Human children can be fussy eaters, working through their meals in a specific order. The same is for the children of Oedemasia leptinoides (until recently Schizura leptinoides), but with O. leptinoides being a moth, the eating habits of the caterpillar don’t exasperate the parents. They might exasperate Carya illinoensis, the Pecan tree, though – as that’s what’s getting eaten.
O. leptinoides likes to nibble around the petiole (the stalk that attaches a leaf to a stem) or a tree stem to make a girdle. What it does next is coat the ring of exposed tissue with a fluid. Research by Dussourd et al. in last month’s Arthropod-Plant Interactions reveals that this fluid is saliva. They way they found out is by getting caterpillars and blocking various glands to see which ones matter. The answer is the labial salivary glands, so they’re effectively spitting into the girdle – but why?
Among the papers Dussourd et al. refer back to is last year’s review in Annals of Botany Mechanisms and ecological consequences of plant defence induction and suppression in herbivore communities by Kant et al.. Kant et al. discuss a couple of kinds of plant defences. Constitutive defences are defences the plant has already made. They might not be active, but they can be turned on and work with relatively little activation cost. However, for these defences to be ready-made the plant has to have allocated resources to making them, obviously. That’s effort that could have been put elsewhere to work for the plant.
The other kind of defences are the induced defences. These are the things that the plant makes when it’s under attack. This means if the plant isn’t attacked it doesn’t waste energy making the defences. It also means the defences can be more potent as chemicals toxic to attackers might not have a desirable effect on the plant either.
Kant et al note there are a couple of direct ways that a herbivore can tackle plant defences, they can either decrease their exposure to toxins, or decrease their sensitivity to them, but the also say there’s something else a herbivore can do. Induced defences take time to be built. If a herbivore could interfere with a plant’s signalling they could stop the defences from happening. Kant et al mention nematodes that can suppress jasmonates and salicylic acid, which are both used by plants as hormones to trigger defences, which opens up new areas for research. In particular, they note:
Plant defence suppression by herbivores is a largely unexplored phenomenon, but, together with induction, it may play a profound role in the plant-mediated indirect interactions that determine community structure in the phyllosphere.
…and this is where Dussourd et al. pick up the baton. They say:
…these compounds presumably function to suppress plant defensive responses normally elicited by caterpillar feeding.
…but they’re not sure exactly what defences are being disrupted. One they suggest is polyphenol production, in particular juglones, that reduce the growth of some caterpillars. Another possibility is the production of volatiles. These could signal to parasites or predators that the Pecan tree is an excellent place to hunt for caterpillars. If that’s the case then the caterpillar would do well to stop those signals from getting out.
They also mention in the conclusion that O. leptinoides do more than make girdles. Sometimes they’ll sever a petiole before they’ve eaten all of a leaf. What they do next is to spend time salivating over the stub, again suggesting they’re getting something into the tree to suppress defences.
Getting a bucket of caterpillar spit to examine what effect it has on pecans seems like a lot of hard work, but it could well be an important project. If botanists can find a way to help a pecan counter a caterpillar’s saliva, then it would effectively help the tree produce its own natural pesticides. Hopefully similar solutions would be possible for other plants, to give people a few more tools to help fight unwanted herbivores.