Potential of entomopathogenic fungi against whiteflies

The whitefly (bemisia tabaci) is a polyphagous pest that feeds on more than 600 plants, including cultivated and non-cultivated plants. In Brazil, for a long time, it was considered an occasional pest in soybean crops, but in 2019 it was elected by the Ministry of Agriculture, Livestock and Supply (MAPA) as a key pest in soybean, cotton and bean crops. The insect has different biotypes, or races, with Biotype B being the most common in Brazil.

The insect causes direct and indirect damage to plants, both in the young phase (nymphs) and in the adult phase. Direct damage is caused by sap sucking and injection of toxins, which cause physiological disorders in plants, such as wilting and premature leaf fall. Indirect damage is caused by the transmission of diseases caused by viruses, since this insect can transmit more than 130 species of viruses. Among them, the transmission of the disease known as “soybean stem necrosis”, caused by the virus Cowpea mild mottle virus (CpMMV), which cause darkening of the petiole, necrosis of the stem, burning of the shoot and deformity in the grains, drastically reducing their productivity.

Furthermore, during feeding, insects excrete large amounts of sugary substances, which serve as food for the growth of the fungus. capnodium sp., which causes sooty mold. When growing on the sugary secretion, the fungus forms a dark coating on the leaves, which reduces the plants' photosynthetic capacity and, consequently, reduces crop productivity. Production losses caused by whiteflies vary according to the crop and population density, and can reach 100% in extreme cases, especially if there is virus transmission. According to Aprosoja 2018, soybean losses in Mato Grosso in the 2016/17 harvest were estimated at 5 to 6 bags/ha.

Whitefly life cycle 

The whitefly is an insect belonging to the order Hemiptera (Family: Aleyrodidae), whose biological cycle is divided into the egg, nymph and adult phases. This insect can have 11 to 15 generations/year, depending on environmental conditions. Generally, male and female are paired at the time of copulation (Figure 1). Each female can lay 100 to 300 eggs during her longevity, which can be up to 19 days.

The eggs have an elongated shape, yellow to orange and measure 0,17 mm. After 5 to 8 days of laying, the first stage nymphs (instar) hatch and move around the leaf until they find a feeding site (Figure 1). After another 1 to 3 days, the nymphs move into the second instar, a stage in which the pairs of legs are atrophied. After 2 to 3 days, the nymphs molt into the third instar, which is soon followed by the fourth instar, when the nymphs have two reddish spots corresponding to the eyes of the developing adults. Adult emergence occurs within 5 to 6 days. Therefore, the cycle from egg to adult can take 15 to 26 days, depending on temperature, relative humidity and host properties.

Whitefly sampling and control level (NC) determination

Decision-making for whitefly control must be based on sampling and surveys of the number of insects in the area. The whitefly has a uniform distribution in the crop, therefore, its sampling can be carried out randomly, following a walking pattern that encompasses the entire cultivation area. The most used type of walking is zigzag and the number of sampling points may vary according to the size of the area. It is recommended to quantify all phases of insect development (eggs, nymphs and adults), in the upper third of the plants, where all phases of insect development are concentrated (Figure 2).

The whitefly control level (NC) varies according to the crop, being different for cotton, soybeans and beans. In cotton cultivation, it is recommended to initiate control when the pest reaches 60% of plants with adults or 40% of plants with nymphs. In the case of soybeans, it is recommended to enter control when the population density reaches between 5 and 10 nymphs or adults per leaflet. In the case of beans, the level of control is based on the presence of 1 insect/plant, a low population due to the insect being a vector of the bean golden mosaic.

Microbial whitefly control 

Microbial control for whiteflies has been carried out using entomopathogenic fungi beauveria bassiana e Cordyceps (previously=Isaria) Fumasorosea. Currently, there are 37 microbial products for whitefly control registered with MAPA, 34 of which are based on beauveria bassiana and 3 products based on Cordyceps Fumasorosea. In the field, these fungi have been applied both in combination with chemicals and in isolated applications.

The action of entomopathogenic fungi (mode of action) in pest control begins with the adhesion of conidia to the integument and subsequent penetration into the insect's body (Figure 3). The conidia attached to the insect's body germinate within a few hours. For germination, conidia need favorable environmental conditions (“high humidity”, >65%; temperature of 25-30 °C and “low solar radiation”). The germination of the conidium results in the formation of a structure called “germ tube”, which develops and forms the “appressorium”, a structure for fixing and penetrating the fungus. The appressorium also releases enzymes “chitinases and lipases” that degrade the insect's cuticle, favoring penetration into the pest's body. Inside the insect, the fungus starts to produce toxins that are released into the insect's hemolymph (insect's bloodstream). As the fungus develops, it consumes all the nutrients from the insect's body, a process that culminates in the rupture of the insect's tissue in the opposite direction (exit of the pathogen), producing new conidia (Figure 4). This process is called “sporulation” and only occurs in environmental conditions of high humidity and temperature between 22 and 30 °C. With sporulation, conidia are spread by wind, water or even insects and can cause the death of other healthy insects not affected by the application, further increasing control effectiveness.

The key point to obtain high efficiency in whitefly control with the application of entomopathogenic fungi is to be careful with the product application technology (mainly with spray coverage, as “the fungus works by contact”) and compatibility with the products chemicals in mixture. To determine the coverage area, we can use technologies already available, such as Checklife (Kimberlit/Bionat) or even water-sensitive paper. The technology Checklife It is based on the use of a specific adjuvant placed in the spray solution. The places reached by the syrup containing adjuvant plus the microorganisms become fluorescent in color (Figure 5, right). However, this coverage/deposition check is only possible by observing the leaf on specific equipment (Checklife). Leaves without equipment, it is not possible to check the coverage area (Photo 5 on the left). The better the coverage area, the better the control efficiency. The compatibility of chemical products with entomopathogenic fungi is of fundamental importance to avoid the death of conidia in the spray tank. Therefore, mixing chemical fungicides and products that are harmful to conidia in the mixture should be avoided.

References

EMBRAPA. Soy Production Technology – central region of Brazil – 2009. Londrina: Embrapa Soy: Embrapa Cerrados: Embrapa Agropecuária Oeste, 262p. 2008.

MoreSoy. Whitefly: Soy and Cotton - 2019. Available at: https://www.youtube.com/watch?v=fSkkC3pFRlI

Pereira, M.F.A; Boiça JR, A.L.; Barbosa, J.C. Sequential sampling (Presence-Absence) for bemisia tabaci (Genn.) biotype B (Hemiptera: Aleyrodidae) in bean (Phaseolus L.). Neotropical entomology, v. 33, no. 4, 499-504, 2004.

Petroli, V. Whitefly infestation causes losses of up to five bags of soybeans per hectare in Mato Grosso in the 16/17 harvest.

Marcos Roberto Conceschi, Bionat Microbiology Researcher

Álefe Vitorino Borges, Bionat Market Development Analyst

Igor Henrique Sena da Silva, Bionat Market Development Analyst