The adaptable and wide-ranging feeding behaviour of the Rice Weevil Sitophilus oryzae is well documented e.g. by Trivedi et al. (2010) who have analysed a range of metabolites in weevils fed on different diets. Along with its ability to fly to new food sources, this adaptability has led to its status as a ‘post-harvest’ pest of stored produce. Although now cosmopolitan in distribution, confusion between this species and the similar S. zeamais (and likely mixing in collections) means that its exact distribution in Britain is unclear (Morris 2002).
However, on 3rd Dec 2010, whilst buying bird-food, I did find some Sitophilus weevils under the rim of a tub of dried mealworms being sold in a hardware store. There were four live individuals which could not be collected, but two were dead or moribund and these came home along with the mealworms (which are currently being feasted on by starlings on my bird-table.) The species’ separation is relatively straightforward by dissection as the median lobe of S. oryzae is dorsally smooth without longitudinal sulcae, unlike that of S. zeamais which has two sulci. The specimens turned out to be S. oryzae.
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| Adult S. oryzae from the hardware store (approx. 2.5mm long) |
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| Underside showing antennal insertions beneath swollen rostral base. |
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| Dorsal view showing punctures, especially of the pronotum. |
Although these will form a useful record of the species, I was interested to know how broad their diet is. Adults and larvae feed on whole grains, and as well as rice, can feed on a range of other cereal crops, dried beans, cashew nuts, wild bird seed, leguminous pods (Pemberton & de Rodriguez 1981), plus cereal products such as macaroni. Females chew a cavity into a seed, laying a single egg and sealing it in using a secretion from her ovipositor. The larva develops within the seed, hollowing it out while feeding. The larvae and pupae complete their development inside a seed kernel or man-made equivalent (such as macaroni with its handy ready-made hollow space within a wheat ‘shell’) and have been known to develop in hard-caked flour.
So, given that S. oryzae has expanded its range of foods to include not only other cereals, but also legumes and nuts, could it adapt further to include animal material? Well, it is unclear whether it could feed on animal matter to some extent, but previous research (Chippendale 1972) suggests that this is unlikely to form more than a passing nibble. This is because it appears that the branched-chain amylopectins of cereal starches acts as both a feeding stimulant and an essential nutrient – without this, the weevil simply doesn’t feed and starvation is seen. So, although its culinary tastes are wide-ranging it is not omnivorous, and it is likely that the specimens I found were incidental on the mealworm tub having arrived along with more suitable grain and seed-based bird food.
References
Chippendale, G.M. (1972). Dietary carbohydrates: Rôle in survival of the adult rice weevil, Sitophilus oryzae. Journal of Insect Physiology 18(5): 949-957.
Morris, M.G. (2002). RES Handbook Vol.5 Part 17b: True Weevils Part 1. Coleoptera: Curculionidae (Subfamilies Raymondionyminae to Smicronychinae). RES, London.
Pemberton, G.W. & de Rodriguez, A. (1981). The occurrence of a strain of Sitophilus oryzae (L.) (Coleoptera: Curculionidae) breeding in Portuguese kibbled carobs. Journal of Stored Products Research 17(1): 37-38.
I am glad to learn more about the life cycle of this weevil - and now I know why my my birdseed used to "come alive" in the bin.
ReplyDeleteDo you have any information on the effect on humans who ingest any of the rice products which could possibly be infected with these critters, dead or alive?
ReplyDeleteDue to celiac disease, my carbohydrate intake comes almost exclusively from rice cakes (sort of rice crispies dried into a patty shaped biscuit). I am sure these products stored wherever, are not immune.
I wonder what the effect is on birds that ingested them in one form or other.
I realize this is outside your scope, but maybe you have come across it somewhere.
I'm certainly no medic, but as far as I'm aware, these insects are widely (if unintentionally) eaten all over the world where rice is consumed. I've seen various points being made - one is that they are a useful source of protein (which to me implies they are generally not a problem - they are so widespread that I imagine many people have eaten them without realising, which again implies no common noticeable problem). However, I'm sure I've read that secretions from stored-food pests may lead to secondary effects e.g. by promoting mould, E. coli, or by harbouring pathogens. I don't know if this is the case (or again, if true, how common or serious it is), but there's a good post about weevils in bird seed (I suspect birds eat them all the time and that mouldy bird food is more of a problem) at http://seabrookeleckie.com/2008/11/25/bird-seed-surprise/
ReplyDeleteThe practical advice seems to be to store rice in airtight containers - I don't know what processing takes place to make a rice-cake, but if it kills any weevils aleady there (or removes them at an earlier processing stage), maybe airtight storage will prevent them reappearing? Also, rice weevils mainly attack whole grains, so they may be less interested in rice-cakes.
I've heard that a different species (the 'Warehouse Beetle', Trogoderma variabile) has bristly larvae and that these bristles can cause irritation (to the mouth, stomach, gut) if ingested. I don't know if this is true, or if so, how common or serious it is.
As you say, a bit outside my scope, so I guess medical advice might be the best idea if you are concerned - if you do find out more, please do post the info here; I'd certainly be interested.