Damage from the bean leaf-cutter caterpillar

The bean leaf-cutter caterpillar (Spodoptera frugiperda) is the species most observed in the initial phase of bean cultivation, responsible for cutting young plants close to the ground and causing severe damage. Its control involves the adoption of integrated measures, which, when adopted together, allow maintaining the sustainability of pest management.

The occurrence of complex caterpillars spodoptera in bean cultivation it has increased and in some it is necessary to be alert so that the injuries caused do not result in important losses in productivity. In this genus, the species cosmioides spodoptera, Spodoptera eridania, Spodoptera latisfacia e Spodotera frugiperda can attack bean plants at different stages of vegetative development.

Although newly emerged bean plants may be attacked by other caterpillars of the genus spodoptera, which pierce or destroy the cotyledons and attack the leaves, the species most commonly observed at this stage of crop development has been Spodoptera frugiperda, the corn armyworm or military armyworm, referred to by bean growers as the bean leaf-cutter caterpillar. This species is one of the most harmful to annual crops in tropical regions.

Damage from the bean leaf-cutter caterpillar

In bean cultivation, it is known as leaf-cutter caterpillar, as it attacks at the initial seedling stage, cutting the young plants close to the ground. The injuries are similar to those caused by the screwworm (Agrotis ipsilon). Outbreaks of Spodoptera frugiperda in bean crops they are usually associated with drier periods and economic damage can occur when attacks coincide with the occurrence of large caterpillars (last instars) and newly emerged plants or in the initial stages of growth. The attack of the pest at this stage of crop development reduces the number of plants per area and can compromise the production and profits of bean producers. In more developed plants, the caterpillars scrape the stem at ground level. The insects can tolerate the damage for longer, but they wilt and may be knocked over by the wind.

Description of the pest

The moths of Spodoptera frugiperda They measure 35mm to 38mm in wingspan, with sexual dimorphism (grayish-brown forewings in the female and darker in the male). Beige eggs are placed in dough. Due to cannibalistic behavior, first instar caterpillars have dispersive behavior. The caterpillars have a variety of colors (light green, dark brown or almost black) and measure around 40mm when they complete development. Those that attack bean seedlings normally come from other host plants, corn and sorghum and various grasses, including pastures. The caterpillar pupates in the soil, at a depth of 2cm to 8cm, and transforms into a moth in around eight to ten days.

Conditions favorable to the occurrence of the pest

In Brazil, it is present in all states and, as it is a polyphagous pest, it is responsible for damage to several crops, including beans. The intensification of cultivation systems in Brazilian producing regions, which allow the harvesting of two and even three crops/year, forming the so-called “green bridge”, associated with the large number of host plants, is the main factor in the permanence of this insect in the field all year round. The occurrence of caterpillars Spodoptera frugiperda that attack bean plants at the seedling stage is related to their presence on host plants, generally weeds or pastures, shortly before sowing.

Recommended management measures

To control the leafcutter caterpillar, Integrated Pest Management (IPM) is recommended, the basic principles of which must be used within the integrated production system approach. The population variation of Spodoptera frugiperda is determined by a complex set of factors, which, as far as possible, must be manipulated to prevent population growth from reaching levels that cause economic damage. However, isolated control methods should not be used, such as, for example, just the use of chemical control, but rather seek the integration of available practices to maintain the sustainability of pest management.

Bean crops cultivated under favorable conditions can better tolerate the attack of Spodoptera frugiperda, and the available recommendations regarding spacing, cultivars, planting times, fertilization, humidity conditions and soil preparation, crop rotation and crop association must be carefully observed when installing the crop.

In this context, it is possible to reduce the infestation of Spodoptera frugiperda by eliminating host plants (weeds, soybeans, corn, among others) at least three weeks before sowing. This will reduce the oviposition of moths in these areas, thus avoiding the presence of large caterpillars (3rd instar) that cause greater damage in the initial phase of bean development.

Natural biological control must always be favored through good agricultural practices, such as the use of selective products against natural enemies. The release of parasitoids such as Trichogramma in culture it is also a practice to be prioritized. They recommend the release of around 100 individuals per hectare when the first spawns or adults of the pest appear.

Thus, profits obtained from bean cultivation can be increased not only through increases in productivity, but also through reductions in production costs, as environmental stability can be increased and pest problems arising from driving can be avoided. inadequate crop production.

How and when to control the pest

The control level for soil pests that reduce bean plant stand is 10% of attacked plants or two plants cut or with wilt symptoms in two meters of row. If the control level is reached, insecticide sprays should be carried out directed at the base of the plant. 

There are no products registered with the Ministry of Agriculture, Livestock and Supply (Mapa) for the control of Spodoptera frugiperda in bean cultivation. When there are outbreaks, the pesticides acephate and carbaryl registered are used to control other bean leaf-cutting caterpillars: cutworm (Agrotis ipsilon) and soybean caterpillar (anticarsia gemmatalis).

Chemical control must be seen as an alternative or tool to be used when other control measures are not sufficient for effective combat, and is therefore complementary, and taking into account the relationships between benefits, risks and costs. The choice of product, dose and number of applications must be based on the severity and population level of the pest, the stage of development and its economicity. Correct applications mean reductions in the quantity of product applied, production costs, environmental pollution and residues in food.

Another important aspect concerns the insecticide application technology, which must also be considered when applying chemical control, paying attention to the maintenance and adjustment of equipment, volume of spray, application time and wind speed. Therefore, it is recommended to use appropriate nozzles, use enough spray volume to cover the area well, carry out applications at the coolest times of the day and ensure that the wind speed does not exceed 10km/hour.

Bean culture 

The common bean (Phaseolus vulgaris), with its different types of grains, in Brazil is cultivated in three different sowing seasons: 1st season beans or “water beans” or spring-summer cultivation; 2nd season beans or “dry beans” or summer-autumn cultivation; and 3rd season beans or “winter beans” or autumn-winter cultivation. Bean crops in the 1st and 2nd seasons correspond to more than 80% of national production. The main producing regions are the Southeast and Center-West, with highlights being the states of Paraná, Minas Gerais, São Paulo, Mato Grosso and Goiás.

The evolution of cultural practices, combined with the development of modern cultivars and technologies developed by research institutions and adopted by bean producers, has allowed significant gains in productivity in recent decades. On the other hand, one of the factors that have affected bean productivity and the producer's profitability is the attack of pests, whose expenditure on insecticides on average can reach 5% of the production cost.

Daniel de Brito Fragoso, Embrapa Rice and Beans

Article published in issue 199 of Cultivar Grandes Culturas.