“Seed weevil” is the kind of imprecise common name that drives naturalists to distraction. To start with, weevils, taken generally, are a vast group of beetles, with not far shy of a quarter-million described species fitting the broadest definition of the term. What we might call “seed weevils” — weevils with life histories that center around eating seeds — occur in several families and include thousands of species.
But if the term has little taxonomic validity, seed predation, or feeding on seeds, is an important strategy, and certain weevils are pros at it. Seeds represent a major investment on the part of a plant: Every seed contains, in addition to the genetic blueprint for the plant’s next generation, a reserve of stored energy to get the new plant off to a good start as well as specialized tissues and structures intended to protect the active parts of the seed from heat, cold, desiccation, or being eaten.
From a weevil’s perspective, then, each seed represents a trove of resources ready to be pillaged. And because each seed represents a potential plant of the next generation, each seed devoured by a seed predator represents a substantial lost investment from the plant’s perspective. In effect, seed-eating weevils have a strong incentive to interfere with the reproductive success of whatever plant species they prefer to feed on.
An example to clarify this point might be Trichapion rostrum, or the wild indigo seed weevil. The plant this tiny beetle feeds on, Baptisia tinctoria, is a common and familiar perennial of dry soils on the Vineyard (and elsewhere;) its clover-like leaves, round habit, and yellow flowers abound at sites such as the fire lanes in Correllus State Forest or the grassland at Katama Air Park.
A prolific producer of seeds, which develop in small, black, bean-like pods, wild indigo has evolved a brilliant dispersal strategy: In fall, Baptisia plants break off from their root systems and roll, wind-blown, across the landscape, dropping seeds as they go.
If, that is, there are seeds to drop. Larvae of the wild indigo seed weevil develop from eggs injected into developing seed pods by female weevils, and the larval weevils typically devour the seeds in their pod completely before reaching maturity. The percentage of seed pods inhabited by weevils can be very high — close to 100 percent, sometimes — and so this tiny beetle is a major player in the population dynamics of wild indigo.
Inspired by the curious life history of the wild indigo seed weevil, I’ve been looking for other examples of similar habits. I recently found a weevil, similar in size and shape to Trichapion, in the seed pods of Tephrosia virginiana, another member of the legume family that, like wild indigo, is a characteristic plant of the Vineyard sandplain. It’s a hardy, drought-resistant, and fire-adapted plant found on sandy soils, common here but generally scarce due to its unusual habitat preferences.
Identifying this Tephrosia-feeding beetle posed some challenges, but I eventually tracked down a report by the Canadian Wildlife Service on Tephrosia in Canada, where this plant is a rarity and the focus of considerable conservation effort. In its section on “Herbivory,” this report refers to Apion segnipes, noting that Apion “larvae have been collected from [Tephrosia] seed pods” and that “Tephrosia is the only documented host for this weevil and no other Canadian weevils are known to share the host.”
This weevil’s scientific name has apparently been updated to Sayapion segnipes, but it was easy to follow up on the lead provided by the Canadian report, find some photos to compare my beetle to, and determine that the weevil I had extracted from Tephrosia seed pods in Correllus State Forest must surely be Sayapion segnipes. No doubt Sayapion regulates Tephrosia populations in the same way Trichapion does those of wild indigo. Reflecting the general scarcity of Tephrosia, I can find few records for Sayapion, and it seems fair to say that the Vineyard is likely an important refuge for both this specialized plant and its specialized seed predator.
There is a paradoxical aspect to the relationship between plants and their seed predators. If a weevil is too successful across a series of growing seasons, the plant it depends on grows scarce, reducing the opportunities for the weevil. And disturbances that might initially seem harmful to the plant — periodic fire, for example — can actually turn out to be helpful because they suppress the weevil population. The plant species and their seed predators engage in a perpetual tug-of-war, with the balance between the insect and its host constantly changing.
From my naturalist’s perspective, all this is both fascinating and daunting. Many Vineyard plants undoubtedly host specialized weevil seed predators — and many or most of these host-predator pairs are poorly understood or simply unknown. There’s a high-stakes game of seed predation, with enormous rewards possible for the beetles and serious risks to reproduction for the plants, playing out almost undetectably all around us. It piques my curiosity.
