How to manage high whitefly infestations

High infestations of the whitefly Bemisia tabaci, biotype B, have been recorded in Brazilian crops, mainly in the Central-West region. Several factors are associated with this population explosion

29.07.2016 | 20:59 (UTC -3)

Whitefly population increase bemisia tabaci Biotype B in the Central-West region has worried technicians and rural producers, as high infestations at the end of the soybean crop cycle are demanding additional insecticide spraying to avoid premature defoliation and not compromise crop productivity. Furthermore, after the soybean harvest, the pest migrates to crops sown in sequence with corn and cotton. As a consequence, severe attacks of B. tabaci in newly emerged plants they have occurred constantly, with the need to treat the seeds with insecticides and complement them with foliar sprays, due to their high incidence, which implies an increase in the cost of crop production.

B. tabaci It is a polyphagous pest, with more than 600 plant species recorded as hosts, and the range of host plants has increased over time, which has been attributed, among other reasons, to the use of irrigated monoculture agricultural practices. This large number of hosts has allowed the whitefly to reproduce and migrate quickly in both wild and cultivated hosts such as cotton, soybeans, tomatoes, beans, potatoes, etc.

Recently, the whitefly was also observed in grasses, completing its cycle (egg to adult) on corn plants. Although corn is not a good host for the insect, some areas cultivated with second crop corn, in Mato Grosso, suffered a reduction in stand. due to whitefly attack as soon as the plants emerge.

When growing cotton in the second harvest, initial infestations are even more harmful to the crop, as the plant is a good host for the pest, which favors the insect's persistence throughout the crop cycle. Furthermore, B. tabaci is a vector of the common cotton mosaic, causing direct damage through sap sucking and indirect damage through the transmission of the virus when weeds such as guanxuma are present (Sida rhombifolia) and broom (AIDS micrantha) infected with the disease. Therefore, good weed control is part of managing this pest.

As the whitefly is present in different crops, it must be treated as an insect pest of agricultural systems, as the production model adopted and the management of the insect in each crop directly imply the infestation of the pest in the subsequent crop.

As an example of the effect of the production system influencing the pest's population fluctuation, we can mention the planting of beans in irrigated areas during the soybean fallow period, with the aim of optimizing land use. However, the bean plant is a good host for the pest, which allows a high population of the insect at a time when the pest population would normally decrease drastically due to the lack of cultivation of host plants. After harvesting the beans, soybeans are immediately sown, which are then infested with whiteflies from the previous crop. In and around these areas, pest infestation at the end of the soybean cycle is significantly higher, as is the use of insecticides to control it.

Another production system that has increased in Mato Grosso and that favors the population increase of the pest is the cultivation of off-season soybeans over the cultivation of summer soybeans. In these areas, the need to control whiteflies in the off-season increases.

Although monoculture and intensified systems (two or three harvests per year) favor population outbreaks of pests, the way the crop is conducted significantly influences pest dynamics. Therefore, the use of Integrated Pest Management (IPM), with monitoring, respect for the level of control of each pest and the use of selective insecticides becomes essential for more sustainable agriculture and with less damage caused by pests.

Among the strategies used to control the crop for pest control and which may be related to the whitefly population outbreak, we can mention the use of preventive spraying, mainly for the control of helicoverpa spp. And Heliothis virescens in soybean cultivation, as several of these insecticides have low selectivity to natural enemies. Predatory insects like geocoris sp, orius sp. Nabis sp, Callida sp, lacewings and ladybugs (several species), in addition to feeding on caterpillars, also prey on whitefly nymphs. These non-selective insecticides have also affected the parasitoid population, mainly the wasp encarsia sp. In this way, they favor the population increase of the pest due to the low presence of these natural enemies.

Another factor that may have contributed to the pest's population increase was the low infestation of phytophagous stink bugs in some soybean areas. In this way, the application of insecticides to control this group of insects decreased, allowing the population increase of B. tabaci, as the insecticides used to control bedbugs also control whiteflies, which indirectly reduces the pest infestation. However, this isolated fact alone would not cause a population outbreak of the plague, but rather the sum of these events.

Regarding the level of control, the recommendation in soybeans is ten nymphs per leaflet for most products used to control whiteflies. As viruses do not yet impact soybean productivity, whiteflies only cause damage when sooty mold appears, caused by the fungus. capnodium which generally occurs in infestations greater than ten nymphs per leaflet. However, there is genetic variability regarding the resistance of cultivars to whitefly damage and studies with several cultivars are necessary.

For cotton crops, except when weeds infected by viruses occur in the area, there are recommendations for control levels that vary from 20% to 40% of plants with the presence of the insect. As with soybeans, new studies are needed for a more accurate index.

In addition to monitoring the incidence of the pest, it is important to evaluate the presence of natural enemies such as entomopathogenic fungi that, in conditions of high rainfall, promote natural epizootics that control the pest such as Aschersonia spp and Isaria sp.

As an example of the viability of IPM, in the Mato Grosso cities of Sinop and Sorriso, where IPM was implemented during the last three harvests, the reduction in the number of insecticide sprays was always greater than 50% when compared to management with preventive spraying. for pests. Furthermore, in the areas where IPM was used, there were no population outbreaks of pests, which can be explained by the increase in the community of natural enemies, as by tolerating a low infestation of insect pests in the crop, it allows the establishment of the natural biological control, which directly influences the population reduction of pests. Furthermore, when there is a need for chemical control, the efficiency of insecticides is satisfactory, as the pest infestation is not as high. It is worth noting that productivity between managements was similar, however, with a lower production cost in areas with IPM.

In areas where preventive management is used to eliminate the pest at low infestations, the natural enemy community has difficulty establishing itself. Thus, the pest population grows rapidly, causing population outbreaks crop after crop.

Another negative factor in the use of preventive insecticide applications (in addition to making it difficult to establish natural biological control) is the selection of insecticide-resistant pest populations, which also contributes to pest population outbreaks. For whiteflies, there is numerous scientific evidence of populations resistant to insecticides.

Therefore, it is important for producers to be aware that the more the production system intensifies, the greater the problems with pests. In this way, the use of MIP becomes essential for maintaining satisfactory and profitable productivity.

Click here to read the article in issue 179 of Cultivar Grandes Culturas.

Mosaic Biosciences March 2024