Many of us in Britain will have heard the name Weevil uttered at some time or another. For me, I first heard it as a child when I opened up a bag of flour to find it crawling with Weevil larvae. Recently, I came across a Weevil infestation in a large quantity of bird seed and I realised that although I knew what the insect was, I didn’t know much else about it. So I set off to learn a few facts on this common insect. There are over 60,000 species of weevil worldwide. The Rice, Granary and Maize Weevils are pests of grain throughout the world and can cause extensive damage to stored grain. The weevil I came across was the Granary Weevil, Sitophilus granarius, and this is the insect we will focus on in this article.
The adult Granary Weevil is dark brown to black in colour, about 3-4mm in length and cannot fly. The Weevil has a distinctive look due to the large snout extending from the head. The larvae are white, legless grubs that eventually change to white naked pupa, before emerging from the grain kernels as an adult beetle.
Adult Granary Weevils can live for up to eight months and during this time female Weevils can lay between 50 and 250 eggs, with all larval stages and the pupal stage occurring within the grain. The female finds a store of grain and sets about to lay her eggs. She will usually lay one egg per grain kernel and she does this by using her strong mandibles to chew a hole into the kernel. Inside, she lays her egg and then seals the hole with a gelatinous secretion. Other female Weevils can tell if an egg has already been deposited within a kernel and will not lay a second egg inside it. Once the egg has been laid it is a short time before the larvae hatch out. This time period depends on temperature, with heat speeding the process up and cold slowing the process down. Once completing the four larval instars, the larvae pupate and emerge from the kernel a short time after. Once leaving the grain kernel, females release a sex phermone to attract a mate. With the right temperature, the Granary Weevil can develop from egg to adult in approximately five weeks, but this can take up to twenty weeks in cooler temperatures.
Infested grain can be identified in several ways. The kernels may show round/oblong holes from where adults have emerged, the grain may be slightly warm from where the insects have been feeding, and it can have increased moisture levels due to the insect’s metabolic processes. All small grains can be infested by the Granary Weevil and they are sometimes found inside stores of flour, pasta and other cereals. It can be extremely difficult for Weevils to complete their life cycle in foods such as flour unless they are compacted, however adults will feed readily on flour and grains. Adult Weevils can live for up to four weeks without food.
Controlling an infestation is near impossible without the use of insecticides, so prevention in the first instance is best. This can be achieved by checking new grain as it comes in, storing all grain in a sanitised environment and storing it in sealed bins. Potentially infested grain stores should be checked fortnightly in warm weather and monthly in cooler weather, looking for signs of mould, damp and live insects.
Despite the fact that the Granary Weevil is a huge pest to stored grain, it is a fascinating insect that has developed a very clever method of finding a safe, secure, food rich environment in which to lay its eggs. And if you haven’t already been impressed with this beetle, it has also evolved with a clever defense mechanism- when adult Weevils feel threatened or are disturbed, they feign death by pulling their legs close to their body and turning on their backs. Overall, a successful survivor from larvae to adult.
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Weevils on Stored Grain — Entomology — Penn State University. 2015.Weevils on Stored Grain — Entomology — Penn State University. [ONLINE] Available at: http://ento.psu.edu/extension/factsheets/weevils-on-stored-grain. [Accessed 20 January 2015].
Granary weevil – Sitophilus granarius ( L.) – Primary insect pest. 2015.Granary weevil – Sitophilus granarius ( L.) – Primary insect pest. [ONLINE] Available at: http://www.grainscanada.gc.ca/storage-entrepose/pip-irp/gw-cg-eng.htm. [Accessed 20 January 2015].
Factsheet – Sitophilus granarius (Linnaeus, 1875) – Granary Weevil. 2015.Factsheet – Sitophilus granarius (Linnaeus, 1875) – Granary Weevil. [ONLINE] Available at:http://keys.lucidcentral.org/keys/v3/eafrinet/maize_pests/key/maize_pests/Media/Html/Sitophilus_granarius_%28Linnaeus_1875%29_-_Granary_Weevil.htm. [Accessed 20 January 2015].
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