The caterpillar Helicoverpa armigera It is a pest species distributed throughout Asia, Oceania, Africa and part of Europe. In many countries on these continents it is considered the most important pest, such as in cotton in China, India, Pakistan and Australia; in corn in Australia and in vegetables in Europe. Its larval stage is extremely aggressive, when it can feed on more than 100 species of naturally occurring or cultivated plants.
The species is resistant to climate variations, such as heat or low temperatures, and is little affected by drought. It is highly prolific, laying between 300 and 30 eggs, most of which are viable. It completes its cycle from egg to adult in approximately 25 days at a temperature of XNUMX°C, resulting in a large population growth capacity and capable of generating outbreaks of the pest, with great risks to agricultural crops.
In places where the pest occurs, there are serious difficulties in controlling it, especially because it naturally tolerates many insecticides and doses that control other species. On the other hand, these local populations develop resistance to many insecticides used on crops very quickly.
This set of characteristics of Helicoverpa armigera places this species as the most important and feared by Brazilian agriculture at all times, as it presents risks to more than 50 million hectares and possibly some billions of reais, whether due to the increased use of insecticides or the losses imposed on crops such as soybeans, cotton, corn, other cereals, tobacco, potatoes, tomatoes, vegetables in general, etc.
Helicoverpa armigera It presents many particularities in its development, which make it so feared by producers. The complete cycle presents the egg, larva/pre-pupa, pupa and adult phases, with a development period of 30 days to 45 days, which at low temperatures can be longer. The moths lay isolated or grouped eggs on leaves at night, totaling between 300 eggs and more than two thousand eggs. The eggs are yellowish-white, becoming dark brown, close to the moment of larvae hatching, which occurs between five and seven days after laying. The first and second instar caterpillars are not very mobile on the plant and have a color ranging from yellowish-white to reddish-brown with a dark brown to black cephalic capsule, measuring 1,4mm to 4mm, respectively. Sixth instar larvae can reach 34mm in length. The color of the caterpillars depends on the diet, being predominant from straw yellow to green with brown stripes on the side of the thorax, abdomen and head. The larval period varies from 17 days to 35 days.
The identification of Helicoverpa armigera It is very difficult, even in the last instars, however, it is possible to separate it from other groups of soybean caterpillars. Heliothines have a particular behavior by recurving their head and abdominal segments towards the ventral region, forming a loop. At the end of the caterpillar stage, the larvae stop feeding and move to the soil to pupate. With the emergence of the adult, moths emerge. Helicoverpa armigera, which have yellowish front wings with a series of dots on the margins and a dark spot in the center of the wing. The hindwings are lighter with a dark brown border at their apical end.
The morphological identification of adults, based on the pattern of the wings, makes it possible to separate helicoverpa spp. in Heliothis virescens (which occurs throughout Brazil), however, does not allow separating the species of helicoverpa, for which it is necessary to examine the genitalia, preferably of male insects. This examination can be carried out with insects obtained from breeding and light traps (males and females) or collected from sex pheromone traps (males only). Identification based on these characters is carried out at the Integrated Pest Management Laboratory of the Federal University of Santa Maria (LabMIP-UFSM).
The adults of Helicoverpa armigera they have a great migratory capacity, which can be proven by their expansion in Brazil, and by their rapid distribution in Rio Grande do Sul, in the 2013/14 harvest. Host selection by adults allows for great polyphagy in this species, whose females are attracted to crops that have sweet substances and nectar in their flowers. Caterpillars feed on both vegetative and reproductive parts of several economically important species. In Brazil, the presence of the species has been reported in cotton, soybeans, corn, tomatoes, beans, sorghum, millet, pigeon pea, wheat, sunn hemp, sunflower, fruit trees, other vegetables, as well as in several weeds. They have the potential to attack many other plant species, including rice, potatoes, tobacco, among others, all of agricultural importance for Brazil.
