- Recognizing Lepidopteran pests in soybean crops
- Integrated Pest Management as a strategy to manage Lepidoptera pest populations
- An effective strategy to manage pest populations of Lepidoptera
The Lepidoptera order, including butterflies and moths, play an important role in natural ecosystems as pollinators and in the food chain. However, their larvae pose a problem, particularly for agricultural vegetation, as they predominantly feed on live plant matter, causing damage to crops and livelihoods.
In this guide, we cover how to recognize, manage, control and monitor Lepidoptera pests in soybean crops, which is a particularly problematic issue in Brazil.
Several species of lepidopteran caterpillars feed on soybean leaves, including the soybean caterpillar, Anticarsia gemmatalis. Considering its abundance and frequent occurrence in soybean-cultivated regions of the country, it is deemed the most significant pest.
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.
Recognizing Lepidopteran pests in soybean crops
Anticarsia gemmatalis pest (Soybean Caterpillar)
|Velvetbean (Anticarsia gemmatalis)
|Velvetbean (Anticarsia gemmatalis)
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.
Chrysodeixis includens pest (Soybean looper)
|Soybean looper (Chrysodeixis includens) caterpillar
|Soybean looper (Chrysodeixis includens) moth
Newly hatched C. includes 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.
Caterpillars of the Spodoptera complex
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.
(Spodoptera eridania) caterpillar
|Southern armyworm moth
|Spodoptera cosmioides caterpillar
|Spodoptera cosmioides moth
|Fall armyworm (Spodoptera frugiperda) caterpillar
|Fall armyworm (Spodoptera frugiperda) moth
Helicoverpa armigera pest (cotton bollworm)
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. You can find pupae 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.
|Cotton bollworm (Helicoverpa armigera) caterpillar
|Cotton bollworm (Helicoverpa armigera) moth
Integrated Pest Management as a strategy to manage Lepidoptera pest populations
What is Integrated Pest Management?
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).
Highlighting control measures post-pest appearance is crucial, considering the relationship between pest density, damage level, and the established Action Level for the crop.
What is the Action Level?
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, one must take control measures.
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. Never apply these selective pesticides preventively, as this intensifies pest problems.
- In Brazil, there are several registered products for use in soybean pest control. Product rotation and the responsible use of insecticides are indispensable factors in managing insect resistance. For more information, please visit http://www.cnpso.embrapa.br/helicoverpa/publicacoes.htm.
Biological alternatives for controlling Cotton Bollworm (Helicoverpa armigera)
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 applied to other crops, including 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.
- To find out which biocontrol options are registered and available for soybean pests in Brazil, visit the CABI BioProtection Portal.
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.
Knowing the Action Level for the Heliothis-Helicoverpa caterpillar complex in soybeans
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.
Action Levels for Lepidoptera pest species in soybeans
Two monitoring options for Helicoverpa armigera
The beating sheet, which is 1 m long, is the main tool for pest diagnosis in crops. Farmers can easily use this simple tool.
- Place the beating sheet between two soybean rows, taking care not to shake any plants in the row chosen for sampling.
- In the case of Helicoverpa, the first step is to check plant growth points within a 1 m space. Pest attacks start at the growth points, which are the preferred places for moths to lay eggs.
- After checking the growth points individually, shake the plants on the beating sheet to find any larger caterpillars in the foliage.
- Count the caterpillars found in a linear direction. Use six to ten growth points per 100 hectares. The greater the number of sampling points, the easier it will be for the farmer to decide whether to use some type of control.
- Using the average number of caterpillars found, the farmer can decide either to apply control or to continue monitoring over the coming weeks.
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.
An effective strategy to manage pest populations of Lepidoptera
In conclusion, integrated pest management is the most efficient method of managing Lepidoptera pests in soybean crops in Brazil. This combines monitoring and preventing pest populations with the application of carefully selected direct control measures only when needed. Promptly identifying infestations correctly is essential to implement appropriate control and prevention methods.
Visit the CABI BioProtection Portal Brazil to find out which biocontrol options are registered and available for soybean pests in Brazil.