Boll weevil management in cotton

The main pest in cotton, the boll weevil scares technicians and producers due to the enormous losses and burden caused to production costs. Regional management, in an integrated manner, with rationality and management in the application of insecticides is essential so that it is possible to live with the insect without making the crop unviable.

Since its first identification in Brazil, the boll weevil has been considered the main pest of cotton. In the beginning, the insect quickly infested crops, which, for the most part, were characterized by being relatively small and with an extremely low technological level when compared to current crops, due to the absence of pest monitors, modern spraying equipment, etc.

With the migration of cotton farming to the Cerrado regions, probably due to the large cultivated areas and combined with crop rotation, it was believed that the pest would not dominate crops, being limited to borders or small percentages of planted areas.

In recent harvests, the ability demonstrated by the pest to dominate crops, generate enormous losses and increase production costs in a frightening manner has worried many producers and technicians involved in the production chain.

The new technologies introduced in cotton farming run a serious risk of becoming unviable from an economic point of view, as the benefit of safety in lepidopteran control could result in enormous losses caused by the boll weevil.

Although applications for boll weevil control are considered cheaper than those for controlling caterpillars, for example, the frequency significantly increases the number of applications.

In the western region of Bahia, the average number of specific applications – or combined ones for a complex of pests, also with the aim of controlling the boll weevil – amounts to approximately 26 in rainfed crops and 40 in irrigated crops. In this management and control model, irrigated crops and those in risk areas already become unviable.

Other factors will also influence the producer's decision to plant cotton or not and this is also regional, depending on the production system adopted.

In Mato Grosso, due to the climate, the largest planted area is safrinha, while in Bahia it is harvest, that is, in Bahia the cotton areas compete for space with soybean and corn crops, while in Mato Grosso cotton crops are planted after soybeans, competing for space only with corn as an off-season option. But replacing costs for caterpillar control with royalties paid, which are added to the growing costs for boll weevil control, could make cotton economically unviable.

Description and biology (brief summary)

Boll weevil eggs are smooth and white, 0,08mm long. The larvae are white in color, with a light brown head, without legs and curved (Figure 1).

The pupae are white, 10mm long. In them, you can see the traces of the different body limbs of future adults (Figure 2).

Adult

Adults are generally gray or brown in color, 7mm long (including the beak), and approximately 2,3mm wide. The body is covered with small, fine golden hairs, giving the insect a fuzzy appearance (Figure 3). Newly emerged adults are reddish-brown in color. The femur of the front legs has two ridges (spines), one larger than the other (Figure 4). The femurs of the median and hind legs only have one ridge. The eyes and beak are dark and the antennae have 12 segments.

The adult is a sexually reproducing insect. Females lay an average of 150 eggs. Eggs are deposited inside flower buds approximately 7mm in diameter. The bracts of the attacked flower buds become yellowish and open within three to four days after the attack (Figure 5). The buds fall to the ground, containing developing larvae that pupate and transform into new adults.

The life cycle from egg to adult is completed in approximately 19 days. Six to eight generations can occur during the harvest.

The origin

In the early 1830s in Mexico, the cotton boll weevil was collected for the first time. In 1855, also in Mexico, the first record of A. grandis as a pest of cultivated cotton (Gossypium hirsutum). In 1892 it was already found as a pest on crops in the United States (Manessi, 1997).

In Brazil, the first record of the insect occurred in 1983, in the municipality of Jaguariúna, São Paulo, Campinas region. From then on, the insect spread to practically all crops in the region and very quickly to other producing areas in Brazil, mainly in the Southeast and Northeast.

In 1985, the boll weevil was already infesting cotton crops in the states of São Paulo, Pernambuco, Paraíba and Rio Grande do Norte (Barbosa et al, 1986). On the map, the colors associate the years in which the boll weevil was first recorded in the producing regions. 

Diagnosis of the current problem

The problem currently experienced is the result of accumulated years, unresolved situations, inefficient or partial suppression or control measures. Among the most varied problems, some are particularities of some regions, such as climatic conditions and size of properties or crops, and others are generalized.

