The grain weevil, also known as the wheat weevil is part of the Sitophilus genus and Sitophilus granaries specie. It is an insect that can be found everywhere in the world and is considered a common household pest. It is part of the Coleoptera order, which is the ones where you can also find beetles. There is an estimate that approximately 1000 weevil species exist only in North America. They have a wide range of colours and body shapes, and the size of their body also ranges in size depending on the specie. They can cause significant damage to stored grains and can even drastically decrease crops.
Description and distribution
Grain weevils can be found in all temperate climates and are widely distributed throughout Europe, being a pest to crops on this continent. The species can be found in storage facilities and infest crops of: rye, wheat, barley, rice, buckwheat and even acorns. Usually, infestations appear as a result of importation in cereal products, caused by the vehicles or the buildings used to transport or storage the products, or even the bags the grains are kept in, as the weevils can easily find harbourage in the seams and weave. Rice and maize weevils can also be distributed in the same way; however, they are more often found in tropical or sub-tropical areas.
When it comes to its appearance, the grain weevil can be recognised by its brown colour and their elongated snouts and chewing mouthparts. They are about 3-5 mm long, have wings, but are incapable of flight. The head and the neck shield are covered in dots, the wings cover the whole back side of the insect and they are glued together, the insect can’t fly. The insect has vertical lines all over its body, and they are dotted. The larvae of the weevils are about 2.3 mm in length and are white, their head is brown, they have no legs, and are curved in the middle, with the surface of the larvae body seeming to have wrinkles.
Breeding and dietary information
The female grain weevil can normally lay between 36 and 254 eggs, while usually one egg is deposited in each grain kernel. The female will chew through the grain and will lay its eggs in it, covering it with secretions to protect the egg afterwards. All the larval and pupal stages of the insect will occur while it is in the grain, as it is feeding from the inside until it is ready for pupation. After this, the now adult insect bores a hole from inside the grain and exits. The adult is then ready to reproduce and the cycle continues. At a temperature of 29°C the evolution from egg to adult will take somewhere between 29 and 34 days. The humidity of the wheat has to be at least 9%, or the reproduction can’t take place. In heated rooms, there can be 3-4 generations of weevils a year. Larvae are never free-living and develop entirely within the grain. They moult four times, finally pupating within the grain after 6-8 weeks. Their lifecycle takes around 5 weeks in the summer, but can last up to 20 weeks in cooler temperatures, and adults can live up to 8 months after emerging. One pair of weevils may produce up to 6,000 offspring per year.
They can affect any type of stored grains, as they prefer these to the ones found in the fields. The most infested crops can be of: rye, wheat, barley, rice, buckwheat and even acorns. In the household, they are often found when opening a bag of flour, although they can easily infest nut, seeds, breakfast cereals, oats and barley. If ingested, they can cause various disease, such as E. coli infections.
Grain weevil as a pest
As mentioned above, most of the damage is caused by the weevil in its larval stage. As the female only bores a hole to insert its egg in the grain kernel, the larvae are the ones that gradually consume the kernels from the inside. The infested grain will be tainted with parts of the weevils and faeces and they need to be taken out. There is a possibility of infestation with mould and fruit mites as well, if the grains get contaminated by the weevils.
They can be considered as primary pests of grain as they are capable of damaging otherwise untouched grain. The range of contamination they can cause will affect the weight and quality of the grains. They can also affect the germ of the grain, and in cases this does not happen and germination does take place, the seedling will be weak and vulnerable to attacks from moulds, bacteria or other insects. The tainting of the grains will also affect their taste and quality. Grains damaged by weevils can be easily recognised by the presence of the holes from where the adults emerged.
When dealing with a grain weevil infestation there are several methods of prevailing against them, but you have to keep in mind that the infested grains will most likely have to be thrown away. Stores should be constructed well enough to ensure maintenance of correct storage conditions and allow for easy cleaning which is necessary quite often if you want to avoid an infestation. Stores should be insulated, well ventilated and also damp-proof. Cracks and crevices, which can easily provide harbourage for these pests, need to be kept to a minimum. Commodities should be stacked neatly above the floor level using pallets, away from walls and should not be touching the ceiling either. Gaps between stacks are also needed, as they will allow ventilation, regular inspections, cleaning and, if necessary, treatment with insecticides. Appropriate stock rotation is important as is the careful choice of packaging, as these can help deter insect attack if properly used. Generally, thick, tough materials with a smooth, shiny finish are preferred. Packs should be strong and well-sealed. It is important to ensure that there are no food residues in which beetles can breed and develop, as they will then infest new materials. All infested commodities should be destroyed or fumigated. All grain taken into store should be dried to a suitable moisture content and temperature in order to prevent any weevils from surviving. You must avoid at all cost aiding the weevils in reaching your stored grains, whether they are in a larger storage facility, or just in the smaller bags in your kitchen.
Grain weevils are normally hard to detect and usually, all of the grain in an infested storage facility will need to be destroyed. To prevent other infestations, you can use methods of control such as pesticides, different methods of masking the odour of the grain with unpleasant scents, or even introducing other organisms that are predators of the weevils. After getting rid of weevils you will need to make sure you keep a high level of sanitation, as they can easily return. In the kitchen, you should store your grains in metallic containers with tight lids, as cardboard, even fortified ones, are easily drilled through. You should only buy small quantities of grains, and you can also refrigerate them, just to make sure any insects are killed.
For more details on ways to deal with a grain weevil infestation you can check our related article ‘How to get rid of grain weevil’, or, if you just want to make sure you have covered all the means in which you can avoid an infestation, we have a lot of tips on how to do this in our article about how you can ‘Prevent a grain weevil infestation’.