Defining stored product pests
Stored product pests fall into two main insect orders:
- Coleoptera (beetles)
- Lepidoptera (moths)
Within each group, some species are considered as primary pests, they attack the whole grain, while others follow the initial damage as secondary pests. Losses can amount to 10% or more of the commodity through spoilage or secondary fungal infection.
Understanding Primary and Secondary Pests
Primary coleopteran pests include:
- grain weevils (Sitophilusgranarius, S.zeamais, S.oryzae)
- the lesser grain borer (Rhyzoperthadominica)
- the saw-toothed grain beetle (Oryzaephilussurinamensis.)
Secondary beetle pests include:
- the flour beetles (Triboliumconfusumand T.castaneum)
The main lepidopteron pests are secondary; they feed regularly on processed foods so are more common in domestic kitchens and larders. A variety of pests infest stored grain. This may occur in succession – primary then secondary. Eggs are laid in the grain or part-processed food (flour, bran etc.) where the larvae feed through to the pupal stage. Moth larvae usually fly to a hidden site away from the food source to pupate, adults then mate and eggs are deposited on or in suitable food. Emerging beetle adults feed on grain or find shelter in the structure of the silo/warehouse prior to invading new food sources.
Species category: Stored product pest
Scientific name: Tribolium Castaneum
There are many different species of Red Flour Beetle ranging from 2.3- 5.75mm in length. As the name suggests, they are Reddish-brown in colour or black/brown. Although small in size, they have a long body and distinctive segmented antennae with three large club-like segments at the end. They are cosmopolitan insects and found readily all across the world. Serious pests of cereal products, including grain, flour, porridge oats and rice bran. Flour beetles are most commonly encountered in bakeries, flour mills and other agricultural stored grain areas. Other products which may be attacked are oil seed, oil cake, nuts, dried fruit, spices, chocolate – even bones and other animal products.
The beetle lays its eggs in damaged grain and are prolific reproducers, laying 100s of eggs in a breeding season. They are not cold hardy, so only overwinter under warm conditions. They reside in the smallest of crevices, and are a particular problem in machinery where cereal and other food residues accumulate.
Flour beetles are a secondary grain pest and increase the feeding damage done by primary pests. When present in large numbers, flour beetles:
- Make flour prone to moulding and will also turn the product grey.
- Taint commodities with secretions from scent glands.
Species category: Beetles & Weevils
There are 4 key species:
1. Ptinus tectus (Australian Spider Beetle)
2. Ptinus fur (White Marked Spider Beetle)
3. Niptus hololeucus (Golden Spider Beetle)
4. Gibbium psylloides (Hump Spider Beetle)
Varying in size by species, these insects are on average 1.7-4mm in length and have a dull-brown/red body with golden hairs. The Hump Spider beetle is different and has a shiny, red-brown to black body with no hairs. Spider Beetles have 11 segmented antennae and all share a number of spider-like characteristics including a stout body, long legs and generally, a hairy appearance.
Gregarious and nocturnal, Spider Beetles spend the day in cracks and crevices amongst packaging and the fabric of a building. They thrive in old buildings where they find safe harbourages. Spider beetle larvae infest all types of dry animal and vegetable matter including grain, spices and fish meal. They will scavenge on debris and bore holes in order to find a safe place to pupate. In doing so, they destroy packaging and contaminate foods. There are 2-4 generations per year in unheated conditions. All stages except eggs and young larvae can overwinter. Peak activity is reached between August and November.
The Australian Spider Beetle is Australasian in origin and now is widely distributed. Most spider beetle species are cosmopolitan and are rarely imported.
They enjoy dark and damp conditions and readily feed on moisture-damaged food. Infestations often originate from birds’ nests. Spider beetles are becoming increasingly common in domestic premises where they are found in attics, wall cavities and floorboard cracks. Granaries and bakeries also offer the perfect conditions and food sources. Hump Spider beetles are tolerant of cool conditions and can survive for long periods without food supplies.
Spider beetles can reduce the quality of commodities by contaminating them with webbing and droppings. The larvae bore into packaging and the grain itself, in addition to other materials such as grain sacks, leaving behind tell-tale holes.
Scientific name: Sitophilus granarius
There are 3 different species:
- Sitophilus granaries (Grain Weevil)
- Sitophilus oryzae (Rice weevil)
- Sitophilus zeamais (Maize Weevil)
Each of these species varies considerably in size but has a distinctive elongated snout which is adapted to the size of its preferred grain. Typically, they reach 2-4mm in length and have a long cylindrical body which is dark brown or nearly black in colour. Grain weevils are encountered in all temperate and warm-temperate climates. They are widely distributed around Europe. Both adults and larvae are cold-hardy. Rice and Maize weevils are widely distributed in tropical and sub-tropical areas and will be carried to temperate areas on imported commodities. The maize weevil breeds on maize in the field but the Rice weevil only breeds in stored grain. Both insects will not normally overwinter in unheated premises or grain stored at normal temperatures.
Grain weevils do not fly but instead, infestations often occur after being imported in grain and cereal products, also from the fabric of vehicles used to transport grain or buildings to store it.
The female will lay a single egg inside the grain, where larva and pupa stages will occur, once developed, the weevil bores its way out leaving a hole in the grain. The Grain weevil can only breed in grain with moisture content of more than 9.5% and at temperature within the range 13-35C.
Grain weevils are primary grain pests, infesting undamaged grain and attacking other hard cereal products such as macaroni and spaghetti. Weevil-damaged grain is readily recognised by the presence of large holes which are the exit holes of the emerging adults. Both the adults and the larvae feed on the grain causing holes and also contamination with their excretions. Grain quality and marketability is reduced.