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The Boll Weevil
A pest is defined as any species that cause injury to humans, whether that injury is physical or financial. And the boll weevil, while it can’t hurt humans directly, most definitely fits this definition of a pest. This insect can cause a great deal of nuisance and harm to humans, and as a result, various methods have been created to help deal with the boll weevil. In this article, we will examine some facts about the boll weevil. Because the first step in dealing with a pest – any pest – is to know your enemy. So read on while we dive into the world of the boll weevil.
What is a boll weevil?
A good place to start would be to determine what a boll weevil actually is. After all, these pests may not be as familiar to people as other bugs like ants and spiders.
The boll weevil – Anthonomus grandis, to give it its scientific name – is, like all weevils, a type of beetle. Weevils, in general, are a herbivorous beetle, and the group contains over 60,000 species, of which the boll weevil is just one. However, many other weevils do not cause significant problems for people, and so are not as well known. The boll weevil, on the other hand, can cause serious problems for us.
Boll weevils are thought to originate in Mexico, but in the 19th century, they began to move northwards into the southern US. Most weevils are very specific in what plants they will or won’t eat, and the boll weevil is typically picky. It will only eat cotton plants. Since cotton is one of the world’s most important crops and was the economic lifeblood of the southern United States for a long time, it is easy to see why this tiny bug has become notorious.
Like all weevils, the boll weevil is small, around 6mm in length when fully grown. Like all beetles, boll weevils have six legs. Its body is almost perfectly round in appearance. They are usually brown or gray in color. If you look closely at a boll weevil, you may see the short hairs that cover its abdomen and thorax. But the most striking thing about the boll weevil’s appearance is its face. These bugs have a long mouthpiece that projects from the front of their head and looks almost like an elephant’s trunk or an anteater’s snout.
Boll weevils get their name from the fact that they destroy cotton bolls – and no, I’m not spelling ‘ball’ incorrectly! The boll is the part of the cotton plant that holds the white tuft of cotton most of us are familiar with. In order to produce cotton, a plant needs to grow these bolls to protect the cotton inside. But the boll weevil consumes any new buds the plant tries to put out, meaning that the plant cannot produce any cotton.
Cotton plants need a warm climate, and boll weevils need cotton plants, so their range is limited by those requirements. However, the spread of boll weevils over the last two hundred years has been dramatic. Boll weevils can now be found in Mexico, Alabama, Arizona, California, Florida, Georgia, Louisiana, Mississippi, New Mexico, North Carolina, South Carolina, Texas and Virginia. It’s not unheard of for these insects to be found outside of this area, but they generally don’t have viable breeding populations in other areas. However, boll weevils will go wherever cotton rows, and as global temperatures creep higher, the potential range of this important pest continues to grow.
Life Cycle of a Boll Weevil
Like all beetles, the boll weevil undergoes complete metamorphosis. This means that the young boll weevils look completely different from the adults of the species. The boll weevil life cycle consists of four distinct stages: egg, larva, pupa and adult. Let’s start at the beginning – with the egg.
Boll weevil eggs are laid on the cotton plant themselves. They are deposited into cotton flower buds or into the bolls themselves. The female boll weevil will seal the hole that she has made to lay her eggs in with frass, which is the chewed-up remnants of the plant, or with droppings. This leaves a characteristic raised brown area at the site of the egg laying. When eggs are laid in the flower buds, it generally causes them to turn yellow and fall from the plant. When eggs are laid in bolls, it may or may not cause the bolls to fall from the plant.
The eggs are laid one at a time. In around 3-5 days after being laid, the eggs will hatch. The larva that emerges from the egg is a small, worm-like creature. This larva will feed on the boll, and this is part of what causes the damage that a boll weevil infestation can do to a cotton plant. The weevil will stay in this larval stage for 7-12 days, during which time its only job in life is to eat and grow. Once this period of time has passed, hormonal changes in the larva will induce it to pupate.
In order to pupate, a boll weevil larva will construct a kind of cocoon around itself. This cocoon protects the weevil while it undergoes the dramatic metamorphosis that will transform it into an adult. While the weevil is pupating, it doesn’t feed or even move. It simply grows inside the cocoon until it is ready to emerge.
After 3-5 days of pupation, the weevil will emerge as an adult, using its newly developed mouthparts to chew its way out of its pupa and the boll it has spent its whole life in so far. At this stage, the weevil is an adult, with all the characteristics of adults of its species, including the six legs and the anteater mouthparts.
In its new adult form, the boll weevil will spend 3-7 days feeding on the cotton plants. After this time, it is ready to mate. Once a female boll weevil has mated, she will lay her eggs one at a time inside the cotton plant, injecting them into bolls and buds with a special organ called an ovipositor. A single female can lay up to 200 eggs in her lifetime, so you can see why these bugs have become such a big problem!
In favorable conditions, where the weather is warm enough, the entire life cycle can be completed in 3 weeks. However, through much of the United States, the weather can be too cold to allow the boll weevil to breed quite so fast. In these cases, adult boll weevils can go into a state called diapause, similar to hibernation. A pre-diapause boll weevil will not mate, but will spend the end of summer eating as much as possible to build up its reserves for the winter. Then it will seek out a warm spot in some leaf litter where it will hide and wait out the winter to emerge in spring.
The boll weevil’s reproductive potential is such that it can produce five or more generations in a single year. The result of this is that a few weevils can quickly become a major infestation and devastate cotton crops.
Damage of Boll Weevil Caused to Agriculture
As we have learned. The boll weevil is completely dependent on the cotton plant for its survival. With the possible exception of diapause, its entire life is spent on or near cotton plants. And all stages of the boll weevil, whether larva or adult, feed on the plant voraciously.
