Insects of Stored Products and Food

Insects found in stored agricultural products and food are classified into several classes. The main insect species belong to the classes Coleoptera and Lepidoptera, but there are also species of the classes Diptera, Psocoptera, etc. In the broadest sense of the term, storage insects are also cockroaches (Blattaria) or “fish” (Thysanura). Also, in places that host food, Hymenoptera and Hemiptera can often be found, as predators or parasitoids of other species. In addition, many species of mites are also found in stored food, mainly of the order Astigmata but also Prostigmata, Mesostigmata etc. Finally, important animal enemies of warehouses are also rodents, which are mentioned in another section of the Guide. Below we will refer only to the two most important orders, the Coleoptera and the Lepidoptera.

The warehouse ecosystem is particularly complex, as it is a unique “closed” ecosystem, which is entirely man-made and is not affected by the external environment (temperature, sunshine, etc.) to the extent that with other related systems (eg greenhouses). Thus, insects (as well as other animal enemies), in most cases, have adapted their biology to the characteristics of each space that hosts each habitat. A characteristic of these insects is their ability to “travel” over long distances (insect travelers) with the same means by which food travels, via land, sea and air routes. For this reason, the main storage insects are now worldwide in distribution. Although evolutionarily, these insects have been field insects in the past, many of them are almost exclusively found indoors and are therefore also relevant to Public Health (see below). Both Coleoptera and Lepidoptera are omnivorous, while as aphids, only Coleoptera feed.

Most species found in stored foods follow a specific ecological succession, which is largely determined by the type and condition of the product. Thus, the infestation often starts from the so-called “primary” species (primary colonizers) which are capable of easily infesting whole products, such as cereal seeds and other related “solid” products (pasta, etc.). Several species are classified here, such as Sitophilus oryzae, S. granarius, S. zeamais (Coleoptera: Curculionidae), Rhyzopertha dominica (Coleoptera: Bostrychidae) and Sitotroga cerealella (Lepidoptera: Gelechiidae), Acanthoscelides obtectus and Bruchus spp. (Coleoptera: Bruchidae).

For these species, the larvae develop inside the seeds and the infestation progresses without being seen. The secondary species (secondary colonizers) are those that grow in already infected products (eg infected seeds from primary species) or processed products (eg flour, bran etc.). For these species, the larvae develop on the outside of the products. Most storage insects fall into this category. Indicative species are Tribolium castaneum, T. confusum, Tenebrio molitor, Palorus spp., Latheticus oryzae (Coleoptera: Tenebrionidae), Ephestia elutella, E. kuehniella, E. cautella, Plodia interpunctella, Pyralis farinalis (Lepidoptera: Pyralidae), Tinea granella (Lepidoptera: Tineidae), Oryzaephilus surinamensis, O. mercator, Ahasverus advena (Coleoptera: Silvanidae) and Cryptolestes spp. (Coleoptera: Laemophloeidae).

These species rapidly develop large populations while some have a particularly large range of food preferences. For example, P. interpunctella is found in flour but also in botanicals, chocolate, legumes and nuts. Likewise, the so-called “tobacco beetle”, Lasioderma serricorne (Coleoptera: Anobiidae), although it is the most important entomological enemy of stored tobacco, is found in flour, bran, chocolate, spices, dried fruits, pasta, etc. In contrast, some species, such as Carpophilus hemipterus (Coleoptera: Nitidulidae), have a narrower range of food preferences, such as dried fruit. Then, another class of insects plays an important role, which are generally called “fungus feeders”. Many species belong here, with the most important species of the Cryptophagidae and Mycetophagidae families. The presence of these species is directly related to the decomposition and degradation of the product and their finding in areas that host food betrays poor hygiene conditions. In general, these species do not grow on the product itself but on fungal spores and hyphae that grow on it. The last stage of the ecological succession are the “scavengers” who cause the further quantitative deterioration and quality degradation of the product. Here belong the species of the Dermestidae family as well as the fungus gnats. Also, several species of the genera Attagenus, Trogoderma, Anthrenus and Dermestes should be mentioned, which readily feed on material of animal origin.

Additionally, one class of insects is the predators and parasitoids, which belong to the Hemiptera and Hymenoptera. These should be considered “beneficial” because they feed on other species present in the product. Nevertheless, the addition of beneficial species to the site and the product does not have the same importance as the corresponding use of beneficials in crop protection, given that it is essentially the addition of other insects to food. Therefore, biological control of stored product insects with predators and parasitoids should be considered with particular care. The same applies in the case of the addition of preparations of pathogenic insects, such as entomopathogenic fungi.

Finally, it should be emphasized that there is also a final category of insects, the so-called “guest insects” which are often found indoors, but are not related to the product. Their presence in warehouses and related areas is therefore random and any decision to take control measures should be decided based on the seriousness of their presence. These are mainly large-bodied Coleoptera but also Orthoptera such as Gryllidae.

Curculionidae

This is where the species of the genus Sitophilus belong. Sitophilus grows primarily on whole grain grains such as wheat, maize and rice but also more rarely on legume seeds. They are also often found in packaged foods such as brown rice and pasta. Only the flourishing stages are visible, while the immature stages develop inside the seeds, without the infestation being visible. Indicatively, it is reported that until the appearance of the flourishing larvae, the larvae can internally reduce the weight of the seeds they attack by more than 50%.

Bostrychidae

It is a family of wood-eating species. However, one species, R. dominica, is one of the most important enemies of stored grains worldwide. It is favored by high temperatures, while it can attack seeds that have a low moisture content (even 8%). The female lays her eggs on the outer part of the seeds and the larva enters it causing serious damage. In addition to infesting stored products, this species has the ability to easily puncture various packaging such as polyethylene and polypropylene.

