Dispersal of boll weevil toward and within the cotton plant and implications for insecticide exposure

Abstract

BACKGROUND Immature stages of boll weevil complete development endophytically leaving only the adult stage accessible for chemical control. We tested the hypothesis that boll weevil colonization of the cotton plants significantly affects their exposure to sprayed insecticides. We determined the adult dispersal toward and within cotton plants, lethal time (LT), and residual control by recommended insecticides (malathion, carbosulfan, thiamethoxam, fipronil, beta‐cyfluthrin, lambda‐cyhalothrin, and thiamethoxam + lambda‐cyhalothrin) through dried residue exposure and residual control regarding the leaf position in the upper and the lower thirds of the treated plant canopy. RESULTS Newly emerged adults from fallen buds reached the cotton plants by walking (80%) and most of the time settling on cotton bolls in the lower part of the plants (78%). Irrespective of sex and mating status, adults released on the upper part of the plant remained longer on the same release site than the lower part, with some individuals remaining up to 50 h on the same flower bud. The shortest LT90 was found with thiamethoxam (106 h). Fipronil and malathion, respectively, provided the longest (>144 h) and shortest (24 h) residual control times and caused boll weevil mortality above 80%. CONCLUSIONS These findings suggest that weevils accessing the cotton plants exhibited within‐plant distribution that minimizes their contact with insecticide residue on plant foliage. Furthermore, short residual control with malathion, the most used insecticide against boll weevil, and the low susceptibility exhibited by the tested population to pyrethroids highlight the current challenges faced for boll weevil control in Brazilian cotton fields. © 2020 Society of Chemical Industry

Spatio‐temporal distribution of Anthonomus grandis grandis Boh. in tropical cotton fields

“…Similar to other studies on curculionid spatial distributions, 11,50 the boll weevil exhibited a moderate to strong aggregated pattern for adults and for infested reproductive structures. This suggests relatively slow in‐field dispersal of adults when plants offer plenty of structures for food and as oviposition sites; 51 hence, there is no impetus to move farther away. Aggregation in boll weevil populations can result from the release of aggregation pheromone, 52 and the prevalence of preferred reproductive structures 36–38 .…”

Field‐evolved resistance to beta‐cyfluthrin in the boll weevil: Detection and characterization

“…This observation supports the hypothesis that resistance selection in the past should be of a low order, considering that boll weevil would be exposed to insecticides only during the cotton season, with most individuals exposed to cotton pesticides likely dying before the next cotton cropping season. However, this hypothesis has been changed recently due to the observed presence of boll weevils in volunteer cotton plants developing in soybean and corn fields 14,51 . This occurrence of volunteer cotton appears to be due to the use of herbicide‐resistant cotton (Roundup Ready cotton), as these plants can survive the herbicide roundup® commonly used in these others 52 .…”

“…Infested squares fall to the ground 5–6 days after oviposition, where the immature stages complete their development. This behavior makes boll weevil a key pest and limits insecticides control to the adult stage 8,10,13,14 . Oviposition occurs over several days, resulting in staggered adult emergence, which requires repeated insecticide applications to limit insect lifecycle 15…”

Stabilized Double-Stranded RNA Strategy Improves Cotton Resistance to CBW (Anthonomus grandis)

“…The use of chemical insecticides remains widespread in the control of this pest. Nevertheless, it is an expensive procedure with limited efficacy due to the endophytic nature of CBW larvae [ 3 ].…”