Integrated management of caterpillars in soybeans

Since the beginning of soybean cultivation in Brazil, caterpillars have been among the main pests of the crop. Many decades later, despite all the technology developed for cultivation, they continue to threaten the national soybean industry and take away the peace of mind of many producers. The species may be different in some cases, but these insects continue to be among the main pests of soybeans. Learning to manage them, with the correct use of available tools, within the Integrated Management of Soy Pests (MIP-Soja), is the most effective way to control them and avoid losses

Until the end of the 90s, the soybean caterpillar was the main one in the crop. After the 2000s, this insect grew in importance and also reached the “status” of a key pest of soybeans, mainly due to the reduction of its natural enemies, among which were entomopathogenic fungi that naturally controlled its population. Currently, the complex of caterpillars of the genus spodoptera, despite not being a widespread key pest throughout the country, it causes fear among soybean farmers because it causes defoliation, attacks plant pods and is not controlled by Bt soybeans that express the Cry 1Ac protein. Damage to crops due to the attack of these caterpillars is largely due to the abandonment of Integrated Soy Pest Management (MIP-Soja) and the use of insecticides in a “scheduled” manner or in “piggyback” with herbicide spraying. and fungicides applied to the crop. Failure to adopt the refuge area for Bt soybeans is also a new threat to the management of these pests. Something that needs to be corrected and its adoption encouraged, as well as the resumption of MIP-Soja. To successfully control these pests, it is necessary to correctly use the main management tools available.

Main tools for handling and correct use

chemical control

Insecticides are among the most important pest management tools today and when used well they bring great benefits to the producer. However, the incorrect use of this tool, with unnecessary spraying, carried out without adopting the appropriate agronomic criteria, can cause great harm such as an abusive increase in production costs, selection of resistant insect populations, outbreaks of pests even greater than would otherwise occur. lack of application, not to mention the risks of contamination of humans and the environment.

When comparing spending on agrochemicals between the 2002/2003 and 2012/2013 harvests (Figure 1) it is possible to visualize the increase in production costs due to the use of pesticides. Part of these expenses can be reduced by avoiding the unnecessary use of insecticides. It is not recommended that soybean producers use control for any pest infestation or preventively. The application of insecticides must be economical and, therefore, only justifiable when the pest population is at levels known to be threatening to the profitability of the crop, known as action levels.

Results with early or super-early cycle soybean cultivars, indeterminate growth and small leaf area index indicate that soybeans remain tolerant to defoliation without losing productivity (Figure 2). Therefore, the recommended action levels (Tabela 1) are reliable, and there is no reason for the producer to apply before these levels are reached.

In addition to applying at the correct time, you also need to choose the right insecticide and dose. These procedures depend on the species of caterpillar or the predominant group, which can only be obtained by evaluating the crop with the patch cloth (Figure 3). Considering that caterpillars are all the same is a big mistake, as the best dose and product to use depend on which group of these insects is standing out in crops. For sampling in the field, the producer needs to separately record at least four groups of caterpillars, which are: spodoptera, false-medium caterpillars, caterpillars from the heliothinae group and the soybean caterpillar (Figure 3).

It is important to highlight that, in general, the insecticides most suitable for controlling caterpillars in soybeans are those that can combine good efficiency in controlling target pests with high selectivity for beneficial insects (bees, predatory insects and pest parasitoids). Insecticides from the group of insect growth regulators (popularly known as “physiological”), biological insecticides based on viruses (baculovirus) and bacteria (Bt) and some newer groups of insecticides such as spinosyns and diamides can be more selective to beneficial insects when compared to older groups of insecticides such as pyrethroids, carbamates, organophosphates, among others, and are therefore more suitable for controlling soybean caterpillars as long as they are duly registered with the Ministry of Agriculture, Livestock and Supply (Mapa) for the target pest.

Furthermore, it is important to “rotate” the use of insecticides with a different mode of action, which will reduce the selection of insects resistant to the pesticides used. Furthermore, the practice of preventive, “scheduled” or “piggyback” applications with other agrochemicals such as herbicides and fungicides should be avoided, as in the long term the problem will intensify, mainly due to the suppression of natural enemies, the selection of strains resistant and the increase in non-target pests.

Bt soy

In the 2013/14 harvest, Bt soybeans were released and used for cultivation in Brazil. This first generation of Bt soybeans aimed to control the species A. gemmatalis, C. includens, H. virescens and armpit drill in addition to the suppression of H. armigera. It is worth mentioning that the caterpillars of the complex spodoptera are not controlled by this technology.

It is important to highlight that while these plants will be controlling target pests 24/7, they are also selecting for Bt-resistant insects 24/7. Thus, in the same way as with conventional insecticides, the use of transgenic crops that express insecticidal toxins (Bt plants) has brought with it the enormous risk of selecting resistant insect populations.

