Pest control in stored grains requires strategic information in order to be certain in the selection of products, without running the risk of facing economic consequences or inefficient control mechanisms. First of all, it is necessary to know the active ingredients registered at SENASA (the body responsible for guaranteeing and certificating the health and quality of farming, fishing and forest production) and, additionally, to be informed about how each active ingredient acts and which is the most convenient use.
The law 27262, recently passed, bans phytosanitary treatments during the loading and transportation of grains. This norm, which already had similar precedent regulations in Córdoba and Santa Fe, promotes good preventive practices with products which, apart from being economically more profitable, allow maintenance of the grains without deterioration of their treatment quality over long periods. In the case of having to resort to curing treatments, these will have to be carried out at the same location, allowing the grace period indicated on the labels to elapse before loading the goods.
Among the most widely used products registered at SENASA, four preventive and two curing active ingredients stand out. In the preventive ones there are two organophosphorus: Chlorpyrifos methyl and pirimiphos methyl, and two pyrethroids: deltamethrin and lambda-cyhalothrin. A preventive of very short duration dichlorvos (DDVP) and the phosphine-based curing fumigants: hydrogen phosphide and magnesium phosphide.
Active ingredients analysis:
- Chlorpyrifos methyl, chemical classification: organophosphorus; Use: insecticide; Action: contact, ingestion; vapor pressure: 5.6 mP at 20°C.
- Pirimiphos methyl, chemical classification: organophosphorus; Use: insecticide, weevil poison and acaricide; Action: contact, ingestion and inhalation by larvae, pupae and adults; vapour pressure: 13 mP at 20° C.
- Deltamethrin: Chemical classification: pyrethroid + benzodioxole; Action: contact and ingestion; Vapour pressure: 0. 002 mP (millipascals at 20° C). This active ingredient for the use on stored grains is always formulated with piperonyl butoxide, which is a synergist. As is mentioned earlier, deltamethrin is a molecule with very low vapour pressure which makes it very stable, and with the effect of this synergy it improves its performance with respect to the control of sitophilus.
- Lambda-cyhalothrin ME: Chemical classification: pyrethroid; Action: contact, ingestion and inhalation; Vapour pressure: 0.2 mP. Since it is micro-encapsulated, its residual power increases.
- DDVP or dichlorvos: Chemical classification: organophosphorus; Action: contact, ingestion and inhalation (curing action); Vapour pressure: 1.600 mP at 20° C (within the first 8 hours after its application the active ingredient degrades by 50%).
- Phosphine: Chemical classification: inorganic; Action: inhalation (A. Curing): hydrogen phosphorus gas liberation, high volatility gas.
From the previous analysis it follows that both Dichlorvos and Phosphine, commonly known as “knock down” (for its rapid action), are curing pesticides that cannot be used in a preventive treatment since the active ingredient will rapidly degrade and will no longer be able to prevent an infection just a few days after being applied.
When Dichlorvos is used in high doses it can have a protection of up to 30 days after treatment. However, it is also necessary to be aware that during the first 8 hours after its application it degrades by 50%. It is usually a product used during loading in order to stop an infection.
When adult, the pests place their eggs on the particles of dust. In these cases, if the facilities are treated with preventive products the control will be effective and when these eggs are hatched the larvae, by passing through this dust containing active ingredients, will immediately die, especially if the selected product is a larvicide.
It is recommended to use the preventive products in mixtures and in grain vein, as well as in the treatment of the facilities. For example, an organophosphorus with a pyrethroid. With the organophosphorus we can control for long periods almost all the pests except for the lesser grain borer (Rhyzopertha dominica) insect, which presents resistance to organophosphorus, therefore it is essential to mix a pyrethroid at low doses in order to control this primary infection pest which causes great damage, especially if it acts for several months in the barn.
There is an old myth about grain storage which says that it is recommended to add DDVP together with a preventive in order to obtain a knockdown effect. This is an incorrect practice. Whatever the organophosphorus preventive may be, and especially pirimiphos methyl due to its vapour pressure, it ensures that within 48 hours no pest remains alive.
Moreover, for treatments carried out at empty facilities it is important to use products which control not only adults such as pyrethroids, but also larvae and pupae too, since it has to be borne in mind that a dead larva does not reach the adult stage, but that an adult places between 80 and 600 eggs, depending on the species. This is simply the way of achieving a successful control at the facilities, which interrupts the pest cycle, preventing their reproduction.
Each pest problem has to be studied in order to determine which product to apply. The first thing we have to take into account is that the product is registered at SENASA for that end. Nowadays we have serious problems with our grain and sub-product exports because the buyers detect residues of pesticides that are not registered for pest control in stored grains.
The products are not all the same and no matter if they are of good quality and well-backed, each of them has to be used as the situation warrants. In this way we will carry out efficient treatments without contaminating the cereals which in the future will be turned into food.