Trees That Emit a Chemical to Ward Off Bugs

Quantitative vs. Qualitative

• The chemical defenses of plants can be roughly divided into quantitative and qualitative kinds. Quantitative chemicals are dosage-dependent: that is, their repellent effect on insects or other herbivores is amplified with increasing amounts. They usually take the form of chemicals such as tannins that make plant tissue less palatable or nutritious to an herbivore. Qualitative chemicals are dosage-independent, effective at basically any concentration; they tend to be actively toxic and potentially lethal to plant-eaters. In general, fast-growing, pioneering, early successional trees tend to utilize qualitative chemicals. Quantitative chemicals, which take time and energy to build up, are usually found in long-lived, slow-growing trees.

Example: Aspen

• Quaking aspen is among the favored hosts of one of the chief notorious forest pests in North America, the gypsy moth, which in total feeds on more than 200 kinds of trees. The caterpillar, sprung from egg clutches coated in the hairs of the female moth, commences nocturnal feeding on leaves in spring. Entire groves may be defoliated by such feasts. Certain quaking aspens by genetic disposition have high concentrations of aspirin-like chemical compounds called salicylates, which can be repellent or toxic to gypsy moth caterpillars. Because of the variability of salicylates’ sequestration in aspens, a given grove afflicted with caterpillars may have some trees skeletonized, others – more resistant – in full leaf.

Arms Race

• Trees (and other plants) play out with their insect herbivores an evolutionary arms race with one another. A plant that develops some chemical compound, quantitative or qualitative, that turns out to deter or kill insect attackers will survive to pass on its genes, thereby introducing the trait into the population. Similarly, an insect that happens to have some resistance to this chemical can pass this immunity on. This will encourage the emergence of an even more potent defense in the plant – and so on.

Other Defenses

• Chemical routes aren’t the only ones taken by plants to ward off insects. Bullhorn acacia trees encourage habitation by ants with hollow, sheltering horns and specialized food sources, while the ants aggressively deter insects and other herbivores from attacking the trees. This is one of the great cases of mutualism, a type of symbiotic relationship where the two participating organisms help one another in some form and benefit both from the interaction.