Jessica B. Mahas

Understanding how biotic and abiotic factors influence vector-borne pathogen spread is essential for effective management. Cotton leafroll dwarf virus (CLRDV), transmitted by the cotton aphid, Aphis gossypii, can cause yield loss in cotton, Gossypium hirsutum CLRDV has a variable incidence across the U.S. Cotton Belt, but factors underlying this variation are unknown. Field surveys of commercial cotton fields conducted in Alabama and Georgia from 2021 to 2022 allowed for collection of data on A. gossypii, natural enemies, CLRDV weed hosts, CLRDV incidence, landscape composition, temperature, and precipitation. A structural causal modeling analysis using a directed acyclic graph framework was used to test the significance of these factors on CLRDV incidence in cotton while controlling for potentially confounding effects. A. gossypii abundance, prevalence of cotton in the landscape, and temperature had significant and positive effects on CLRDV incidence, whereas other variables were not significant. Temperature had a significant and positive effect on A. gossypii abundance, which likely contributed to the overall effect of temperature on CLRDV incidence. Cotton is a host for both the vector and virus, which may increase the abundance of both. Novel findings from this study identify factors associated with regions at high risk for CLRDV. Future work can leverage these findings to better predict the spatial and temporal variability in disease incidence.

The introduction of cotton leafroll dwarf virus (CLRDV) (Family: Solemoviridae, Genus: Polerovirus) into the southeastern U.S. Cotton Belt has prompted research into understanding the epidemiology, including identification of potential insect vectors. Aphids (Hemiptera: Aphididae) are vectors of cotton-infecting poleroviruses. Of the eight aphid species reported to feed on cotton in the U.S., only Aphis gossypii Glover has been reported to transmit the phloem-limited virus, CLRDV, to cotton. Population dynamics of alatae aphids and additional information regarding CLRDV spread throughout the year may help identify other vector species. Using insect traps to monitor populations of these aphids is more effective for the seven species that are seldom observed, or difficult to sample because they feed underground on roots. However, current identification keys for aphids rely on using apterous colonies already infesting known host plants or are not specific to cotton fields. Here, we present a modified key for identifying cotton infesting aphids captured in pan traps around agricultural fields in the southeastern U.S., including A. gossypii, Protaphis middletonii Thomas, Aphis craccivora Koch, Rhopalosiphum rufiabdominale Sasaki, Macrosiphum euphorbiae Thomas, Myzus persicae Sulzer, Aphis fabae Scopoli, and Smythurodes betae Westwood. Pictures are provided to aid in the identification of these species.

Thrips (Thysanoptera: Thripidae) can injure seedling cotton (Gossypium hirsutum L.), soybean (Glycine max (L.) Merr.), and peanut (Arachis hypogaea L.) crops in the southern United States. The planting window and timing of thrips infestations into these crops overlap in the region, but thrips preference for oviposition has not been investigated. We evaluated thrips preference for cotton, soybean, and peanut by counting eggs, immatures, and adults at the cotyledon to 4 true leaf stages. Peanut was significantly more attractive for oviposition than cotton and soybean. Oviposition in cotton was significantly higher in the cotyledons than the other leaves. The highest oviposition in soybeans also occurred in the cotyledons but differed only significantly from the fourth true leaf. In all crops, there was no significant difference among oviposition in the true leaves. The highest number of immatures were found on cotton, followed by peanut and then soybean, while adults were evenly distributed among crops. Our results suggest that while peanut is preferred for oviposition, this crop may not facilitate immature development and survival as effectively when compared with cotton. This study presents an initial examination of crop selection by thrips under field conditions and suggests peanut may be the preferred oviposition host.

Cotton leafroll dwarf virus (CLRDV) is an emerging aphid-borne pathogen infecting cotton, Gossypium hirsutum L., in the southern United States (U.S.). The cotton aphid, Aphis gossypii Glover, infests cotton annually and is the only known vector to transmit CLRDV to cotton. Seven other species have been reported to feed on, but not often infest, cotton: Protaphis middletonii Thomas, Aphis craccivora Koch, Aphis fabae Scopoli, Macrosiphum euphorbiae Thomas, Myzus persicae Sulzer, Rhopalosiphum rufiabdominale Sasaki, and Smynthurodes betae Westwood. These seven have not been studied in cotton, but due to their potential epidemiological importance, an understanding of the intra- and inter-annual variations of these species is needed. In 2020 and 2021, aphids were monitored from North Carolina to Texas using pan traps around cotton fields. All of the species known to infest cotton, excluding A. fabae, were detected in this study. Protaphis middletonii and A. gossypii were the most abundant species identified. The five other species of aphids captured were consistently low throughout the study and, with the exception of R. rufiabdominale, were not detected at all locations. The abundance, distribution, and seasonal dynamics of cotton-infesting aphids across the southern U.S. are discussed.

Cotton leafroll dwarf virus (CLRDV) is an introduced Polerovirus (Family: Solemoviridae) of cotton, Gossypium hirsutum L., in the U.S. The only vector known to transmit this virus to cotton is the cotton aphid, Aphis gossypii Glover; however, there are seven other species of aphids (Hemiptera: Aphididae) reported to colonize cotton in the southeastern U.S.: Protaphis middletonii (Thomas), Rhopalosiphum rufiabdominale (Sasaki), Aphis craccivora Koch, Macrosiphum euphorbiae Thomas, Myzus persicae (Sulzer), Smythurodes betae Westwood, and Aphis fabae Scopoli. Little to no information is available on annual population dynamics of these species in the southeastern U.S. The timing of CLRDV spread to cotton plantings is also unknown. The objective of this study was to monitor the population dynamics of eight cotton-feeding aphid species concurrent with the spread of CLRDV at three different locations in Alabama. Aphids were monitored weekly for two years with yellow pan traps, and sentinel plants were deployed weekly to monitor CLRDV spread throughout the cotton-growing season. During the two years, most CLRDV spread at all locations occurred when A. gossypii was actively dispersing in the field. Early season spread at sites in south and central Alabama, when A. gossypii was not abundant, suggests additional aphid vectors are possible.

Cotton leafroll dwarf virus (CLRDV) is a yield-limiting, aphid-transmitted virus that was identified in cotton, Gossypium hirsutum L., in the United States of America in 2017. CLRDV is currently classified in the genus Polerovirus, family Solemoviridae. Although 8 species of aphids (Hemiptera: Aphididae) are reported to infest cotton, Aphis gossypii Glover is the only known vector of CLRDV to this crop. Aphis gossypii transmits CLRDV in a persistent and nonpropagative manner, but acquisition and retention times have only been partially characterized in Brazil. The main objectives of this study were to characterize the acquisition access period, the inoculation access period, and retention times for a U.S. strain of CLRDV and A. gossypii population. A sub-objective was to test the vector competence of Myzus persicae Sulzer and Aphis craccivora Koch. In our study, A. gossypii apterous and alate morphs were able to acquire CLRDV in 30 min and 24 h, inoculate CLRDV in 45 and 15 min, and retain CLRDV for 15 and 23 days, respectively. Neither M. persicae nor A. craccivora acquired or transmitted CLRDV to cotton.