Among the various measures to delay the selection of resistant individuals, planting the refuge is of particular importance. The refuge, therefore, is the cultivation of 20% of the area (in the same plot) with the same non-Bt crop (preferably an isoline) with the aim of reducing the selection of insects resistant to this technology (Figure 4). 

It is important to be clear that in the refuge area it is strategic to form individuals susceptible to the toxin that can interbreed with possible resistant insects from the Bt area and thus create insects that will continue to be killed by technology (Figure 4) which are heterozygotes. Therefore, the abusive use of insecticides in the refuge area must be avoided, destroying the insect population present in the area. The refuge area needs to produce economically, but in a sustainable way, where in addition to producing soy there is also the production of insects susceptible to technology. Therefore, it is essential that the management of the refuge area is carried out in accordance with the MIP-Soja. Technologies such as sampling using a sweeping cloth as well as the application of insecticides only when pest action levels are reached (rational application of insecticides) will continue to be essential in the refuge area as well as in the area Bt. Plants Bt They came to add to MIP and not to replace it!

b

Biological control of caterpillars in soybean can be carried out using entomopathogens, such as baculovirus and the bacteria Bacillus thurigiensis the most common examples, or with the use of egg parasitoids. Among the tactics that have shown good results, especially in the management of lepidoptera, is the use of egg parasitoids of the genus Trichogramma and more specifically Telenomus remus for the control of spodoptera spp. The main benefit of releasing these parasitoids is the control of pest eggs in all regions of the plant before any injury occurs. However, the success or failure of releases depends on knowledge of the bioecology of these parasitoids and their interaction with selective agrochemicals so that the main recommendations can be made.

Based on the research results obtained to date, it is possible to state that T. pretiosum has the potential to remain effective in field control for four days after release. The evaluation of its biological characteristics and thermal requirements demonstrate that this species is capable of developing throughout the soybean cycle at the most diverse temperatures. In the field, the average radius of action and the area of ​​dispersion of T. pretiosum in soybeans they were 8 m and 85,2 m², respectively. Therefore, the number of parasitoid release points, determined through the effective dispersion radius, must be 117 points per hectare, so that there is a homogeneous distribution in the treated area in the first 24 hours and thus obtain greater efficiency in parasitism and consequently in the pest control. This large number of points/ha can be an obstacle to the use of this technology, but the aerial release of parasitoids, using unmanned aerial vehicles (UAVs or drones), is already carried out in some locations and can meet this need.

As for the number of T. pretiosum to be released, it can be concluded that the best density of parasitoids for release was at the rate of 25,6 parasitoids/pest egg. After the release of this density of parasitoids, high parasitism of lepidopteran eggs occurred in all parts of the plant. However, as the evaluation of eggs in the field is impractical in commercial farming, another methodology to indicate the best time to release the parasitoid still needs to be evaluated and is the main challenge for the use of egg parasitoids in soybeans at the moment. The use of pheromone traps to detect moths may be an alternative, but the diversity of species in the crop still presents its challenges for the correct use of this technology.

In field trials, with the release of T. pretiosum in different phases, it can be observed that the release of the parasitoid was a good complementary tool in the suppression of Lepidoptera pests in soybeans when associated with other already established IPM-Soy practices. Even after the release of Trichogramma chemical control with insecticide may be necessary. In view of these factors, the use of selective insecticides that preserve the T. pretiosum in addition to other parasitoids and other beneficial insects, it is crucial to successful management. Although there are some gaps in knowledge regarding the best use of egg parasitoids in soybean crops to control lepidopteran pests, it is now possible to satisfactorily employ these biological control agents in the crop.

b

When lepidoptera management is carried out correctly, it becomes possible to rationalize insecticides with the same control efficiency. In the state of Paraná, MIP-Soja has been conducted in several municipalities in a state program called “Plante your Future”. In these reference units (URs), where Emater technicians provide assistance and introduce MIP-Soja to producers, it has already been possible to reduce the use of insecticides by approximately 50% in addition to increasing the first application interval by approximately double (from 35 days to 63 days) allowing greater preservation of natural enemies and providing less impact from the application of chemical insecticide (Figure 5).

The success of MIP-Soja in Paraná is due to Emater's great effort to show the importance of the most different control methods being developed and remaining available to the soybean farmer, since the integration of these technologies in a harmonious way is crucial for the success of pest management in soybeans. Therefore, even in the age of biotechnology, biological control, insecticides and other available pest control tactics will continue to be necessary to combat soybean pests and need to be preserved and used in a sustainable and integrated way.

Adeney de Freitas Bueno, Embrapa Soja; Débora Mello da Silva, Iapar

Article published in issue 206 of Cultivar Grandes Culturas.