Today's entry is courtesy of Hannah Bottomley from Simon Fraser University, who has recently co-authored a paper on today's subjects. We've Hannah to thank for the write-up and thermographic images and Stephen Takács for the conventional photographs. Hannah writes:
Pinus monticola (western white pine) cones glow warmly in contrast to cool conifer needles in the infrared spectrum (top right; bottom left). Cones can be up to 15˚C warmer than needles (as indicated by the temperature bar on the right) and emit significantly stronger infrared radiation. Infrared radiation is a type of electromagnetic radiation that the human eye is unable to perceive; it has longer wavelengths than visible light (380-750 nanometres), but shorter wavelengths than microwaves (1 millimetre to 1 metre).
These thermographic images of Pinus monticola cones were taken with an infrared camera, exposing a previously unknown way in which insects are able to hone in on their host plant. Recent research by Takács and his colleagues reveals that Leptoglossus occidentalis (western conifer seed bug) has infrared receptors and is able to exploit the difference between cones and needles in the infrared spectrum, and zero in on cone-laden conifers from afar. This insect is a specialist herbivore that feeds on the contents of developing conifer seeds; in the second photo, it can be seen feeding on a Pinus monticola cone.
This phenomenon of warm cones is not limited to Pinus monticola - it has also been observed in Pinus contorta var. latifolia (lodgepole pine), Pseudotsuga menziesii (Douglas-fir), Picea engelmannii (Engelmann spruce) and Larix occidentalis (western larch). It is attributed in part to the fact that larger objects retain more heat than smaller objects, as well as to the tendency of a cone's surface to reflect solar radiation. In all likelihood, seed development (and associated metabolic activity) is also generating warmth, contributing to the relatively high temperature of conifer cones.
Although there are a few recognized infrared-detecting insects, this is the first study to show that herbivorous insects are using infrared emission from a specific part of a live plant as a foraging cue. This research is yet another testament to the complexity of plant-insect interactions and reminds us that there is a world of nature that exists beyond our own perception.
Daniel adds: For a popular summary of the paper, see "Heat Sensors Guide Insects to a Hot Meal" from ScienceNews. To view the scientific paper, see: Takács, S. et al. 2008. Infrared radiation from hot cones on cool conifers attracts seed-feeding insects. Proceedings of The Royal Society B. 276(1657):649-655. doi: 10.1098/rspb.2008.0742. For those of you who are particularly keen, I note that mast-seeding is mentioned in the abstract as a hypothesized method of producing a cone-crop large enough to exceed the capabilities of the insect herbivores to eat them all.
Horticulture / Garden Design link: Les jardins de Quatre-Vents, a garden I first learned about yesterday from the guide (thank you, Luana!) at Montréal Botanical Garden. Here are some photographs of the landscape and the plants. Virtual tours (in English) are available here: Virtual Tours of Les jardins de Quatre-Vents.