Pests on stored soybean seeds

Maintaining the quality of soybean seeds during the storage period requires, in addition to observing physiological and sanitary parameters, the correct management of pests such as Lasioderma serricorne, Oryzaephilus surinamensis, Cryptolestes ferrugineu e Ephestia kuehniella, responsible for grain deterioration. Purging with phosphine biocidal gas is one of the alternatives to prevent and treat the incidence of these insects.

The evolution of soybean cultivars and the population management of the crop for grain production have demanded the supply of seeds with high physical, physiological and sanitary qualities. Maintaining quality during the storage period, in addition to physiological and sanitary parameters, also requires pest management.

The quality of soybean seeds in storage can be affected by the action of several factors. Among these, storage pests, especially beetles Lasioderma serricorne, Oryzaephilus surinamensis e Cryptolestes ferrugineus, and the moth Ephestia kuehniella, may be responsible for seed deterioration.

Knowledge of the feeding habits of each pest is an important element in defining the management to be implemented on seeds during the storage period. According to this habit, pests can be classified as primary or secondary.

The primaries are those that attack healthy seeds and, depending on the part they affect, they can be called internal or external primaries. The internal ones pierce and penetrate the seeds to complete their development. They feed on all seeds and enable the installation of other deterioration agents. An example of this group is the species L. serricorne. The external primaries destroy the outer part of the seed (tegument) and, later, feed on the inner part without, however, developing inside. The seed is destroyed only for food purposes.

Pests that depend on already damaged or broken seeds for food are considered secondary, as they cannot attack them when intact. They occur in cracked, broken or even damaged seeds by primary pests and generally infest from the period of receipt to the processing of the product. They have the characteristic of multiplying quickly and, in most cases, cause high losses.

    Pests in stored soybeans can represent a concern for storers, due to: (a) insect migration is facilitated by the storage of other seed species in the same processing unit; (b) loss of seed quality attacked by insects; and (c) loss of all work and economic value of the seed that must be sold as grain for industry. The main species that currently cause losses in soybean storage are Lasioderma serricorne, Oryzaephilus surinamensis, Cryptolestes ferrugineu and Ephestia kuehniella.

Lasioderma serricorne (Coleoptera: Anobiidae)

This pest originates from stored tobacco, which is why it is called tobacco beetle and, recently, it has started to occur frequently in soybean grains and seeds during storage. It is cosmopolitan, found in practically all countries, feeding on stored dry products. In Brazil, it has been reported in all regions and in all producing states, in storage of cereals and oilseeds, such as soybeans.

Adults live up to 20 days and do not feed. The larvae dig into the products, in this case stored soybeans, where they form galleries. It is not capable of attacking living plants, although it affects a large number of products in storage, such as dried fruits, tobacco, condiments, cereals, oilseed grains, bran, flour, pasta, biscuits and feed. It is often observed in manufactured products of plant origin, such as cigarettes and cigars. Research carried out at Embrapa Soja showed that the pest consumes, survives and develops adequately on a diet of soybeans. It was possible to obtain more than 800 adult insects, after 140 days of development, in glass jars with 220 grams of soybeans. The consumption of soybeans increased as the initial infestation increased, due to the multiplication of the species, demonstrating its wide adaptation in soybeans during storage. These results highlighted the development of  L. serricorne in soybeans during storage, with high potential for destruction and consumption of the stored product.

Oryzaephilus surinamensis (Coleoptera: Silvanidae)

It is a cosmopolitan species that occurs in practically all regions of the world, always in stored products. In Brazil, the pest is present throughout the grain producing region, well adapted to the climatic regions of production, with a preference for hot climates where it is best distributed.

It is considered a pest that infests a wide variety of commodities, especially cereals, dried fruits and oilseeds. Thus, grains of corn, wheat, rice, soybeans, barley, oats, among others, are the most sought after by the species. It is also a pest of storage structures, such as hoppers, cleaning machines, elevators, dryers, tunnels, silo bottoms and shipping boxes.

This is a secondary pest that attacks broken, split grains and grain residues. It can damage the grain mass, being significant when it occurs in high population density. It appears in practically all storage units, where it causes grain deterioration due to a sharp rise in temperature. It is a species that is very tolerant to chemical insecticides, being one of the first to colonize the grain mass after applying these products.

 

Cryptolestes ferrugineus (Coleoptera: Cucujidae)

It is a cosmopolitan species found in various parts of the world where dry products exist. In Brazil, it occurs throughout the grain and seed producing region. Generally associated with other pest species of stored products, in warmer regions of the country.

It is a secondary pest of major importance in the storage of soybeans, corn, wheat, rice, barley and oats, in addition to infesting nuts and nuts. It is also a pest infesting storage structures such as hoppers, cleaning machines, elevators, dryers, tunnels, silo bottoms and shipping boxes.

It is a pest that can destroy split, cracked and broken grains, penetrating them and attacking the germ. It consumes broken grains and grain and flour residues, causing a rise in the temperature of the grain mass and deterioration. The same way that O. surinamensis, appears in large quantities in warehouses, after treatment with insecticides, as it is tolerant to these treatments. It is necessary to study and determine the damage potential of this insect, taking into account the ease of reproduction in masses of stored grain and the level of resistance to the insecticides used.

 

Ephestia kuehniella (Lepidoptera: Pyralidae)

The grain moth normally occurs in all regions of the world. In Brazil, it is distributed throughout the grain producing region from North to South and from East to West of the country. It occurs in the storage of products throughout the year, as long as there is food supply. They are reported in cocoa, tobacco, dried fruits, dehydrated vegetables, cereals and oilseeds. Grains and seeds from soybeans, corn, sorghum, wheat, rice, barley and oats are preferred, as well as processed products such as cookies, cereal bars and chocolates.