Main causes of the boll weevil population increase in producing regions in recent harvests 

Behavior

During many harvests, all management techniques suggested for controlling or coexisting with the pest considered a standard behavior, where the boll weevil, under normal environmental conditions, occurs in a standardized way, seeking shelter at the end of the previous harvest with the fall temperature and “shortening” of days. In this behavior, its favorite places to shelter are flooded areas, native forests that border or are close to the attacked crops and refuge crops (eucalyptus, coffee plantations, etc.), also marginal or close to the attacked crops, where it enters a hibernating state and awakens at the beginning of the season. following harvest, returning to the cotton through the borders, feeding on the cotyledonary petioles and, after nourishing itself, it migrates to the interior of the crops which, with the beginning of the cotton reproductive phase, begins “colonization”. Currently, part of the behavior cited as standard is still true, such as the search for shelter at the end of the harvest, with the drop in temperature and the “shortening” of days, and their favorite places continue to be flooded areas, native forests and refuge crops, where they enter a hibernating state, waking up in the following harvest. However, under conditions of altered environment, the boll weevil has the option of cotton ratoons and bushes on highways, roads and crop residues (Figures 6 and 7), where it remains active during a period in which it should be hibernating. In this activity, we observe the maintenance of adults, which feed on leaves and petioles, and even the multiplication of the pest, when ratoon and tiguera plants have productive structures (flower buds, flowers and apples).

The aggravating situation occurs in soybean and corn crops, planted in succession to cotton crops from the previous harvest, with the presence of resprouting plants and tigueras. When this occurs, infestations in neighboring cotton crops do not only occur along the borders and in the initial period, as in standard behavior, but also in central areas, due to the presence of nourished adults capable of flying, and on an ongoing basis, for account of the presence of attacked productive structures that constantly generate new adults. 

Crop management

Activities to combat the boll weevil require committed and disciplined people, as well as definitions of method, management and attitude. The insect must be respected and recognized as a highly prolific, silent and extremely skilled pest. When there is community or regional control, this facilitates individual control for each producer.

In a sequence of actions to manage the boll weevil in attacked areas, properties or regions, trapping is suggested to create a history of captures over the years, to determine risk levels for the next harvest and to identify the routes of hibernating adults. ; planting bait (Figure 8) (*) on the routes of hibernating adults (previously identified by trapping) to attract and kill dispersing adults; the sowing time, considering the characteristics of the varieties (vegetative cycles), keeping the adults on the already known hibernation routes; applications in B1 (Figure 9) (**) to promote the “suppression” of adults who display standard behavior during the colonization phase; intermittent applications on borders to maintain “control” of dispersing adults in the highest risk areas of the crop; the collection of attacked flower buds that have fallen to the ground (Figure 10), when concentrated on borders or in duly identified patches; batteries in part of the crop or in the entire area from the identification of the first outbreak (***), defoliate with insecticides (****) to reduce hibernating adult boll weevils, reducing the risk of high infestations for the following harvest ; the harvest associated with the sowing season, maintaining the routes already known; the destruction of cultural remains in an adequate and efficient manner (Dead plant); the installation of tubes to kill boll weevils or bait ratoons also to reduce hibernating adult boll weevils, reducing the risk of high infestations for the following harvest, and crop rotation with efficient control of regrowth and tigueras, also contributing to other crops ( soybeans and corn) and ensuring the sustainability of cotton farming in the production system.

Bait planting consists of anticipating the cultivation of strips by 15 days before planting the crop, installing these strips on the adult entry and exit routes, and must respect the sanitary void period. Phase B1 (Figure 9) is highly attractive to hibernating adults who exhibit standard behavior. The number of insecticide applications from B1 will be determined by the trap capture rate. If the cotton grower has not trapped the areas intended for planting, the number of applications must be fixed at three sequential applications with three-day intervals.

Hibernating adults with standard behavior are more sensitive to insecticides, therefore, discipline in carrying out this activity is extremely important and will be reflected in crop management during the flowering period.

The duration and intervals of battery applications will be determined by the intensity and time of adult dispersal in the crop. It is extremely important to defoliate with insecticides. It is also essential to control which products were used in defoliation so that different products are used in the first applications of the following harvest.

Phosphorus products with a strong smell, even if they are not very toxic to mammals, should be avoided during defoliation, as, in addition to bothering people involved in the harvesting process, the odor of these products could contaminate the milk of animals treated with cottonseed.

Alternative management in cotton crops and rotation areas

It is recommended that properties create Boll Weevil Day, dedicated to an insect hunting operation or damage to cotton plants (crops or volunteer plants), mobilizing the property's entire technical team.

Inspect the preferred bud (Figure 11) (most visible structure on the plant, located at the transition from the middle third to the upper third).

The recommended action level for crops attacked during the period prior to Cut-out is initiated with battery applications at five-day intervals, with the presence of an adult or 3% of attacked preferred buds (OA or OP).

The recommended level of action for crops attacked during the period after Cut-out is battery application, with an interval of five days until defoliation.

When to apply 

- Sporadic application of insecticides to control stink bugs with simultaneous control of the boll weevil on volunteer soybean plants.

- “Mandatory” battery application at five-day intervals during soybean senescence.

- “Mandatory” battery application at three-day intervals on the cotton borders during the soybean senescence period.

If the presence of reboleiras or adults is detected inside cotton crops, carry out three battery applications with an interval of five days.

Products

There are few product options and they are all considered old. Malahion and Parathion have presented the best performances in field results and in comparative trials, however, some other options such as methidathione and fipronil present interesting results. Considering the pest complex at the beginning of the crop, during phase B1, the products carbosulfan and acephate, when interspersed with specific applications, are an interesting option for weevil management.

Pyrethroid products should be directed towards the end of the crop cycle and when populations are controlled or present a low risk of damage or for pre-harvest, added together with defoliation.  

Some products previously considered efficient in weevil control are currently being criticized and their effectiveness is highly questioned, including the likely loss of efficiency of pyrethroid products. Probably these products, when used in old infestations, much lower than current infestations, were rotated with more efficient products and sufficient to maintain the pest at acceptable levels of infestation for the standards of the time.

Currently, the high population rates of initial infestations, as well as the high population pressure of continued dispersions, are not considered safe for the use of these products (broad spectrum and moderate boll weevil control). Several factors give reason for these questions, and the main one is the visual effect after application or the increasing infestation percentages, even after several applications. The product's logical reaction is to considerably increase the dose and reduce application intervals, which certainly speeds up the selection process. However, in combined applications, a significant improvement in controls is observed.

As a suggestion for applications for infested crops, infestation levels are considered, classifying them as moderate to low, medium to high and high with frequent dispersion.

When infestation levels are classified as moderate to low, conventional land applications with flow rates of 50L/ha to 100L/ha are considered efficient for “controlling” populations, but discipline in meeting the predetermined dates for applications is the fundamental element for the success of the activity. When, due to the climatic conditions of certain periods or short intervals, terrestrial applications cannot meet the deadlines determined for the batteries, even in low infestations, terrestrial applications may not be sufficient.

When considering infestation levels of medium to high intensity, the best methodology for “control” of the boll weevil is aerial applications with water and low flows of 3L/ha to 10L/ha (its greater efficiency is due to the low volume of water, which results in a greater concentration of the product in the syrup, combined with better penetration of the syrup into the bracts and flowers, which are the target points for adult attacks).

When considering high levels of infestations and frequent dispersions, the most appropriate method for “control” and “suppression” of boll weevil populations are aerial applications with UBV (Ultra Low Volume) or BVO (Low Oily Volume), with flow rates of 2L /ha at 3L/ha, which confer a longer residual period of the spray to control adults (newly hatched or dispersing). However, aerial applications with water, if continued, are great tools for “suppressing” the pest.

In summary, terrestrial applications are recommended only for low intensities of attack and in any condition of boll weevil infestations and dispersions, the most efficient method of control and suppression are aerial applications (with water, UBV or BVO), with applications in UBV and BVO are the most suitable for weevil control in high infestations and constant dispersion from sources to crops. 

Paulo Edimar Saran, Technical Consultant, Soil and Plant Technical Consulting, Owner of Paulo Saran Trainings Técnicos

Article published in issue 194 of Cultivar Grandes Culturas.