Bioinsecticides in caterpillar management

The adoption of control tactics biological and, in particular, the use of Baculovirus-based bioinsecticides to Management of caterpillars of great economic importance has proven to be an important tool complementary and in some cases even alternative to other forms of combat to pests.

It is known that abundance and the distribution of arthropods is influenced by the activity of enemies that occur voluntarily in unmodified ecosystems. Although, in anthropized ecosystems, or so-called agro-ecosystems, these interactions biological species become severely limited or obstructed and certain species lose their natural regulation and become pests.

This way, human intervention is necessary to try to restore the balance among existing species. It is in this context that Integrated Pest Management comes into play. (MIP), with its different control tactics. Among these tactics, control biological has a decisive role, as it is a tool with numerous advantages agro-ecological practices that provide sustainability to the system, as long as it is respected all of its prerogative of use.

Talking specifically Baculovirus, an important category of biological control, It is possible to state that its use in the control of key pests has great potential for employment in Brazilian agriculture, as in addition to being efficient, They are specific and do not harm the environment, animals or humans. The Baculoviruses meet the basic requirements as an alternative and/or complement to other control tactics.

The use of Baculoviruses as biological insecticides to control lepidoptera in programs of integrated pest management dates back to the 1970s. In Brazil, the pest management program of the soybean caterpillar, Anticarsia gemmatalis, through Baculovirus (AgMNPV) has become a globally recognized program for its effectiveness and reach in different Latin American countries

The species of Baculoviruses are highly specific for target invertebrates. Under the point of security view are unable to multiply in microorganisms, cells of plants, vertebrate cell culture and in vertebrates. Furthermore, the family Baculoviridae, the most researched and used worldwide as a bioinsecticide, does not affect other nonspecific insects and invertebrates. Thus, they are agents ideal in IPM programs in annual, perennial and forestry crops.

O first Baculovirus-based bioinsecticide (HzSNPV) was commercially registered in 1975 (Virion-H, Biocontrol-VHZ, Elcar) and employed on a large scale to control the species complex of the family Heliothinae, for example the Helicoverpa zea and Chloridea virescens, important key pests of cotton cultivation in the United States. At the moment, approximately 50 Baculovirus-based bioinsecticides are used in different parts of the world.

You nucleopolyhedrovirus (NPV) of Lepidoptera, as the baculovirus of the genus Alphabaculovirus, are formed by double-stranded circular DNA associated with capsid proteins (nucleocapsid). Surrounded by a protective membrane, nucleocapsids form virions. In turn, the virions are protected in a matrix protein giving rise to occlusion bodies. The formation of occlusion bodies (OB – Occlusion Body) provides physical and biological protection to viral particles against, for example, deactivation through climatic factors such as temperature and solar radiation. Composing the occlusion body can be observed multiple virions (multiple nucleopolyhedrovirus-MNPV) or just one virion (simple nucleopolyhedrovirus SNPV).

After the foliar application of bioinsecticides, the caterpillars ingest the occlusion bodies deposited on plants, which begins the first phase of infection. You occlusion bodies dissolve with the alkaline pH in the midgut of caterpillars, releasing viral particles. Viral particles penetrate the nucleus of intestinal epithelial cells, where they replicate. With replication new viral particles are generated, but they are “naked” and are not inserted into protective membrane. Without protective armor, extracellular viruses (BV- Budded virus) easily disperse through the hemolymph and tracheas, invading other tissues of the infected caterpillar. New occlusion bodies form in the nucleus of the other cells, characterizing the second phase of infection. Big ones quantities of nucleocapsids protected in occlusion bodies are generated, resulting in the rupture of cells and tissues, releasing viruses into the environment and beginning EPIZOOTIA (horizontal transmission) (Figure 1).

To the Baculovirus particles appear in the form of rods, called virions, which are covered by a protein matrix. That The virus begins to act only after ingestion of the target pest. In other words, the caterpillar needs to ingest the viral particles deposited on the leaves. To the viral particles are dissolved within the caterpillars' gastrointestinal tract, enabling the release of virions. In turn, these virions penetrate the membrane of the intestinal wall and reach the hemolymph, spreading and multiplying in the cell nucleus of different tissues. The process, from the beginning of the infection until the target pest dies, it can last around 3 days to 7 days, depending the size of the caterpillars and the environmental conditions (Figure 1).

Caterpillars infected with Baculovirus lose their motor and feeding between two and four days (depending on the age of the caterpillar) after ingestion of contaminated substrates, such as leaves, branches or fruits, and will die within 6 days to 7 days after infection. Discoloration of the ventral part (whitish to yellowish) is a typical symptom, which spreads progressively all over the body. Despite decreasing their mobility, infected caterpillars They commonly move to the upper part of plants and die hanging through the abdominal legs. At this stage, the caterpillar has a flaccid body that later it darkens and breaks easily, releasing a large quantity of polyhedra that will serve as inoculum for contamination of other caterpillars of the same species (Figure 2). The virus has the ability to disperse long distances, whether through wind, dust, rain, and/or contact with other insects (e.g. flies). With the rain, a large number of polyhedra accumulate in the upper layer of soil, where the virus can remain for a year for another, promoting future natural epizootics in the treated areas.

