Measuring caterpillar behavior

The importance of knowing the behavior in plants, population fluctuation and vertical distribution of the caterpillar in soybean crops to define the best management and control strategies for these insects.

The false caterpillar Chrysodeixis includens (Walker) (Lepidoptera: Noctuidae) was once a secondary pest in soybeans. However, the From the 2003/2004 harvest onwards, this species became a key pest in the crop due to frequent population outbreaks and great severity of damage in areas cultivated. Control difficulties in soybean cultivation are very related to the behavior of this insect.

The false measurement tends to occupy the lower canopy of soybean plants, which makes their control difficult through spraying with insecticides. Pest monitoring constitutes the basis of integrated pest management and define whether or not to implement control tactics. The use of traps baited with sex pheromone synthetic is also considered a practical and efficient method for pest monitoring. Carrying out studies on vertical distribution of insect pests could improve monitoring and assist in the development of control tactics. This information may indicate the best time or place for depositing insecticides applied in spraying, in order to maximize pest control.

With this objective it was a study was conducted at Embrapa Agropecuária Oeste, during the 2014/2015 harvest, with the aim of obtaining information about population fluctuation, knowing the relationships between adults and immature W. includens in soybean cultivation, as well as calculating the vertical distribution of eggs and caterpillars in the crop's leaf canopy, with the aim of providing subsidies to be used in integrated management of this pest.

caterpillar adults false gauge were monitored during the period from October 2014 to October 2015 in soybean cultivation. The capture of moths was carried out using Delta-type traps with adhesive floors, baited with sex pheromone Bio Pseudoplusia. Weekly, the traps were inspected to count the number of moths captured, when the adhesive floors were also changed, while the pheromone septa were replaced every 21 days.

Eggs and caterpillars were visually monitored on soybean plants shortly after their emergence. In the sampling of caterpillars, the beating cloth was used, carrying out two three times a week with five wipes close to each trap. pheromone. The captured caterpillars were classified as large (≥ 1,5 cm) or small (< 1,5 cm). At each sampling time, two nearby plants from each trap were also removed and taken to the laboratory. entomology for the inspection of eggs on leaves and stems. The capture data adults in traps and their immature forms found on soybean plants were subjected to linear regression analysis.

The caterpillars of C. includens were also sampled in soybean plants during the flowering of the crop from six o'clock in the morning morning with the aim of studying the vertical distribution of these insects during the day in the soybean leaf canopy. For this, ten plants were collected and sectioned into three extracts (lower, middle and upper), which were bagged separately and taken to the laboratory for egg and egg counts. caterpillars. The same procedure was repeated at 8 am, 10 am, 12 pm, 14 pm, 16 pm, 18 pm and 20pm each day. For the analysis of the movement of caterpillars between the extracts of plants throughout the day, to the upper, middle and lower parts of the plants, the treatments to be evaluated and the number of plants, the repetitions of the trial conducted in a completely randomized design.

Results obtained

Population fluctuation and relationships between adults and immatures in soybeans

During the entire period of monitoring, 1.199 moths were captured C. includens, with the presence of this species being observed in all Months of the year. During the soybean cultivation period (October/2014 to February/2015) the number of moths captured was notably higher, with the highest peak of occurrence of adults being observed in the months of January to February (Figure 1), when soybeans were in the reproductive stage. In April it was found the second peak of moths, when corn was being cultivated in the area safrinha.

The false measure is considered a polyphagous pest, as it feeds and develops in approximately 170 species of host plants, belonging to 39 families. This species has already been observed attacking other plants such as beans, tomatoes, tobacco, sunflower, cauliflower, lettuce, as well as soybeans, cotton and corn. However, studies have demonstrated that C. includens he has greater preference and adaptation to soybeans when compared to other crops. At the Brazil, outbreaks of C. includens occur frequently in western Bahia, Goiás, Mato Grosso, Mato Grosso do Sul, São Paulo, Paraná and Rio Grande do Sul, but there is a lack of information on mobility of this species in tropical conditions, especially during the off-season of soy.