In soybean crops, although they feed on the leaves and stems of plants, they prefer reproductive structures such as flower buds, legumes and grains. Damage occurs when the plants are emerging, when the cotyledons are exposed and the unifoliate leaves are developing, the caterpillars cut the plants under or over the cotyledons. From the V3 stage onwards, the caterpillar behaves as a defoliator and attacks the terminal shoot. During the reproductive period, it feeds on both the flower buds and the grains, making a small circular opening in the legume right over the grain where it feeds. Helicoverpa icetopoeon, which occurs in Argentina, can consume between eight soybeans and 15 soybeans during the last two larval stages, demonstrating the great harmful potential of this caterpillar (Igarzábal, 2008). This species is present in southern Brazil as well and probably shares spaces with H. armigera.
Monitoring populations of Helicoverpa armigera It is a key procedure for the successful management of this pest. Monitoring the population evolution of the pest, throughout the crop cycle and between crops, must consider the egg, larva, pupa and adult stages. It is based on this monitoring that management strategies will be defined, determining decision-making on the use of chemical control, spraying scheduling and the choice of insecticides and doses to be used.
When monitoring moths (adults), environmental factors are correlated with the capture of the species and can influence the efficiency of the method used. Adults can be monitored using light traps that capture males and females or with sex pheromone traps (LabMIP-UFSM Project and Nufarm Monitora 2013/14 harvest) that capture only males. By capturing the moths in pheromone traps and positive identification for Helicoverpa armigera, means that in these areas there may be mating and then laying by the population indicated in the traps. After laying the eggs, within ± five to seven days the caterpillars hatch, which are initially very small, consume little leaf area and are protected from spraying. These data correlating the occurrence of adults and caterpillars of Helicoverpa armigera were proven in chickpeas, in which a positive response of individuals in the population to high temperature and relative humidity was observed.
In the LabMIP-UFSM and Nufarm Monitora Project - 2013/14 harvest, delta-type traps, baited with the sexual pheromone Iscalure armigera ((Z)-9-hexadecenal (Z90C16Ald; (Z)-9-tetradecenal (Z9-C14Ald); (Z)-11-C16Ald), were placed in soybean areas in 47 municipalities in Rio Grande do Sul. In each municipality, a soybean crop was randomly chosen and a wooden structure was set up to support the sex pheromone trap. They remained in the field between the 23rd and 29th of November 2013. Of the sampled locations, only six collected insects. The adult males were dissected at LabMIP-UFSM and by genitalia it was possible to confirm the species. Of the traps in the sampled locations, only six collected insects. The adult males were dissected at LabMIP-UFSM and it was possible to confirm the species using their genitalia. The positive samples for Helicoverpa armigera were from Ipiranga do Sul, Cruz Alta, Tupanciretã, Estrela Velha, Santa Maria, Rosário do Sul. In a new set of samples , whose traps remained in the field between November 30 and December 12, 2013, traps in Entre Ijuís, Espumoso, Eugênio de Castro, Giruá, Santa Cruz do Sul, São Sepé, Santo Augusto, Soledade and Vacaria. Two aspects result from these findings. The first means that within a few days of proving the occurrence of adults (between ten days and 15 days) the caterpillars could be damaging the soybeans; and the second is the expansion of the pest in the soybean crop in Rio Grande do Sul, in December 2013, with its risks and impacts on production and sustainability. As the results of this monitoring are released every 14 days, in an online report on the occurrence and distribution of the pest, there is still a need to monitor the caterpillar populations in the areas, especially to verify (1) whether there were no populations previous; (2) the size/stage of the caterpillars; (3) the species and (4) mainly the density and distribution of the population.
Therefore, in addition to monitoring adults with traps, it is necessary to thoroughly check the leaves and the entire soybean plant from the beginning of development to the end of the cycle, at least twice a week, to count and identify eggs. and caterpillars present and the definition of the management strategy.
LabMIP-UFSM recommends monitoring the pest in different ways throughout the soybean cycle:
These monitoring phases are proposed depending on the crop's phenology, soybean leaf area, reproductive phase and vegetable production. Each sampling must be repeated at different points of the soybean crop, in order to represent the different plots and also the differences in the management applied there, allowing an overview of the response of Helicoverpa armigera the environment and the management practiced during the harvest. This separation into phases/forms of monitoring should also precede the traditional times of spraying soybeans, whether for weeds and/or diseases. For a long time, these phytosanitary problems were a priority; however, it is now necessary to give priority to the pest that poses the greatest risk to soybean crops, adjusting the timing of spraying. Inspection and egg counting should be carried out on the leaves, central stem and lateral stems; on flowers, vegetables and especially on shoots.
