Local Variation in the Chemical Defence of European Pine Sawfly (Neodiprion sertifer): Effects of Host Tree Chemistry and Social Environment

Chemical defences are diverse in nature, and this variation can be adaptive or non-adaptive (i.e., environmental noise). Identifying the sources for this variation helps to understand ecosystem interactions and, to make more accurate predictions of species distributions and local abundances. The European pine sawfly (Neodiprion sertifer) is an example of a chemically defending forest pest insect that sequesters resinous acids from the host plant for its antipredator defence. It is also a social insect that feeds and defends cooperatively in a group during the larval stage. In this master’s thesis, I studied the variation in chemical defences from three different natural populations of N. sertifer from outbreak areas (Kaavi, Pieksämäki, and Puumala). I also studied the effect of host plant quality, specifically the monoterpene concentration of pine needles, on the chemical defence and defensive behaviour of N. sertifer larvae and female oviposition decisions (group size and sex allocation). Additionally, I analysed how the social environment (group size and sex ratio) correlates with larval defensive traits. To study these, I measured the effect of host plant’s chemical concentration on larval chemical defence traits at both on the group level (proportion of individuals performing defensive movements and deploying defensive fluid in a group) and on the individual level (deployment, monoterpene concentration, and volume of defensive fluid) in response to a simulated predator attack in laboratory conditions. My main hypothesis was that higher levels of monoterpenes in host trees would lead to stronger antipredator defences in N. sertifer larvae. I also expected that, since in some sawfly species, females have been shown to adjust the sex ratio of their offspring based on host plant quality, tree quality may covary with the sex ratio of larval colonies. Firstly, my results show that larvae’s defensive behaviour and the monoterpene concentration and volume of defensive fluid varied among outbreak populations. However, monoterpene concentrations in pine needles and larval social environment did not vary significantly across populations. Secondly, the monoterpene concentration of the host plant was positively correlated with the monoterpene concentration and volume of the defensive fluid of larvae. Thirdly, the sex ratio of the larval group affected chemical defences; the monoterpene concentration in the defensive fluid (measured from females) decreased as the proportion of males in the group increased. The increase in group size also caused a decrease in the proportion of larvae participating in group defence in defensive fluid deployment. Finally, host plant chemical quality was not associated with female oviposition choices as larval group characteristics (group size and sex ratio) were not affected by it. Altogether, my results suggest that host plant quality influences the strength of chemical defence, affecting both secretion volume and monoterpene content. However, differences in chemical defence among populations cannot be explained solely by host plant quality but may also be a result of the social environment or different selection pressures not measured here (e.g., predation risk).

Grantor dc:publisher

Helsingin yliopisto

Year dc:date.issued

2025

Author dc:creator

• Jämsä, Liina-Lyydia

• cooperation
• herbivory
• chemical defence
• monoterpene
• phenotypic variation
• secondary compound
• social behaviour

Statement dc:rights

• In Copyright 1.0

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10138/596630

OAI identifier oai:identifier

oai:helda.helsinki.fi:10138/596630