Cotton growers in Kansas have benefitted from an aggressive boll weevil eradication program in Oklahoma that has helped ensure weevils have not be a problem in Kansas. Cotton industry leaders and growers want to keep it that way.

That’s why the Kansas Cotton Association will be implementing an extensive boll weevil trapping program in Kansas during the 2020 growing season. Rex Friesen, crop consultant for the Southern Kansas Cotton Growers Association, says he is putting out information via newsletter about weevils, what they look like and what the damage they cause looks like to help assist early detection and extermination.

“As we work on the trapping program, I would like consultants, field scouts and farmers to know a little more about them so as they walk through cotton fields, take sweep samples, inspect squares and fruit, you would be able to recognize them or their damage it you were to see it,” Friesen says. “The more eyes on the lookout, the better.”

Biology

Friesen says it would be more accurate to call it a “square weevil” rather than a boll weevil because the preferred feeding and egg-laying sites are squares with green bolls being the second choice. But long ago the first grub was discovered in a boll, so the insect became the boll weevil.

Boll weevils have four stages: egg, larva (grub), pupa and adult; from egg to egg-laying adult takes approximately 18-21 days.

The adult boll weevil is typically about ¼ inches long from rump to the end of its snout. Their color starts as a reddish-brown when newly emerged and changes to more of a brown gray.

The mouth of the weevil is at the end of the long snout, which they use to bore into the squares and bolls, feeding on the pollen (squares) or seeds (green bolls) inside.

Adult females feed for 3-7 days after emerging from the pupa before mating and commencing to lay eggs. After feeding in the square or boll, the female turns around and deposits an egg inside the feeding wound, and then seals it with a plug made of frass (weevil poop — looks like a zit or a really small cork).

Typically, there is only one egg laid per fruit body, unless fruit is limited. The female marks it with a pheromone to keep others from laying additional eggs in it. A typical female weevil can lay 100-300 eggs, spread out over a period of 10-12 days.

The infested squares fall to the ground where the grub finishes its development and pupates; infested bolls don’t typically fall off. A lot of damaged mid- to full-sized squares on the ground are a possible sign of their presence. Tear a few open to see if they are infested, having a white grub or pupa inside — not to be confused with squares shed from drought, plant bugs or other causes.

Remember — the likelihood they’re due to boll weevils is truly near zero, so don’t freak out if you see some squares on the ground. But do check a few just to be safe.

Late in the season, as the days shorten and begin to cool, the boll weevils’ physiology changes into a diapause (hibernation) mode, where the adults feed to store up fat and then take flight to search for a place to hide for the winter months. They typically go under leaf litter, wood piles, shingles, etc. It’s reported that it takes sustained temperatures of 23 degrees F or less to start killing them.

Frisen says one study even said it takes an hour at 5 degrees.

“That doesn’t sound too hopeful for a good winter kill, even in Kansas,” he says.

Adult overwintering boll weevils start emerging in spring and look for cotton squares to feed on. If they find some, the males feed and release an attractant pheromone to bring in the females for feeding, mating and egg laying. The cycle then repeats.

There can be between three to 10 generations in a season, depending on geography and the season. In Kansas, Friesen says it would be closer to three generations.

“Multiple, overlapping generations are why boll weevils are so insidious — once they start reproducing, you literally get new adults emerging daily. To break the cycle, you would need to treat every three to five days until all immature weevils have finished emerging, because the adult stage is the only stage you can kill by spraying,” Friesen says. “This aggressive schedule is needed to kill the newly emerged ones before they have enough time to become sexually mature, mate and begin laying eggs.”

Knocking their numbers down is only a Band-Aid, he adds. To ultimately win the war against their damage, they have to be eradicated.

“That’s why producers and the USDA have spent untold millions over the last 40 years working to eradicate them — they’ve been pushed back to South Texas and Mexico, their place of origin,” he says.

Friesen says he has plenty of horror stories from the past when boll weevils were at their worst in the southern U.S

Growers literally lost half their crop, even after spraying 10 to 20 times, he says. Many growers in those days tell stories of having to periodically stop the harvesters to brush weevils off of the air cleaners.

“You get the picture,” Friesen says. “We simply can’t let them even get started in Kansas.”

There are some interesting YouTube videos on the topic if you enter “Boll weevil eradication” in the search window.

A few insect 'factoids'

Of all groups of living organisms, there are more species of insects than of anything else — around a million identified, plus at least that many yet to be; by comparison, there are around 4,000 mammal species, Friesen says.

Of all the insect orders, beetles (Coleoptera) have more species than any other order in the insect world, around 97,000 species.

Within the order Coleoptera, there are more species of weevils than any other types of beetles, an estimated 60,000 species (most are in the family Curculionidae, which is the largest family of beetles). There are over 1,000 species in the family Curculionidae in North America alone. Boll weevils are members of this family.  

“There’s a good chance that with the thousands of species of weevils, that farmers will see a weevil of some species when they start really looking for them,” Friesen says. “Boll weevils are the only one we are worried about, so don’t get to excited about a beetle with a snout.”
Friesen suggests that farmers who find a weevil they are worried about take good closeup photos and send them in, or actually collect the specimen to mail or bring in.

“A picture can truly be worth a thousand words,” he says. “Verbal descriptions over the phone don’t work very well.”

To help with identification, Friesen says one fairly unique identifying feature is “teeth” or “spurs” on the front legs. Some other species have those spurs, but most do not. All boll weevils do have them.

“A good hand lens magnifier really helps on seeing some of these things in the field,” Friesen says. “Two local species with which boll weevils might be confused are the alfalfa weevil — similar in size and shape, light gray, but with a darker stripe part-way down the back. Also, the snout is shorter and thicker. Also, the pecan weevil, which is typically about the same color, a little larger in body-size, but the snout is much longer and thinner.