Soybean producers in the Middle North of Mato Grosso faced problems with whiteflies Bemisia tabaci (Gennadius) biotype B, also referred to as B. argentifolli Bellows & Perring. The municipalities of Lucas do Rio Verde, Nova Mutum and Sapezal already had the presence of the pest since the beginning of the harvest, but in some areas, the population increased significantly at the end of the harvest due to errors in managing this insect.
In the 2012/13 harvest, many producers were already facing the problem and there was a lot of abandonment of areas with high whitefly infestation. Another important point is that the whitefly has a large number of hosts. Its cycle is just not as intense in grasses. Areas with cotton cultivation also favor the pest, even with the application of chemical pesticides. Furthermore, the climatic factor also interferes with where the whitefly remains on crops even in periods of drought, making it a preferred environment for this species.
Whiteflies are one of the pests that cause damage to soybean and cotton crops in the state of Mato Grosso. By sucking the sap from plants, adults and nymphs cause changes in the vegetative and reproductive development of the plant, weakening it and reducing its productivity.
Whitefly biology is complicated by the apparent existence of different biotypes, whose evolutionary and practical significance remains unclear. In Brazil there are different biotypes of B. tabaci and B. argentifolii (also called Biotype B of B. tabaci), whose separation is only possible through molecular markers or special breeding techniques.
Adults measure 0,8 mm in length and, apparently, are generally white in color, but have white wings and a yellow body. Under favorable climatic conditions, its life cycle can be two to four weeks, and it can produce up to 15 generations per year. They are preferably located on the underside of the leaves, where they oviposit, on average, 150 to 300 eggs per female. Preference for oviposition on cotton is affected by leaf structure, gravity (positive) and light (negative, except for adults in flight). Other factors that may be involved are interactions between gravity, light, leaf structures, environmental conditions and natural enemies. Temperature is the main external agent that influences the progressive metamorphosis of the whitefly, meaning that its life cycle can vary from 15 to 24 days. The nymphs are yellowish-green in color and go through four instars. The 1st instar nymph is mobile, that is, it has legs and is able to move around the leaf or plant. From the 2nd instar onwards, the nymphs lose their legs and are immobile. They remain in this form until the adult emerges. From the 3rd instar onwards, the nymphs are easily visible, resembling mealybugs. The nymphs of whiteflies of the genus Bemisia They are characterized by not having fringes formed by waxy filaments around the body.
Damage
In soybean cultivation, direct damage is caused by both nymphs and adults, which, when they settle on the underside of the leaf, suck the sap, extracting carbohydrates and amino acids, which can cause leaves with chlorotic spots, wilting and falling. Plant depletion occurs due to sap extraction, compromising the plant's vegetative and reproductive development, reducing productivity and quality in production and, in severe attacks, bringing forward the cycle by up to 15 days.
During feeding, the insect excretes a “honeydew” that favors the development of sooty mold, a black fungus that grows on the leaves, darkening them, impairing photosynthesis and, consequently, can interfere with productivity depending on the stage of the plant and population. present of the pest. It is the nymphs that release a large amount of this sugary substance, allowing greater growth of sooty mold on the leaves which, turning black, absorb a lot of solar radiation, causing “burning” and falling of the soybean leaves.
Severe attacks cause yellowing of older leaves. When plants are young, drought and even leaf death can occur. Damage can worsen if there are periods of summer or inadequate irrigation.
In cotton cultivation, adults and nymphs feed by sucking the sap from the plants' phloem, removing nutrients from the plants, which can lead to their death or a drop in production. The attacked leaves turn yellow, dry and, as the main symptom, fall. During feeding, a sugary and sticky substance is excreted, known as “honeydew”. This secretion leads to the formation of sooty mold on branches, leaves and fruits, resulting in a reduction in the photosynthetic capacity of the plant, contamination of the cotton linter and damage to the fiber quality, in addition to interference in the harvest.
Indirect damage
The whitefly is a vector of viruses from the geminivirus, carlavirus, closterovirus and luteovirus groups. For cotton crops, viruses from the geminivirus group are the most common. As insect vectors they can cause more serious damage, even when present in low populations. Although other phytopathogens can be transmitted by this insect, viruses cause the biggest problems. In soybeans, the main damages are related to the transmission of viruses, characterized by the symptoms: severe stunting, leaf curling, intense chlorosis and decreased grain production.