The first eggs and caterpillars of C. includens they were observed in the month of November between the V2 and V3 phenological stages of plants of soybeans, while population peaks of adults, eggs and caterpillars in soybeans were observed during the month of January (Figure 2). These results show that the population explosion of this pest occurs when soybean plants, generally, they enter the reproductive phase and with a “closed” canopy. The month of January presented lower precipitation (82 mm), when compared to other months of soybean cultivation (Figure 1). It is known that C. includens is favored by low precipitation conditions or by periods of drought that precede outbreaks of this pest, as observed in this search.

The relationships between moths captured in traps baited with pheromone and the densities of immature samples sampled from soybean crops were not significant. However, a linear and significant relationship was found between egg and egg densities. caterpillars of this pest (Figure 3).

Based on the results obtained, it can be inferred that sampling eggs in soybean plants could be a good option for estimating caterpillar infestation in crops. At the However, eggs C. includens they are difficult to monitor because they are very small (they measure approximately 0,5 mm in diameter) which makes this parameter inapplicable for monitoring caterpillars in soybeans, making it more practical to carry out Sampling the caterpillars with the beating cloth. Under the conditions in which this work was carried out, pheromone traps were effective in detecting adults from C. includens. This information is important, as the presence of adults in the culture precedes the occurrence of eggs and caterpillars. However, no significant relationship was observed between the adult population and immature densities that made it possible to estimate caterpillar infestation or eggs in soybean plants, using the population density of adults found in pheromone traps.

Vertical distribution of eggs and caterpillars in soybeans

The moths of C. includens oviposited in all three extracts (lower, middle and upper) from soybean plants, but the eggs were deposited in greater quantities in the lower extract, when compared to the middle and upper extracts, which presented a similar oviposition rate (Figure 4).

When soybean plants are at the stage of full flowering, the culture is closed. In this condition, the plants grow and move closer to each other, closing the canopy foliar, leaving the lower part of the plants more protected. The moths preferentially deposited their eggs in the lower part, followed by the average of plants, probably due to the protection that this environment provides to their immatures, which are less exposed to climatic adversities, dehydration, as well as natural enemies and chemical control.

In studies of vertical distribution of caterpillars W. includens 282 insects were sampled, 186 small caterpillars and 96 big ones. Regardless of the size of the caterpillars, they were distributed in the profile of the plant, preferably in the lower third, followed by the median third to the similarity to that observed for oviposition behavior (Figures 5 and 6). To the caterpillars prefer the lower part of plants, probably because in this environment they are more protected from natural enemies and adversities climate. It should be noted that this behavior of the caterpillars makes it difficult for them to control through the application of insecticides, as these caterpillars are less exposed to contact with insecticides when applied to soybeans, especially when the crop is in the reproductive phase in which the top of the plants is closed, forming a barrier to the penetration of spray droplets, a fact known as the umbrella effect.

In studies on the vertical distribution of caterpillars W. includens in the profile of soybean plants throughout the day, it was found that both large and small caterpillars migrate to different extracts of plants according to the time of day (Figure 5 and 6). During the hottest periods of the day, between 10 am and 16 pm, most caterpillars migrate from the upper and lower strata medium to the lower extract of plants. However, during the day cooler temperatures (6am, 8am, 18pm and 20pm), the caterpillars return to the extract medium and even the upper reaches of soybean plants. At these times, the extract superior has a proportionally higher percentage of caterpillars, especially large ones, than in other periods. This information is from extremely important, especially for the management of large caterpillars, C. includens in soybean cultivation, since These caterpillars are more difficult to control than small ones.

Final Words

Based on the results obtained in this research, it can be inferred that adults of Chrysodeixis includens can be found practically during all year round in the region of Dourados, Mato Grosso do Sul, however the peak of its occurrence occurs in the months of January and February, flowering season soy;

There is a linear relationship significant and positive difference between the egg density of C. includens and the densities of small caterpillars and caterpillars large in soybean plants.

The females of W. includens have a preference for ovipositing in the lower canopy of soybean plants

The small and large caterpillars of W. includens generally remain in greater proportion in the lower canopy of soybean plants, in relation to the middle and upper part. However, throughout the day, these caterpillars move along the profile of soybean plants, becoming more exposed in the middle and upper strata at times of cooler temperatures mild, especially in the case of large caterpillars. In this way, you can infer that chemical control of large and small caterpillars should preferably be carried out in the early hours of the day or at dusk since in these periods the caterpillars are more exposed to the contact with products sprayed on the crop.

Article published in issue 227 of Cultivar Grandes Culturas, April 2018.