Tiny threats, big losses: why grain store pest control matters

After a season of care and attention in the field, grain pests can devastate a crop once it’s in storage. James Phelps from Barrettine Environmental Health shows how to identify the worst offenders and what steps can be taken to avoid or mitigate loss.

Cereal crops in the UK are constantly subject to the threat of field pests and diseases, but the risk also continues post-harvest. Without a comprehensive understanding of the behaviours and biology of the insect pests which can inhabit stored grains and harbour in the storage area year after year, they can cause ongoing issues with loss of quality in stored grain and potential livestock health implications.

Below is a list of several commonly encountered stored product pests. Although this is not exhaustive, these species are certainly among the usual suspects.

Weevils:

Weevils are primary pests of stored grain; the female bores into whole kernels using her rostrum and deposits an egg inside the germ. The larvae then develop inside the grain, consuming the germ as they grow, before metamorphosing and emerging as an adult to begin the cycle anew. Weevils act as a pilot species when grain becomes infested, as the damage they cause helps to create an environment suitable for other pests. Commonly infested commodities include wheat, barley, oats, rye, maize and rice.

Risk factors: warm grain, moisture above safe storage levels, poorly ventilated storage.

Key indicators: hollow grains, round emergence holes, fine dust residues and live adults in samples.

Saw toothed grain beetle:

Saw toothed grain beetles are incapable of biting through an intact kernel and are therefore secondary pests, feeding on broken grains and dust. Eggs are deposited loosely among food material and develop externally from grain. These beetles are numerous, breeding rapidly in warm conditions and hiding in cracks or crevices where food sources accumulate between harvests.

Risk factors: contamination, further loss of grain protein and carbohydrate content.

Key indicators: numerous small active beetles, infestation in cracks and machinery, dust accumulation.

Red flour beetle:

Red flour beetles are a secondary pest, feeding on debris and damaged grains resulting from primary pest infestation. They are easily distinguished from the saw toothed grain beetle by the lack of tooth-like projections on the thorax. Eggs are laid in food material, with larvae appearing yellowish and worm-like, and infestations are often accompanied by a strong unpleasant/musty odour. Warm, humid stores are their ideal habitat.

Risk factors: warm indoor facilities, accumulated grain dust and prior damage from primary pests.

Key indicators: numerous beetles, larvae in dust/debris, musty odour.

Confused flour beetle:

Confused flour beetles are closely related to red flour beetles and it is difficult to tell the two species apart without microscopic examination; however, T. confusum’s inability to fly can provide a basic identification. Infestations are similar in character to those of the red flour beetle.

Risk factors: warm indoor facilities, accumulated grain dust and prior damage from primary pests.

Key indicators: numerous beetles, larvae in dust/debris, musty odour.

Angoumois grain moth:

The angoumois grain moth is a primary pest. Adult females lay eggs directly on the kernel; the larvae then hatch and bore into the grain, feeding on the germ and pupating inside before emerging as adults to reproduce.

Risk factors: warm grain, poor aeration, grain stored for prolonged periods.

Key indicators: small moths flying around grain, circular exit holes, hollow grain, strongly associated with warmer storage environments.

Indian meal moth:

The Indian meal moth is a secondary pest; the adult female lays eggs among dust and debris created by other pests, where the cream/white larvae with a brown head feed and develop. These larvae produce silk webbing which creates clumps of damaged grain before they crawl away to pupate.

Risk factors: warm indoor facilities, grain held for prolonged periods, poor stock rotation.

Key indicators: adult moths flying, silk threads in stored grain, larvae migrating away from food sources to pupate.

Psocoptera:

Psocids are a group of species commonly referred to as ‘booklice’. They are a tertiary pest, feeding on the microscopic fungi and mould found in grain with poorly controlled moisture or temperature. Psocids do not directly consume grain but are often a consequence of infestation by primary or secondary pests. They reproduce rapidly in favourable conditions and cause further contamination as numbers increase.

Risk factors: high humidity (>65% RH), condensation, presence of mould/fungus.

Key indicators: tiny insects on the surface of grain, damp odour, visible mould/fungus.

Grain mites:

Grain mites are a group of arachnid species (the same family as spiders), very small and often difficult to spot with the naked eye unless a significant infestation is present. They are typically tertiary pests, associated with moisture, feeding on microscopic moulds and fungi. Populations can grow dramatically in a short time. Infestations can elevate temperatures, produce a musty odour and cause allergic reactions in both humans and animals.

Risk factors: grain moisture >15%, high humidity, poorly ventilated storage.

Key indicators: surface crusting, damp hotspots, ‘moving dust’ in cases of heavy infestation.

Grain store preparation

Prior to moving grain into the storage area, whether enclosed or exterior, it is always necessary to clean the area thoroughly to ensure quality is maintained for the duration of storage. Initially this can be achieved with a thorough wash and sweep to remove larger debris and dust. However, the only way to ensure that insects harbouring in the building are eliminated and prevented from re-establishing is through chemical treatment with CropProtect 25EC, which is proven to kill all pests other than grain mites, or an equivalent product.

Application of this pyrethroid insecticide will flush out any insects harbouring in cracks and crevices and act on contact to eliminate them. Once applied, the active ingredient remains on the surface for up to 12 weeks, preventing new insects from migrating into the storage area before grain is moved in. Alternatively, CropProtect 25EC may be applied directly to the grain as an admixture to provide the same protection for up to 12 months, ensuring any insects trying to colonise the pile are controlled shortly after making contact. When combined with a propionic acid treatment such as Propcorn NC, the grain can be further protected from any fungal or mould growth which may attract tertiary pests, creating a comprehensive solution for whole and rolled grains stored aerobically where moisture levels are higher than recommended for standard storage.

The risks to crimped grain

Untreated grain is at the highest risk of infestation; properly ensiled crimped grains treated with CrimpSafe 300 or CrimpSafe Hi-Dry should deter most common grain pests. However, if grain is contaminated with pests prior to clamping, infestations may occur and cause loss of nutritive value – mainly carbohydrate, protein and fat content. This decreases the performance of the resulting feed and can cause microbial contamination, affecting animal health. For this reason crimped grains should ideally be processed, treated, ensiled and sealed within 24 hours of combining.

A properly stored and fermented clamp will naturally hinder most common grain store pests from feeding, as the low oxygen, increased acidity and temperature are unappealing to grain borers and moths. They may however be found in small numbers feeding near the face if the clamp is improperly stored or fermented. The greatest risk to ensiled feed is the tertiary pests listed, particularly grain mites. These are hardier and more attracted to fungal growth, and due to their small size may be harder to spot until an infestation is severe. Their presence can disrupt the fermentation process through the introduction of pathogenic microbes and fungi carried on their bodies, and grain mites in particular are known to cause allergic reactions through exposure. CropProtect will not eliminate mites in crimped cereals, but good management of the face at feedout – such as using a shear bucket or grab, crossing and removing loose infested material first – can significantly reduce losses.