The thought gives rise to several others: How long would it take them to return to pre-eradication levels? How much would it cost to control them? The second question is the most difficult questions to answer, because there is less experience on which to base an answer.
As to the first question, we do know that it only took boll weevils seven years to colonize the entire state of Mississippi — to move from the Natchez area to the northeast corner of the state. That was a very different situation than currently exists in the north Delta.
In that first case, weevils had to move several hundred miles and then reproduce. Although the eradication effort has reduced boll weevils to very low numbers in the north Delta, boll weevils still are present. In the absence of an organized eradication/maintenance program, it is very likely that the low numbers could return to pre-eradication levels in only two to three seasons.
One factor that would facilitate a quick rebound in boll weevil populations is the widespread planting of transgenic Bt-cotton. Since its introduction in 1996, use of Bt cotton has reduced in the number of insecticide treatments applied to control caterpillar pests and less coincidental control of boll weevils.
How much would it cost for individual producers to control boll weevils in the north Delta if they were allowed to return? Before Bt cotton, producers in the Mississippi Delta applied four to seven sprays per year just to control caterpillar pests. In almost all cases, those sprays contained an insecticide that also had activity against boll weevils.
Even though those sprays provided much coincidental control of boll weevils, many Delta growers still had to apply an additional two or three sprays specifically targeting boll weevils. Thus the total number of sprays applied to control boll weevils was in the range of six to nine sprays, depending on the season.
Current varieties of Bt cotton receive an average of less than two caterpillar sprays per season, and it is likely that the new dual gene varieties that are being introduced now will require very few, if any, caterpillar sprays.
Of course, Delta cotton does receive a number of sprays each year to control tarnished plant bugs, but few of the insecticides that are effective against plant bugs also control boll weevils (Bidrin is one product that does control both species). This means that, again depending on season and location, the number of sprays required to control boll weevils in the North Delta, could range from four to nine.
Assuming an approximate insecticide cost of $6 per treatment (this assumes that methyl parathion would be the primary boll weevil treatment used and allows about $3 for the insecticide and $3 for the costs of application), the season's cost could be from $24 to $54 per acre per year.
Those figures would not include any yield loss to the boll weevil, which, despite aggressive control efforts, can range from 1 to 3 percent. On a two-bale yield, that is 10 to 30 pounds of cotton, which at 60 cents per pound represents an additional $6 to $18 per acre lost to boll weevils. Taken together, the cost of allowing boll weevils to return could range from $30 to $72 per acre.
Blake Layton is a Mississippi Extension entomologist.