One of my responsibilities while house-sitting for my parents is keeping the bird feeders well-stocked. With the period of extremely cold temperatures (at least for this time of year – we’d be laughing come February) the feeders were a hive of activity, and the seed levels dropped steadily. The sunflower and thistle seed feeders were filled again without trouble, but when I dug the scoop into the bag of mixed seed, I noticed a sharp, distinctly mouldy smell. Peering closer, it looked like perhaps the peanuts had started to turn, and it was maybe spreading to the other seed, but I couldn’t really tell. It didn’t look really bad yet, so I stood there for a moment or two, holding the container in my hand, staring at the seed inside as I pondered whether it was still okay to put out for the birds (mould can be poisonous to birds once it’s progressed).
I finally decided better to be safe and not put the seed out, but while I was standing there contemplatively staring at the seeds, I noticed something else. Some movement. The seeds were starting to wiggle. And then…
…from between them, out crawled a tiny beetle. And then another. And a third. And before I knew it, the top was crawling with a dozen or more little beetles. More accurately, little weevils, as I could clearly see the thin snouts protruding from the front of their heads. Weevils are sometimes also known as snout beetles because of this feature. There are some 60,000 species in the weevil superfamily, Curculionoidea. Most belong to the family Curculionidae, and about 2600 species from this family are found in North America. Most are herbivorous, and many are crop pests.
Probably the interaction most people will have with weevils is the opening of a bag of something to discover an infestation of them. They are common outdoor bugs, but are generally small and inconspicuous unless they happen to land on your drink glass or some other coincidental meeting. However, they do occasionally make it home in bags of grain or seed products. There are three species, all in the genus Sitophilus, who are encountered this way, but they tend to be so similar in appearance that identification to species is best left to an expert with a microscope. They have food preferences, but there is a lot of overlap and they will opportunistically infest other sources when their preferred food isn’t readily available.
The three species are Rice Weevil (Sitophilus oryzae), Granary Weevil (S. granarius), and Maize Weevil (S. zeamais), and their common names reflect their preferred food (here in North America, maize is more often called corn). They often come home in infested bags of bird seed (Granary and Maize weevils may prefer mixed seed, Rice Weevils are often found in sunflower), but will sometimes be found in packages of rice, beans, peanuts, or whole-grain cereals. I couldn’t tell which seeds specifically had been targeted in the package, although the amount of mould growing on the peanuts made me suspicious of them. I think that the decay of the seeds that the weevils had been feeding on, whichever ones they were, had lead to the mould.
The life cycle of these weevils takes about a month to complete, and requires temperatures of a minimum of 17 oC (62 oF), but ideally 27 oC (80 oF) and above, and moderate to high humidity. The adult weevil lays their eggs on appropriate seed or grains which will become the food source for the developing larva. When the egg hatches, the larva tunnels into the grain and sets up shop inside. It takes about 3 days for an egg to hatch from laying, and then the larva may be in the grain for another 18 days, at which point it develops into a pupa. Once the adult emerges from the pupa, some 6 days later, it stays in the relative protection of the grain until its exoskeleton has completely hardened and matured, about 3-4 days.
Because they spend most of their cycle inside the grain itself, it may be possible to be harbouring these little bugs in a stored product for a few weeks without even knowing they’re there. Generally speaking, the incidence of infestation is rare, and probably even if they are present the product is consumed before the eggs get a chance to develop and we’re never the wiser (consider it added protein). By the time the bugs reach adult, stage, however, consuming them or the secretions they produce can sometimes result in E. coli infections, depending on the weevil’s particular diet.
I dumped a few out on a blank piece of white paper to try to get some uncluttered photos of individuals, but I had minimal luck. They were just too quick! I found that initially they would curl up their legs and play dead, for instance if I shook the paper to knock them all back to a central starting point. But moments later they’d unfold and start hustling across the paper. Interestingly, their direction of movement wasn’t random. They all moved with a purpose, and while their particular direction varied, it was always directly towards the edge of the paper. I thought perhaps they were trying to get away from the bright halogen that was hanging over the center of the paper to provide illumination for the photos.
None of them ever tried to fly, which I think perhaps rules out Rice Weevil, which is supposed to be winged and attracted to lights. The Granary Weevil has poorly developed wings and can’t fly, and is also not attracted to lights. On the other hand, Rice Weevils are reddish-brown and have 4 pale marks on their wing covers (Maize Weevils are similar), while Granary Weevils are reddish-brown to black and unmarked. Going by that, it looks like I have both in this group. So who knows! When I submitted the images to BugGuide.net, the person who identified them thought it safest just to leave them at genus.