Local feeder (Coleoptera) in rachises of Ambrosia

This species forms a subtle swelling in the central stalk (rachis) of the spikelike raceme bearing the male flowers of giant ragweed. The swelling is approximately 1 cm long, irregular or spindle-shaped to ovoid, and noticeably darkened relative to the light greenish color of the unaffected rachis tissue above and below it. The flowers may need to be removed from the rachis in order to view the swelling more clearly, but even when not removed, they may appear more densely packed at the location of the swelling, betraying the swelling's presence from afar.

I first located examples on 28 August 2016, by which time the culprit had already vacated the plant material. Taking samples a bit earlier in the month, on 15 August of the next year, yielded at least one example in which the larva was still present inside its swelling. Likewise, in 2018, two swellings collected on 17 August each still contained a larva inside, and by 21 August the larvae were crawling around in the rearing container, having evacuated the plant material.

Larvae were coleopteran, less than 10mm long at maturity, pale yellow in color with an orange-brown head capsule, the head capsule relatively large and dome-shaped, the abdomen tapering to the posterior, and the thorax bearing six short knobby bumps on its underside where legs would be expected. The overall impression was that of a somewhat slender weevil larva.

Larvae burrowed into the moistened potting mix provided to them, suggesting the typical pupation location is in the soil, and no adults emerged by the end of the growing season, suggesting that the larva or pupa overwinters. I have not yet managed to rear adults.

In Iowa, MJ Hatfield has reared a brentid weevil tentatively identified as Fallapion sp. from the lower parts of main stems of giant ragweed (Hatfield 2021), and has also collected a Smicronyx sp. in abundance on ragweed in early August (Hatfield 2013), but it is not clear whether the maker of the swellings in the flower cluster stalks might be related to either of these.