Laemophloeidae και Silvanidae

Here belong the species of the genera Cryptolestes and Oryzaephilus, respectively. They are found in starchy products, but also in dried fruits. These are secondary species, i.e. they are usually found in already affected or processed products and for this reason their larvae are found on the outside of the products. It should be mentioned, however, that many species are able to feed on the “soft” parts of the seed, i.e. the embryo, with great ease. They also prefer products and places with high humidity.

Tenebrionidae

A large number of species are included here, with the main species of the genus Tribolium. These species are mainly found in starchy products, such as flour and bran, but also in cereal grains. They rapidly develop significant populations while being highly resistant to many classes of insecticides with various modes of action.

Bruchidae

The so-called “worms” are insects found exclusively in stored legumes. In fact, each type of legume is also attacked by a different blight. For example, A. obtectus is found only in beans and soybeans, Bruchus lentis in lentils and B. pisorum in peas. As primary insects, they attack the seeds from their inner part. In many of the Bruchidae the infestation starts in the field, just before the harvest, and for this reason it is recommended to use insecticides in the pre-harvest stage.

Dermestidae

The Dermestidae include species that feed on materials of animal origin, such as the species of the genera Anthrenus and Dermestes. Their presence in warehouses is also linked to the simultaneous presence of other species. They infest skins, carpets, woolens as well as paintings, entomological collections, etc. The infestation is done exclusively by the larvae. However, this family also includes species that are herbivores, i.e. they feed on stored materials of plant origin, such as species of the genera Trogoderma and Attagenus.

Public health

Due to the relationship of these insects with food, but also with the places that host them, it is clear that their presence is directly related to Public Health. Their presence has direct effects on humans as well as chronic and cumulative effects. In general, the implications for Public Health can be divided into four categories.

A first category of animal enemies are those that cause asthma, dermatitis and rhinitis in humans. For example, Dermestidae larvae have hairs that can cause severe dermatitis, blisters and itching. Similarly, cockroaches, after dust mites, are a major cause of asthma.

A second category is the species that can transfer dangerous organisms to the products, such as fungi, which produce aflatoxins, or enterococci but also other bacteria that are very important for human health. Here, many species of Coleoptera (e.g. Tenebrionidae) but also mites or Psocoptera, seem to be able to “contaminate” the product with other organisms which will cause, depending on the case, liver damage, diarrhea, etc. .

A third category is the intake of insects (and other animal pests) with food. Although the intake of insects, i.e. their body parts and their derivatives, with food seems to be a frequent phenomenon, it is not always associated with harm to human health. Since most of the species mentioned above grow inside the product, the insects themselves, excrement, droppings and their secretions often end up in the food, through processing. The most common symptoms are vomiting, diarrhea and indigestion, but also allergies, spastic colitis, etc.

Finally, a last category is the species that produce the same toxins. For example, some of the Tenebrionidae found in flour, pasta and bran produce so-called quinones (benzoquinones), which can cause dermatitis, erythema, vision loss, etc. in humans. Even more important, however, is the fact that quinones appear to be associated with carcinogenesis and are now classified as “broad-spectrum” toxic substances. It should be noted that quinones react directly with certain substances in food, such as proteins, making even their detection difficult.

Modes of Management

Prevention: By the concept of prevention we mean the set of measures that must be taken, so as to prevent the entry of insects into the premises and the product. In addition to processing and storage areas, we should not forget that houses are also small food processing and storage units. Therefore, the condition of the space and the product should be such as to prevent infestation. Important interventions can take place at this level, such as so-called “hermetic” storage.

Monitoring: There are two sampling techniques. The first, so-called “absolute” sampling, is based on taking product samples, by various methods (often using a sampler). In this case, the number of insects is reduced per unit of product. The second, so-called “relevant” sampling, is based on various trapping devices, which may also have some attractants, pheromone or not. In general, traps used in stored agricultural products and food vary according to use, from probe and pitfalltraps which are used without an attractant, to hanging sticky traps to which pheromone is applied. In this case, the traps are checked at regular intervals and depending on the catches, control measures are decided.

Treatment: There are several ways to treat storage insects, with the main one, even today, being chemical control. Chemical control is based on two broad classes of insecticides, contact insecticides and fumigation or gaseous insecticides. The first are applied to the premises, usually by spraying, while some are approved for direct application to the product (e.g. to cereals). Most of them are neurotoxic, that is, they act on the nervous system through contact or through ingestion (stomach). In contrast, gaseous insecticides work through the insects’ respiration. These include phosphine, which is the most important insecticide in the category of fumigants, widely used in cereals and other products.

Today, the development of other methods is considered imperative, due to the development of resistance by many insects and also due to the adverse effects of chemical insecticides on health. Thus, various alternative methods have been developed in recent decades. In these, the applications of various gases such as carbon dioxide and nitrogen are classified. Similarly, other gases such as sulfuryl fluoride, ozone and propylene oxide have been successfully evaluated.

In general, apart from the toxicity of the above gases, the death of insects also comes from the reduction of oxygen (usually below 3%).

Another promising method is the use of pheromones to disrupt the biology of insects in order to reduce their population.

Extreme temperatures and especially the so-called “thermal disinfestation” are today one of the most important methods for dealing with storage insects in food processing areas. Finally, the application of vacuum, infrared radiation and the use of packaging that does not favor the growth of insects, also have important prospects for the immediate future.