18%，体型则分别缩小1%、8%和60%。综合来看，本研究结果表明，取食低质量食物会导致个体生长速率放缓，但食物质量对生命率、生长与发育的影响效应在不同龄期间并不均等。幼体发育过程中，生活史性状因食物质量差异产生的变化，可能会改变生物与环境的互作模式，包括其被捕食、被寄生的易感性以及竞争能力。

## 实验方法
从四纹豆象（Callosobruchus maculatus）的种群饲养群中收集4日龄卵，将其接种至3种不同质量梯度的人工豆粒培养基（颗粒）中，培养基分别由100%、95%和90%的黑眼豌豆粉配制而成。实验开始后的第5天（即豆象幼虫发育至5日龄时）起，每日从每个重复组的各处理组中取出1粒培养基，持续取样45天（覆盖从卵孵化至幼虫发育满50天的时长）。将取出的培养基置于-10℃冰箱中冷冻，以终止幼虫的生长发育进程，从而获取每24小时间隔的幼虫发育与存活情况的横断式数据集。使用解剖刀在体视显微镜下从冷冻后的培养基中分离出幼虫。本实验共设置40个生物学重复，总取样幼虫数为40个重复×46天×3种处理=5520头。

针对每枚经解剖的培养基，我们记录幼虫的存活情况（即个体是否存活至冷冻取样当日），通过解剖过程中收集到的头壳蜕壳数量鉴定幼虫龄期（L1、L2、L3和L4），测量幼虫头壳的最大宽度，并定量测定幼虫的干重。将幼虫样品置于76℃烘箱中干燥72小时，干重测量重复3次，取三次测量的平均值用于后续所有分析。