How to recognize, manage, control and monitor Lepidoptera pests in soybean crops.
The Lepidoptera order, including butterflies and moths, play an important role in natural ecosystems as pollinators and in the food chain. However, their larvae are often considered problematic – specifically to vegetation in agriculture, as their main source of food is often live plant matter, which can ruin crops and livelihoods.
Several species of lepidopteran caterpillars feed on soybean leaves, including the soybean caterpillar, Anticarsia gemmatalis. It is considered the most important pest given its abundance and more frequent occurrence in those regions of the country where soybeans are cultivated.
Two other insect species have recently been observed causing problems in crops. Changes in crop management have resulted in some Spodoptera species acting as a crop defoliator since 2003, mainly where soybean crops grow close to pastures, corn, or other grasses.
Additionally, the discovery of the invasive pest cotton bollworm Helicoverpa armigera between 2012 and 2013 has been worrying soybean farmers in Brazil and other countries, such as Paraguay and Argentina, due to the severe damage it causes to corn, cotton, and sorghum crops.
The first instar of the soybean caterpillar, A. gemmatalis, is green, with four pairs of abdominal legs, two of which are vestigial and another anal pair, causing the caterpillar to move in a way that makes it seem as if it is measuring distances, with the result that it is often confused with the soybean looper (Chrysodeixis includens). Larger caterpillars (>1.5 cm) can be either green or dark, with three white longitudinal lines on the back. The pupae are brown and are usually located in the ground. Adult wingspans range from 30 to 38 mm, and their colors vary from light grey to dark brown. A diagonal light brown line joining at the tip of the first pair of wings aids recognition.
Newly hatched C. includens caterpillars are light green and have white longitudinal stripes with black dots. In each instar, the caterpillars change from a light brownish green to a translucent lime green. The pupa occurs under a web, usually on the abaxial surface of the leaves. Unlike A. gemmatalis, it has a pale yellow to light green color at the beginning, and quickly develops irregular dorsal pigmentation that lasts for up to 48 hours before adult emergence. Adults have a wingspan of 35 mm, with the wings arranged in a slanted shape. The forewings are dark in color and have two bright silvery spots in the center, and the hindwings are brown.
Spodoptera cosmioides (Walker) and Spodoptera eridania (Cramer) caterpillars are the most common species in soybean crops, especially at the beginning of the reproductive crop phase, and cause defoliation as well as attacking soy pods. Spodoptera frugiperda (Smith) and Spodoptera albula (Walker) can also occur in newly germinated plants, when late-instar caterpillars cut the plants close to the ground.
Newly hatched Helicoverpa armigera caterpillars are light in color, with small spots that become darker as the larvae develop. They also have lines along their bodies, a dark saddle-like protuberance on the fourth segment, and dark legs. Late-instar caterpillars have white hairs around their heads. Pupae are found in the soil beneath the crops. Adults have a wingspan of 30 to 45 mm. Females are brown to reddish brown, while males are opaque greenish to yellow or light brown. The hindwings are light in color and have a wide, dark outer margin with a small light spot.
Knowing how to recognize the main pests, monitoring in the field, and acting from the action levels recommended upon current research allows farmers to decide which management tactics to apply.
In this case, it is important to adopt Integrated Pest Management (IPM), as this an effective way to fight pests. IPM works to coordinate efficient pest control, social and environmental responsibility, and productivity using such tactics as biological control, cultural control, plant resistance, and even environmentally conscious chemical control. Most definitions of IPM focus on the use of control strategies that aim to minimize crop losses through scientific knowledge, technological support, and common sense.
Tactics in an IPM-Soja context include monitoring at least once a week to check the number of pests as well as the size of the caterpillars and the level of damage caused (defoliation percentage, number of attacked plants).
It is important to highlight which control measures should be taken after the appearance of the pest, taking into account the relationship between pest population density, level of damage, and the Action Level established for the crop.
The Action Level is a parameter that defines the most opportune time to apply pest control measures. If pest population levels are below the recommended Action Level, it is not necessary to use control measures, but monitoring is important! When the population reaches or exceeds the recommended Action Level, control measures must be taken.
In addition to the timely application of pesticides, selective insecticides are essential to ensure the preservation of beneficial organisms that act as biological pest controls. These selective pesticides should never be applied preventively, as this intensifies pest problems.
In the state of Bahia, Cotton Bollworm (H. armigera) larvae were naturally infected by the fungus Nomuraea rileyi (Hypocreales: Claviccipitaceae), reaching a 33% mortality rate.
The use of parasitoids has also been growing in Brazil. During the 2013-14 growing season, a species called Trichogramma pretiosum was used on about 250,000 hectares of soybeans to control the number of H. armigera and C. includens eggs. It was also used on other crops, such as cotton, maize, beans, and fruits. In addition, Trichogrammatoidea annulata showed an important potential for parasitism.
Baculoviruses have been imported from countries such as Australia for use in Brazil. In 2019, HearNPV—a species of nucleopolyhedrovirus (NPV) closely related to baculovirus species from Australia, India, South Africa, and China—was reported for the first time. Its insecticidal properties indicate that it can be used to manufacture bioinsecticides for H. armigera control in Brazil.
Another sustainable option may be the adoption of Bt soybeans (i.e., soybeans that express the Cry1Ac protein), which are widely available in Brazil; however, it is important to emphasize the need to establish a refuge area with non-Bt soybeans, which can take up 20% to 50% of the overall field, to ensure the Bt soybeans are effective.
Since H. armigera was reported in Brazil in 2012 and 2013, periodic crop inspections were recommended at least once a week in the vegetative phase, and up to twice a week during the reproductive phase (R1 to R6). H. armigera attacks the reproductive structures of the plants during the reproductive phase, and thus becomes more of a problem.
Because H. armigera (an invasive pest), Helicoverpa zea (corn earworm), and Heliothis virescens (tobacco budworm) caterpillars are very similar, visual identification is almost impossible, so the level of action and management should be designed and carried out for all three of these pests.
Selective insecticides can be a viable option but should always be applied with respect to the Action Levels in order to minimize expenditures caused by unnecessary applications. Selective insecticides are less harmful to beneficial insects that frequent crops and provide biological pest control (natural enemies), especially predators and parasitoids, as well as to enteropathogenic agents such as viruses, bacteria, fungi, and nematodes, which are responsible for the reduction of pest insect populations.
(Modified from Helicoverpa armigera: prevention and management actions)
The beating sheet, which is 1 m long, is the main tool for pest diagnosis in crops. It is a simple tool, and can easily be used by farmers.
The other method used is the monitoring sheet, a didactic tool on the EMBRAPA website, that shows photographs of the main soybean pests, with tables to record the number of small and large caterpillars, as well as information on the main natural enemies that can assist the farmer.
Conclusively, the most efficient method of pest management of Lepidoptera in soybean crops in Brazil is integrated pest management, combining monitoring and preventing pest populations with the application of carefully selected direct control measures only when needed. It is essential that any infestations are promptly identified correctly so that appropriate control and prevention methods can be implemented.
To find out which biocontrol options are registered and available for soybean pests in Brazil, visit CABI BioProtection Portal Brazil.
How to recognize, manage, control and monitor Lepidoptera pests in soybean crops.
CABI showcased the benefits of the BioProtection Portal during a Bioprotection Day conference in Brazil held in collaboration with Embrapa
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