As a result, the boll weevil remains the most destructive pest of cotton in the United States. It is estimated that since it arrived in the United States, the boll weevil has cost cotton producers $13 billion dollars. Recently, the cost of the damage the boll weevil causes has been calculated at $300 million per year.
Boll weevils have the ability to completely destroy a cotton crop. By eating the buds of the plant, they make it impossible for the plant to grow and produce cotton. Furthermore, any cotton the plant does manage to produce is ruined by boll weevils either eating it or laying eggs in it. When boll weevils first arrived in the United States, they completely decimated cotton crops, ruining the livelihoods of countless farmers and destroying entire harvests of cotton.
These days, various strategies have been employed to try to reduce the damage caused by the boll weevil to cotton crops. One of these methods has been the use of pesticides. In the years following World War 2, DDT was extensively used against the boll weevil, and for a while had some success. But soon, the boll weevil’s high reproductive rate meant that the weevils developed resistance to the pesticide, and in the 1970s, DDT was banned due to its deleterious effects on the environment.
Along with the damage they directly cause to cotton plants, boll weevils create another problem for cotton farmers. Because of their high reproductive rate, it is necessary when using pesticide applications to control boll weevils to perform multiple applications throughout the growing season. But applying pesticides so frequently has a number of negative effects. Applying pesticides reduces the numbers of other insects that would normally prey on the boll weevil and help control their numbers, such as aphids and plant bugs. So not only does the presence of boll weevils threaten the cotton plants themselves, but it can also result in a situation where the weevil’s predators cannot survive. In this way, improper pesticide applications can do more harm than good.
Perhaps the only good thing about the boll weevil is that it doesn’t cause damage to food crops. Its dependence on the cotton plant as both food source and breeding ground is such that these weevils have no interest in the plants that humans need to feed ourselves, so they stay away from our food crops. But the economic impact of these bugs is devastating for cotton growers, and cannot be ignored.
Facts about Boll Weevils
– When male boll weevils feed on cotton, they produce a specific pheromone that attracts both female and male boll weevils. Because the males can only produce this pheromone once they have found a food source, it attracts females that want to mate with them and use that food source as a place to lay their eggs. It also attracts males that simply want the food source for themselves. In this way, a few boll weevils can quickly bring more of their kind to damage cotton crops.
– Because of the pheromone-emitting behavior of male boll weevils, a control method that has recently been developed is the use of pheromone traps. These traps use the same pheromone that the male boll weevils produce to lure other boll weevils into a trap from which they cannot escape. Because this pheromone only works on boll weevils, it has no adverse effects on the beneficial insects that also prey on boll weevils, and are often killed by large-scale pesticide applications.
– Because boll weevils spend most of their lives hidden in cotton plants, they are safe from most predators. However, one predator of boll weevils is the fire ant. These ants get their name from the venom they inject which cause a burning sensation in humans. Because of this behavior, fire ants are seldom welcomed in areas where humans live and work.
– Another very effective predator of the boll weevil is the parasitic wasp Catolaccus grandis. This wasp, native to south eastern Mexico, uses the boll weevil’s own pheromones to detect them. After paralyzing the weevils with its sting, the wasp will lay its eggs inside the weevil. Once the larvae hatch they will consume the weevil. Because these wasps are a tropical species, they cannot survive in the southern US states where the boll weevil has become a significant problem. However, cotton growers have had some success in controlling boll weevil populations by occasional releases of specially bred populations of Catolaccus grandis.
– Cold winters are one of the biggest killers of boll weevil populations. Temperatures below 23 Fahrenheit, or -5 Celsius, are lethal to boll weevils. A series of harsh winters in Missouri in the 1970s virtually eliminated the boll weevil in that state for a while, but the populations eventually recovered after more boll weevils spread from other states. With global temperatures rising, winters cold enough to kill off the boll weevil in large numbers are becoming rarer, making this formerly potent weapon against these insects less effective than it used to be.
– Since the 1970s, the boll weevil has bee the target of the National Boll Weevil eradication project, a coordinated effort by the United States Government to completely wipe out its populations of boll weevils. While this program has had some success in certain areas, it has so far proved unable to remove this invasive pest of cotton plants from the states infested.
– The spread of the boll weevils is often cited as one of the factors behind the Great Migration, the widespread movement of African Americans from the southern states northwards in the 1900s. The boll weevil had such a devastating effect on the economies of the cotton growing states that people were forced to move to find new sources of work. Since a disproportionate number of African Americans were employed in growing cotton, the boll weevil had a greater effect on them than on other populations, forcing large numbers to move north and changing the demographic make-up of the United States.
– An early attempt to eradicate the boll weevil population involved the release of one million sterile male boll weevils. The idea was that these males would mate with the females instead of the wild boll weevils, but being sterile, would not produce any offspring. However, the sterile males proved unable to compete with the wild males, and the boll weevil population continued to grow despite this ambitious but ultimately ineffective attempt at control.
– There is a monument to the boll weevil in the town of Enterprise, Alabama. Erected in 1919, the statue shows a woman holding a pedestal with a giant boll weevil on top. The statue was made to commemorate the major boll weevil infestation that plagued the county in the 1900s. While cotton crops failed and livelihoods were ruined, the people of Enterprise decided to diversify the crops that they grew, starting with peanuts. The peanut farming was a great success, and the people of the town came to see what had happened to their cotton crops not as a disaster, but as something that prompted them to explore a new opportunity. The statue is designed to celebrate the resilience of the local people in the face of misfortune.