Author ORCID Identifier

https://orcid.org/0000-0002-8883-3634

Year of Publication

2020

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Entomology

First Advisor

Dr. John Obrycki

Second Advisor

Dr. Ric Bessin

Abstract

In 2013, a new aphid pest of sorghum, the sugarcane aphid Melanaphis sacchari Zehntner (Hemiptera: Aphididae), was found in the southern US, and caused significant yield loss in sorghum. Since then, M. sacchari has spread to most of the sorghum growing regions. Unmanaged populations can cause complete crop loss in grain, forage and sweet sorghum. M. sacchari cannot survive winters without a live plant tissue and must recolonize much of its pest range annually. Sweet sorghum (Sorghum bicolor) is an important crop for many growers in Kentucky, producing $16-25 million a year from its syrup. Biological control of M. sacchari may be most effective when aphid densities are low. Several species of natural enemies attack M. sacchari, however, they are generally not in high enough numbers to suppress the pest’s population outbreaks.

Habitat manipulations can be made to improve natural enemy abundance and fitness in the field. