Integrated boll weevil management

An old acquaintance of farmers, the boll weevil is an insect that causes serious damage to cotton crops. Its control involves the integration of joint measures, adopted regionally and in an integrated manner, such as the destruction of ratoons, respect for sanitary emptiness and the rational application of insecticides.

For more than 30 years, the boll weevil (Anthonomus grandis) is present in cotton crops in Brazil, causing serious losses and increased costs. It is a pest of great economic importance due to its rapid reproductive capacity, high destructive power and the difficulties in its effective control must always be considered. 

The adult boll weevil is a beetle that measures between 3mm and 8mm in length and 2mm and 3mm in width. The color of adults varies from reddish-brown for newly emerged ones to dark brown for aged ones. The adults of the boll weevil have two spurs (aristas) on the tibias of the first pair of legs, one larger than the other. The eyes and beak are dark and the antennae have 12 segments.

Females lay, on average, 150 eggs in flower buds. The incubation period is three days. Eggs, larvae and pupae are found inside flower buds and in apples. The larvae go through three instars of growth that last, on average, eight days, they are white, legless, curved and, when grown, they are between 5mm and 7mm long. The larvae then transform into pupae and, after five days, the adults emerge. The life cycle from egg to adult takes approximately 19 days, with five generations occurring per harvest.

Adults feed preferentially on flower buds, but in their absence or under strong population pressure, apples are also attacked. By introducing its beak, the adult produces feeding and egg-laying holes in flower buds and apples. Over the laying hole, the female deposits a waxy substance covering the cavity, visible and felt by touch. The attacked flower buds and small apples fall to the ground containing developing larvae. Inside flower buds and apples, larvae and pupae develop, largely protected from the action of natural mortality agents. During the maturation phase of the cotton crop, boll weevil adults accumulate fat reserves, which enable them to survive for long periods. At the end of the crop cycle, the adults move to areas that remain vegetated (forests, savannas, furnas, other crops, etc.), existing in the vicinity of the cultivated area. In these places of refuge, they reduce their activities, feeding sporadically on pollen grains from the different plant species existing in the area. The adult mortality rate is almost always high during the off-season, but a sufficient number of beetles will always survive to reinfest subsequent crops. The longer the period of sanitary void, the shorter the survival of the pest. However, it is always necessary to consider that weed population densities will depend, mainly, on the greater or lesser presence of ratoons and/or tigueras in cotton productive environments.

The newly emerging plants attract the weevils that survived the off-season. The first boll weevil adults that arrive at crops before the flower buds are emitted tend to gather on the borders. Initially, the adults feed on the vegetative parts of the plants (leaves, petioles and apical bud), while waiting for the flower buds to appear, which appear after 30 days, for most varieties. Until the beginning of flowering, the insect moves little, remaining in concentrated areas of the borders, but from this stage onwards the weevil begins the process of dispersing throughout the crop and neighboring areas.

Driving

In Brazil, the boll weevil is the main pest of cotton in almost all producing areas. And, more frequently, increasing losses caused by the pest are observed, and many crops in the savannahs present, at the end of the crop cycle, with the upper third of the plants damaged by the boll weevil.

Brazilian farmers are aware of the importance that the boll weevil represents for the sustained continuity of cotton farming. Initiatives are emerging, generally led by producer associations, of regional models of boll weevil control through practical programs to control the pest. These plans are focused on collective actions, considering that the lack of local control of the pest affects the entire producing neighborhood. The success of these plans depends on the participation and adherence of producers and technicians who guide the crops, so that procedures can be standardized. To carry out these plans, traps containing pheromone are important devices with the objective of identifying and quantifying, through the capture of adults, the immigrant boll weevil population in newly established crops. The traps must be installed on the perimeter of the plots, at approximately 200m intervals, and placed at least 30 days before sowing, remaining inspected until the flower buds are fully emitted. Readings of adult captures in traps should be weekly, with the data used to establish insecticide application plans from the emergence of the first flower buds. The average catch, equal to or greater than two adults/week, will determine the need for three applications at intervals of three to five days, after the first flower buds appear.

For areas known to be infested by the boll weevil, management tactics are structured into fundamental technical measures such as: concentrated sowing per region, within 30 to 40 days; applications of insecticides on the borders (30m) from the 2nd expanded leaf until the first firm apple stage, at intervals of five to seven days; three applications of insecticides from the emergence of the first small flower buds, at intervals of three to five days. Next, monitoring the crop, on a weekly basis, should be carried out by inspecting 250 flower buds/plot, one flower bud/plant measuring approximately 0,6cm in diameter, noting the occurrence of feeding or laying holes. The level of control to decide the timing of applications should be up to 5% of flower buds attacked by the boll weevil.

Adults are the only stage of life exposed to the action of insecticides, these products being the main weapons for controlling the pest. Various chemical groups of insecticides can be applied alternately, such as: organophosphates, carbamates, pyrethroids, etc. Insecticides in UBV formulations and also in BVO applications have shown better results in weevil control. The addition of cottonseed oil to applications may favor pest mortality.

 The regional establishment of sanitary emptiness, with a real absence of cotton plants, and the effective destruction of ratoon crops must be understood as collective and fundamental obligations to reduce the damage caused by the boll weevil. Factors such as low environmental and soil humidity affect the quality of destruction of clogs through mechanical and/or chemical methods, requiring subsequent transfers. Common resistance to glyphosate in soybean, corn and cotton crops has added difficulties and limited options for controlling tigueras and clogs.

The cotton remains remaining in the areas will retain a large part of the adults that will remain refugees in these plots during the off-season, constituting potential suppliers of boll weevil for all future cotton crops. Preparing the soil in advance in plots with a history of high weevil infestations causes a “dislodging” effect on the remaining adults in the area, who leave in search of refuge. Plowing and harrowing operations partially reduce the population through mortality and minimize the focusing effect that these plots exert among cotton crops. In successions or rotations, cotton ratoons and tigueras that remain vegetating in plots with soybeans or corn will be maintainers, reproducers and suppliers of boll weevil for nearby cotton crops. A scenario that is becoming quite common in regions that grow cotton, with it being noted that the largest weevil infestations for new crops come from areas previously cultivated with cotton. Soybean crops that follow cotton should be inspected to detect any surviving cotton ratoons and bushes, which should be promptly eliminated. When these soybean crops finish their cycle, and especially during harvest, there will be increasing flows of boll weevil adults invading the cotton areas. These flows must be monitored and effectively combated. Tiguera plants and cotton ratoons need to be quickly eliminated at the beginning of the establishment of soybean or corn crops. Planting corn after cotton cultivation should be avoided as it increases the level of boll weevil infestation in neighboring cotton crops.

Applications of insecticides at the end of the crop cycle, the establishment of strips of bait ratoons (sprouted plants) to attract and combat the boll weevil with sequential applications, observation of the sanitary void and the installation of the weevil-killing tube contribute as complementary measures for the population reduction of the pest for the following harvests.

Walter Jorge dos Santos, boll weevil specialist, IAPAR researcher 

Article published in issue 197 of Cultivar Grandes Culturas.