It is a secondary pest, as the larvae develop on grain and flour residues left by the action of other pests. Its attack harms the quality of stored grains and seeds, due to the formation of a web on their surface or even in the bags during storage. It penetrates into the seed lots, forming a position in the seams of the bag. It is responsible for the large number of thermofogging treatments in the units, during the storage period of seed lots.

 

Seed purging

Purging or fumigation is a technique used to eliminate pests in stored seeds and grains using gas. It must be carried out whenever there is an infestation in the batch, silo or warehouse. This process can be carried out in the most different locations, as long as the perfect sealing of the area to be purged and the safety standards for the products in use are observed. The gas released or introduced into the seed lot must remain in this environment in a lethal concentration for pests. Therefore, any air outlet or inlet must always be sealed with appropriate materials, such as purge canvas.

Phosphine is a general biocide, a highly toxic gas, which is released in the presence of relative humidity in the air. Although its use in seeds has been used for many years, it has only recently started to be used in soybean seeds, due to the presence of pests during storage. All handling of phosphine to carry out purging must be done with appropriate PPE (gloves, masks with filters for toxic gases, etc.), always maintaining operator safety. Both when placing the phosphine tablets and when releasing the purge (removing the tarpaulin or opening the silo), people must be protected with PPE, and the place must be ventilated. Special care must be taken when removing the tarpaulin from the silo or batch of seeds, as there is a high concentration of phosphine gas, which must be ventilated immediately so that it dissipates and degrades in the atmosphere with oxygen. The warehouse doors must be open and with forced ventilation to allow the removal of remaining gas. The presence of people without PPE is prohibited in warehouses where purge operations are being carried out, due to the imminent risk of poisoning.

The distribution of the gas must be uniform at all points of the mass of grains or seeds to be treated, thus controlling all pests, in their different life cycle forms. The release rate of phosphine gas (PH₃) from the fumigant tablets, will determine the time necessary for the total mortality of the pests and the efficiency of the purging process.

In studies monitoring the concentration of phosphine gas, it was observed the gradual and uniform release of phosphine gas over time, indicating that the PH gas release reaction₃, coming from the fumigant tablets, reached high concentrations until the end of the experiment. Phosphine gas was released from 0,5 hours after introducing the tablets into the chamber, slowly increasing until 8 hours, and quickly after 24 hours of release, reaching levels above 1.200ppm of PH gas.₃ after 72 hours. This high concentration remained until the end of the experiment, 240 hours after the release of the fumigant tablets. The temperature and relative humidity of the air, throughout the evaluation period, were 24ºC ± 1ºC and 40% ± 5%, respectively, which are considered medium to low for the rapid release of phosphine gas from the fumigant tablets. When the temperature of the area to be purged is below 10⁰C or relative air humidity lower than 25%, purging is not recommended due to the difficulty in the phosphine gas release reaction.

For a purge to be efficient, that is, for all life stages of the insect (egg, larva, pupa and adults) to be eliminated, the phosphine concentration must be maintained at at least 400ppm for at least 120 hours, and the gas distribution inside the silo must be uniform. According to these authors, the phosphine concentration, after releasing the tablets, was above 400ppm from the first 24 hours onwards and remained at this level in all formulations evaluated, until the end of the experiment, at 240 hours.

Research carried out with soybean seeds with two levels of vigor determined through the tetrazolium test, from the cultivars Embrapa 48 (93% and 82%) and CD202 (69% and 62%), concentrations of 1g, 2g and 3g of PH were used.₃/m³, achieved by applying 3g, 6g and 9g of the commercial product. Daily, for seven days, the concentration of phosphine gas inside each chamber was monitored using the Silocheck meter. The phosphine concentration was maintained in the chambers throughout the experiment, allowing the seeds to be exposed to the gas at the determined concentrations. Even the lowest dose maintained a concentration above 400ppm, which is the technical reference minimum concentration for the elimination of all stages of insect pests from stored seeds.

The physiological quality of the seed was evaluated through germination tests, seedling length, hypocotyl length and accelerated aging, the results of which reported in Table 1 do not allow detecting effects between treatments, meaning that phosphine did not harm seed quality.

The seedling length and hypocotyl tests, which would indicate whether there was phosphine toxicity in the cultivars, at both vigor levels, did not detect any effect of the phosphine concentrations used in the seeds. 

Considerations

The purging of soybean seeds with phosphine can be carried out with an exposure period of 168 hours without any loss in physiological quality.

The phosphine gas used to purge soybean seeds, despite being highly toxic, is quite safe for operators and effective in controlling all stages (egg, larvae, pupa and adults) of stored seed pests.

The person technically responsible for storing soybean seeds must follow the technical recommendations recommended by the Integrated Pest Management of Stored Seeds (MIP Sementes), using when necessary to purge the seeds with phosphine. This purge must comply with a minimum concentration of 400ppm of phosphine in all places where the treatment was carried out, and a minimum period of never less than 120 hours of exposure to the gas.

The measurement of the concentration of phosphine gas during purging must be done with the aid of measuring equipment designed for this purpose, already available on the Brazilian market.

Irineu Lorini, Francisco Carlos Krzyzanowski, José de Barros França-Neto and Ademir Assis Henning, Embrapa Soja

Article published in issue 193 of Cultivar Grandes Culturas.