Effect of tillage on cotton aphids (Homoptera: Aphididae), pathogenic fungi, and predators in south central Georgia cotton fields

Authors: Marti, Orville; Olson, Dawn

Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2006
Publication Date: 9/1/2007
Citation: Marti, O.G., Olson, D.M. 2007. Effect of tillage on cotton aphids (Homoptera: Aphididae), pathogenic fungi, and predators in south central Georgia cotton fields. Journal of Entomological Science. 42(3):354-367.

Interpretive Summary: Two kinds of fungi that are pathogens of cotton aphids were studied in 6 south Georgia cotton fields in 2003 and 2004 using conservation or conventional tillage methods. Aphids and ants on cotton plants were counted in both years and aphid predators (ladybird beetle larvae, lacewings, and spiders) were counted in 2004. Plant sampling began the first week of June and continued for 16 weeks in 2003 and for 14 weeks in 2004. Numbers of aphids on cotton plants increased until the first week of July and then declined. More aphids were present on plants in 2004 than in 2003 for the first 9 weeks of sampling. More aphids were present on plants under conservation tillage than under conventional tillage only between weeks 3 to 8. One fungus species was detected in aphids in mid-March, before cotton was planted, by monitoring aphids on field margins. Another fungus species was not observed until 18 June and 16 June in 2003 and 2004, respectively. Both fungus pathogens peaked at the same time as the aphid population, and then declined. Ants, primarily the red imported fire ant, occurred in larger numbers in fields under conservation tillage, apparently because of greater disturbance of soil and weed habitat under conventional tillage. In both tillage types, numbers of ladybird beetle larvae peaked early in the season and then declined and lacewing larvae and adults peaked late in the season under conservation tillage. Larger numbers of ladybird beetle larvae were present in fields under conservation tillage. Although there was a peak in number of lacewing larvae late in the season under conservation tillage, overall numbers of lacewing larvae were not different under the two management types. Numbers of spiders increased gradually throughout the growing season, with only slightly higher numbers present under conservation tillage. The effects of different tillage methods were more evident on the aphids and their predators than on the fungal pathogens.

Technical Abstract: Two species of fungi, Neozygites fresenii and Pandora neoaphidis, pathogens of the cotton aphid, Aphis gossypii, were studied in 6 south central Georgia cotton fields under conservation or conventional tillage in 2003 and 2004. Aphids and ants on cotton plants were counted in both years and aphid predators (coccinellids, lacewings, and spiders) were counted in 2004. Plant sampling began the first week of June and continued for 16 weeks in 2003 and for 14 weeks in 2004. Numbers of aphids on cotton plants increased until the first week of July and declined thereafter. More aphids were present on plants in 2004 than in 2003 for the first 9 weeks of sampling but did not differ thereafter. More aphids were present on plants under conservation tillage than under conventional tillage only between weeks 3 to 8. P. neoaphidis was detected in aphids in mid-March by monitoring with pan traps on field margins. N. fresenii, in contrast, was not observed until 18 June and 16 June in 2003 and 2004, respectively, and peaked simultaneously with the aphid population and then declined. The incidence of P. neoaphidis was less than that of N. fresenii, but peaked simultaneously with it in the first week of July. Ants, primarily the red imported fire ant, Solenopsis invicta, occurred in larger numbers in fields under conservation tillage, apparently because of greater disruption of soil and weed habitat under conventional tillage. In both tillage types, coccinellids peaked early in the season and lacewings late in the season, with larger numbers of coccinellids present in fields under conservation tillage. Although there was a sharp peak in lacewing numbers late in the season under conservation tillage, overall lacewing numbers were not different under the two types of management. Spider numbers increased gradually throughout the growing season, with only slightly higher numbers present under conservation tillage. The effects of different tillage methods were more evident on the aphids and their predators than on the fungal pathogens.