O The success of programs using Baculovirus depends on a a number of factors, including the selection of more virulent isolates, the timing application (both in relation to application time and size of caterpillars and the population density that must be estimated through assessment frequency of the population, appropriate application technology, favorable climate, among others). The moment of application is extremely important, both in relation to the application schedule such as caterpillar size and population density which must be estimated through frequent monitoring of the population. A application should be carried out at the end of the day, to give viruses time to act during the night, when the caterpillars are most active in their feeding. The application must be ‘preventive and inoculative’, that is, it must be carried out as soon as possible. observe the first small caterpillars in the field.

One of the obstacles to the wider use of this category of products lies in the fact that be highly specific, which can lead the producer to have some difficulty to employ these tools in their pest management programs, as Often two or more species can be found in the same field and at the same time Same time. Another obstacle has been the immediacy of the expectation of obtaining results, since viruses act slower than chemical insecticides and should be used in an inoculating way, as basic tools and as regulators of pest populations. The use of the virus is a valuable tool within a Integrated Pest Management program. Thus, when a producer decides to technological changes, these changes must follow the technical positioning correct, in order to maximize the benefits that this tool can bring to short and long term.

The Brazilian program of use of Baculovirus in soybean cultivation recommended by Embrapa was adopted in approximately two million hectares in 2003/04. This example is often cited internationally as a real success in controlling biological based on Baculovirus. However, due to numerous factors, MIP Soja fell forgotten in recent years and the use of Baculovirus has decreased significantly regrettable, currently restricted to some isolated areas in the south of the country. Producers began to adopt erroneous practices and consequently, there was return of indiscriminate and/or scheduled chemical applications in the crop with inestimable socio-environmental losses. Despite growing populations in A. gemmatalis in soybean crops, especially in the Cerrados, the adoption of Baculovirus remains insignificant, unfortunately, like any other tactic other than chemistry.

A pertinent and more recent case was what happened with the identification of the Helicoverpa armigera in Brazil in 2013. Everyone bet that biological alternatives would take off again, especially in crops such as soybeans, cotton and tomato. Therefore, Brazilian and foreign companies invested mainly in the use of bioinsecticides to control caterpillars, as they are There are many reports of success with the adoption of biological tactics, such as Baculovirus in the countries of origin of H. armigera. However, what was seen, despite all efforts to publicity about the best management tactics for this pest, was a setback even greater, as they began to use chemical insecticides in a indiscriminate, always with the thought that they would be the most viable for a caterpillar like H. armigera.

The shock caused in 2013 by the high infestations with inestimable losses and still poorly accounted for, caused many think that the adoption of MIP itself would be boosted in a widespread in Brazilian crops, but it was not quite like that. The alternatives biological, behavioral, legislative, among others, were once again forgotten. After all, for many chemical insecticides, whether of legal origin or illicit, were enough and what is worse, some even risked saying that the H. armigera it would no longer be a problem for Brazil.

Another pest that urgently needs to be addressed in a broad context in relation to MIP is the Spodoptera frugiperda. In addition to being a serious problem in grasses in generally and more specifically in corn, this pest has become frequent in other crops such as cotton, soybeans, tomatoes, beans, among others. Future perspectives regarding seriousness of this pest are frightening due to its high capacity to adaptation and rapid evolution of increasingly resistant populations, until even state-of-the-art technologies such as cultivars that express Bt proteins. In this alarming context, the only viable and sustainable alternative will, without a shadow of a doubt, be the adoption of Integrated Pest Management programs, with biological control as a fundamental and indispensable tool.

Certainly chemical insecticides are not and never they will be the definitive solution for pest control. The chemical tools need to be seen as just one of several Integrated Management tools of Pests, because for good coexistence with these "super caterpillars" it is The integration of various control tactics is essential. Of the same Therefore, biological control should not be used as an isolated practice, and it is essential that is inserted within a pest management program. In this sense, the biological control must be used in a “continuous and integrated” manner with other control tactics usually adopted by producers. To this end, it must be easily accessible, especially in relation to supply and cost, two historically limiting issues that are gradually being surpassed by companies committed to development and manufacture of these products.

In short, the adoption of biological control tactics and, in particular, the use of bioinsecticides based on Baculovirus for the management of caterpillars of great economic importance - that grow in seriousness and availability of use each year in Brazil - represent a real possibility of reducing production costs, as in addition to being efficient, are highly complementary to other forms of control and significantly reduce environmental risks, which can bring great benefits to Brazilian agriculture.

Cecilia Czepak, School of Agronomy/UFGl; Paula G. Marçon, Marcelo Lima, Rafael Ferreira Silvério and Janayne Rezende, Agbitech; Matheus Le Senechal Nunes, School of Agronomy/UFGl

Article published in issue 229 of Cultivar Grandes Culturas, June 2018.