For caterpillars, considering the difficulties of identification in the field, they must be collected, separated by groups (Anticarsia gemmatalis, Plusiinae, spodoptera spp. and Heliothinae) and quantified with the objective of deciding the management to be adopted. The Heliothines, especially the helicoverpa spp., can be sent to LabMIP-UFSM, where they will be raised to adulthood and identified by species.
The level of control for Helicoverpa armigera in soybeans is under study and has not yet been established in Brazil. Based on the experience of other countries such as Argentina and for other species of helicoverpa that attack fruits and grains, it is possible to estimate that the control level values will be very low, that is, few caterpillars/m2 due to the aggressiveness of the pest.
With the occurrence of the caterpillar helicoverpa In Brazilian crops, many doubts arose about the management of this pest. Initially, the Ministry of Agriculture, Livestock and Supply (Mapa) authorized the use of registered insecticides on crops for other species and is studying other measures to reduce risks and insecurity due to the lack of management alternatives. For chemical control, in the absence of knowledge about the pest in the country and in our cultures, one of the most delicate points is predicting how the insecticides will behave, what efficient doses are and for how long they will be effective. An emergency parameter that can be used in this case is to consult work carried out in other countries, with similar crops and, in this way, have a basis with greater support on what are the best alternatives for controlling the pest in soybean crops in this harvest, here in Brazil.
There are insecticides of practically all modes of action groups that control the caterpillar helicoverpa, such as organophosphates, carbamates, pyrethroids, spinosins, avermectins, oxadiazines, anthranilic diamides, among others. However, there are many insecticides that require high doses to control the pest, making their use unfeasible; and others to which the pest has already developed resistance in many places, such as especially pyrethroid insecticides, for which their use is heavily restricted in many countries. Table 1 lists some of the most used and most efficient insecticides for Helicoverpa armigera, evaluated on cotton and in laboratory bioassays. The results allow us to estimate that in soy it is possible to use doses close to or below those evaluated, since in many of these experiments control efficiency above 90% was obtained, allowing safer results.
One of the most important points regarding chemical control of Helicoverpa armigera is the application technology. As this pest species occurs from very early in soybean crops (V1) until the final stages of grain filling (R6), the parameters for spraying insecticides cannot be the same at different times. It must be considered that there are three different moments of control of this pest (Figure 1). The first moment (V1 to V3), when the main limiting factor is the small leaf area of the crop and the rapid growth of the plant, which can make some sprays inefficient, such as those that use pyrethroids, IGRs, among others that need to be deposited and ingested to act. The second moment (V4 to Vn), which can be considered as one of the most important moments of pest control, with the objective of preventing the establishment of the population, before the closure of the inter-crop line. Finally, insecticide applications carried out after flowering until the final period of grain filling (R1 to R7) require efficient droplet coverage, mainly because after the appearance of vegetables the pest preferentially attacks these structures, which are located distributed in the lower, middle and upper thirds of the plants. For control to be efficient during the crop cycle, it is necessary to adjust the spray volume, especially in combinations with herbicides or fungicides.
The occurrence and impact of the caterpillar Helicoverpa armigera can bring a lot of economic, environmental and even sustainability damage to soybean cultivation in Brazil, however, this serious problem will allow the development of new practices and new technologies, with the improvement of soybean pest management. The soybean producer is the one who could suffer the greatest losses, while official research and public administration work at a speed much slower than the expansion of the pest and the risks it carries. Among the important and indispensable records of this moment is the conviction that there will be no miraculous solutions, arising from scientific pride, haste or recklessness. Effective and lasting solutions to the problem of Helicoverpa armigera they can only come from collective and cooperative work, collaboration and international experience with Brazilian agriculture.
* By Jerson Vanderlei Carus Guedes, Jonas Andre Arnemann, Clerison Régis Perini, Adriano Arrue e Alberto Roehrig, UFSM
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