Planted forests improve the quality of life on the planet
By Elcio Trajano Junior, director of HR, Sustainability and Internal Communication at Eldorado Brasil Celulose
Aggressive and polyphagous pest, capable of feed on different host plants, Helicoverpa armigera is responsible for annual losses of around 5 billion dollars. Present in all Brazilian states, studies on biological aspects of the development of this insect in different crops, such as soybeans, cotton, corn and wheat is of fundamental importance for success in management.
Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is one of the main pests agricultural crops in the world, as it is considered a polyphagous species, that is, if feeds on different host plants, which easily adapts to different types of environments, have great dispersion capacity, self reproduction rate and high power to cause damage to crops.
As a result, its presence has been recorded in various regions of the world, such as Africa, Europe, Asia and Oceania. In 2013, caterpillars H. armigera were found feeding on different crops and weeds in some Brazilian states, causing a reduction of up to 80% in the production of cotton and significant losses in soybeans and corn, which required greater application of insecticides for their control, thus increasing the production cost of these cultures. The losses caused by this pest annually reach 5 billion dollars. Its presence has already been found in more than 170 genera of plants, including crops such as cotton, soybeans, corn and wheat. The caterpillars of this species, although they ingest leaves in the first larval instars, they have a preference for feed on the reproductive structures of hosts, such as flower buds, fruits, apples, ears, panicles and inflorescences.
It is estimated that H. armigera is currently present in practically all Brazilian states. Due to its impact on Brazil and the high risk of losses caused by its attack, it is important know the pest well, know how to correctly identify it and how to implement some management techniques, in order to obtain good productivity in crops. To the different plants in which this species lives influence its survival, duration, number of instars, intensity of larval consumption, as well as fecundity and adult longevity. Thus, studies on biological aspects when H. armigera develops in different hosts is of fundamental importance for the success of its management.
It is possible to calculate the capacity for population growth H. armigera when raised in different cultures. This is essential for understand their mortality rate, survival rate, duration of development and reproductive capacity. These ecological data are fundamental to evaluate the suitability of different host plants with relation to the development of the pest.
Work carried out in Embrapa Agropecuária Oeste showed that the creation of H. armigera on an artificial diet, as well as on different plants hosts presented different results regarding duration and survival of this insect pest (Table 1). Possibly the best nutritional characteristics of the artificial diet were decisive for there was this improvement in the development and viability of the larvae, indicating that this food is suitable for feeding larvae of H. armigera, compared to host plants tested. These results confirm the success of creating H. armigera in laboratory conditions using the artificial diet. On the other hand, although insects have completed the cycle of life in corn and wheat crops, duration assessments and viability larvae indicated that these hosts are less suitable for development of H. armigera, in compared to soybeans and cotton.
The caterpillars of H. armigera they barely fed on corn and wheat leaves, which did not occur for the reproductive parts of these hosts (corn grains and wheat panicles). Probably the physical and chemical characteristics present in corn and wheat leaves were responsible for superior larval longevity and lower larval viability compared to the artificial diet (Table two). The larvae in H. armigera when fed with corn leaves alone may also not complete this phase of development.
The duration of the larval period is a parameter important, as it indicates whether the food used is suitable or not for the larvae feeding. Furthermore, the highest rates of larval mortality in corn and wheat can be justified by the hardness of the leaves and/or high content of hemicellulose, which are normally present in the leaves of these hosts, in especially corn.
It was found a unsuitability of corn and wheat hosts for feeding caterpillars H. armigera, reflected in the pupal stage, which presented higher pupal mortality. Pupal weight is also a parameter important as it can indicate whether a host is suitable for the development as well as the fecundity of the insect. There was a difference regarding fecundity when the insect was reared with different types of food (Table 3). The highest daily fecundity of females in H. armigera recorded on cotton indicates that this host is suitable for the development of this pest. O The same was also observed for total fertility, when adults raised in cotton oviposited 64% and 95% more, respectively, compared to corn and wheat. A capacity to convert the food assimilated in the larval stage and the characteristics of the insect itself can have consequences on the fecundity of adults from H. armigera.
The net reproduction rate (Ro), that is, the average number of females generated per female throughout the oviposition period was superior in cotton, since presented a number of offspring produced/female and survival rate of high female adults (Table 4). The smallest values of Ro and rm (speed population growth) are recorded for corn and wheat, which also had low fertility and higher mortality in the period reproductive. This emphasizes the low suitability of these hosts for the development of H. armigera.
The number of individuals added to the population/female/day indicate that cotton and soybeans are the hosts also more suitable for the development of the pest. The time that the population of H. armigera takes to doubling in number (TD) was greater in wheat and maize due to low fecundity and high adult mortality at the end of the reproductive period. This time of doubling was lower in cotton and in the artificial diet, suggesting that the insect, when raised on these foods, it tends to increase in number more quickly.
The proportion of male:female H. armigera us evaluated hosts was 1:1, therefore, with a sex ratio of 0.5. For the emergence rate of male and female adults of H. armigera, most insects emerged no later than eleven days of pupal development (Figure 1). More than 80% of postures were performed in the first two weeks of oviposition for all the evaluated hosts. The oviposition peak of moths reared on corn and wheat occurred in the first two weeks and progressively decreased after that period (Figure 2). In soybeans and cotton the posture was also concentrated in the first two weeks, but there was a more uniform and stable pattern throughout the oviposition period.
Based on the results obtained in this research, it can be inferred that the management of H. armigera in corn and wheat cultivation, mainly in relation to the application of agricultural pesticides, it must be taken into account that in these crops there may be a reduced initial population of this pest in the field, in compared to soybean and cotton, since these hosts presented high larval and pupal mortality and a low biotic potential. When considering that the moths oviposited for a longer period in cotton and soybeans, this may increase the potential for damage caused by caterpillars in these crops.
The hosts cotton and soybeans showed greater viability in the development phases, higher rate of survival of female adults in the reproductive period and higher fecundity to other hosts tested and similar to insects raised on a diet artificial, showing that these two hosts may be suitable for creation of H. armigera how to diet Natural.
The table parameters fertility lifespan indicate that corn and wheat are the least suitable for the development of the population of H. armigera, since they presented lower values of Ro (net reproduction rate), rm (capacity to increase in number) and λ (finite rate of increase) compared to other hosts.
Article published in issue 223 of Cultivar Grandes Culturas, December 2017/January 2018.
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