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Silk-based attachment of caterpillars on plant surfaces: biomechanics, and cost-benefit analysis of silk production

Supervisors: Dr Walter Federle [co-supervisor Professor Paul Brakefield]

Project summary:

This project will study how the silk-based attachment of caterpillars affects their herbivory and fitness. We will investigate (1) costs and benefits of silk production, (2) the trade-off with growth, (3) the dynamic control of silk secretion, (4) the trade-off with proleg-based attachment, and (5) biomechanical adaptations for silk-based grip. The cost of silk production will be quantified from the time allocated for silk-laying and the proportion of available nitrogen (N) incorporated into silk. Trade-offs between growth and silk-based attachment will be studied by experimentally selecting caterpillars for strong attachment or rapid growth. We will compare silk use across different caterpillar species and study how silk production is controlled depending on N availability and substrate conditions. We will study the biomechanical adaptations of caterpillars to climb using silk carpets.

Lepidoptera are one of the major groups of herbivores worldwide. To feed, their larvae must cling firmly to the surface of host plants, which are often too smooth or slippery for caterpillar attachment devices (thoracic legs and abdominal prolegs) to grip. Many species use an alternative attachment strategy: they spin dense silk carpets onto the surface of their host plants, providing their claws with points to hook on. However, silk is a protein-based secretion and therefore costly to produce for herbivores consuming low-nitrogen diets [1]. Preliminary evidence shows that in some caterpillars, silk production is tightly controlled, and depends on both plant diet and substrate conditions. The costs and benefits of silk-based attachment, their effects on feeding and growth, the trade-off between silk and leg/proleg-based attachment, the control of silk secretion, and the required adaptations have not been studied. Research into caterpillar attachment mechanisms and silk production is essential for understanding caterpillar herbivory, and may help identify plant traits preventing insect attachment, with implications for crop protection and insect pest control [2].

What the student will be doing:

The student will quantify silk-laying intensity by silk staining and video-tracking of Bicyclus anynana caterpillars. Mass spectroscopy will be used to quantify the amount of N consumed by caterpillars, and the allocation of N to body tissue, frass and silk. 15N-tracer experiments will reveal the extent of N recovery from silk spun on leaves by the same or other caterpillars. Caterpillars will be experimentally selected for strong attachment and studied for effects on development time (growth). Conversely, selection lines for rapid/slow growth will be tested for differences in attachment performance and silk production.Experiments will test how N availability and substrate slipperiness influence silk-laying intensity, the amount of silk secreted and the strength of silk threads. Silk use will also be quantified in selected species differing in leg/proleg morphology and attachment performance.The mechanics of how caterpillars grip on silk and relevant adaptations will be studied by force measurements, light and scanning electron microscopy (SEM), and video recordings of locomotion.The evolution of silk-related traits will be traced by mapping them onto available phylogenies.

References:

1. Berenbaum MR, Green ES, and Zangerl AR. 1993. Web costs and web defense in the Parsnip Webworm (Lepidoptera: Oecophoridae). Environment. Entomol. 22: 791-795.
2. Whitney HM, and Federle W. 2013. Biomechanics of plant-insect interactions. Curr. Opin. Plant Biol. 16: 105–111.
3. Mortimer B, Holland C, and Vollrath F. 2013. Forced reeling of Bombyx mori silk: separating behavior and processing conditions. Biomacromolecules 14: 3653-3659.