Integrated management
Integrated Pest Management (IPM) is the use of different strategies to control and coexist with the pest. The search for selective insecticides or their selective use and the extension of the useful life of efficient insecticides against this insect stand out. Both options can be implemented by evaluating new products, improving application methods and constantly monitoring whitefly resistance levels in specific areas, to rotate insecticides, in the context of other IPM practices. Due to the insect's characteristics, chemical control has been the most used method, however, its intensive use in some production systems can lead to the emergence of resistant populations.
High populations become difficult to control, as well as repeated exposure to insecticides without changing the mode of action, making the evolution of resistance to products very likely. Therefore, an integrated strategy with biological control, cultural control and chemical control must be used.
The preservation of natural enemies in agroecosystems is of fundamental importance as a factor in the dynamic balance of insect pest species populations, minimizing human intervention in control. Therefore, basic product selectivity studies are necessary, as the information obtained can be used in decision-making regarding the product to be used.
The currently possible biological control consists of preserving the whitefly's natural enemies through the use of selective insecticides. Several species of natural enemies have been identified in association with whitefly species complex. In the group of predators, 16 species were identified from the orders: Hemiptera, Neuroptera, Coleoptera and Diptera. Among the parasitoids, 37 species of microhymenoptera were identified. Among the parasitoids, the genera encarsia, Eretmocerus e Amitus, commonly found. Regarding entomopathogens, there are several more virulent isolates of fungi Verticillium lecanii, Paecilomyces fumosoroseus, Aschersonia aleyrodis e beauveria bassiana, with action on whiteflies. In a biological control program, after the identification of natural enemies, the use of these agents must be established more efficiently, probably through inundative releases at the very beginning of the crop or through releases of parasitoids in adjacent weed plants. In several countries, several biological control agents for whiteflies are being identified and studied, the results of which have been very promising for the management of this pest.
Some native species of the Malvaceae family are reservoir hosts for the virus, mainly Sida rhombifolia (racaxuma), S. micrantha (broom) and Nicandra physaloides (joá-de-capote), in addition to other cultivated plants such as beans, soybeans, okra and tomato plants. Control is carried out by eliminating native malvaceae near the planting, uprooting symptomatic plants and chemical control of whiteflies. No resistant or tolerant cultivars have yet been reported.. Other weeds such as forage turnip, milkweed, black pickerel, viola string, trapoeraba must be eliminated from the areas as they are hosts of the insect.
During this harvest, high whitefly infestations were observed in Crotalaria ochroleuca being preferred over other species such as C. spectabilise C. juncea.
Another IPM strategy is the use of commercial crop cultivars that offer tolerance to insect attack or non-preference. In the 2011/2012 and 2012/2013 harvests, the whitefly population was quantified in some soybean cultivars used in Mato Grosso. In the first year of the work, infestation was high from the beginning of the crop cycle and it was observed that the cultivars TMG1176RR, TMG132RR and P98Y11 showed lower preference for oviposition and whitefly colonization in relation to the others evaluated, which had a higher population (TMG1188RR, NA8015, TMG1288RR, TMG133RR and TMG127RR). In the 2012/2013 harvest, the whitefly population increased during the crop cycle and the cultivars TMG1288RR, TMG1176RR, TMG132RR and P98Y11 showed slower and lower development of this population during the crop cycle. However, at some moments during the development of the crop, all cultivars evaluated presented high populations, which indicates that when there is high insect pressure, it is not possible to differentiate the preference or not for different soybean cultivars. This study should be continued in order to identify other cultivars in relation to whitefly non-preference.
Preventive and/or successive applications of phytosanitary products to control whitefly in soybeans can favor the selection of resistant insects and an increase in the pest population. One of the most important tactics for managing pest resistance to insecticides is rotation by mode of action and to achieve this objective it is essential to create a rotation program with products that have different mechanisms of action. Therefore, technicians need to know the mode of action of insecticides and acaricides on the market, to include them in their recommendations for chemical pest control.
In general terms, it is important to carry out preventive measures that discourage the development of whiteflies in cultivation areas, such as: eliminating host plants for the pest within the culture and in neighboring areas; survey and count the pest in the field; use selective products to keep natural enemies out; destroy cultural remains; avoid planting cultivars that are very attractive to the pest; training of the monitor and product applicator; selection of products considering: rotation of mode of action, effect on natural enemies and development phase of the whitefly.
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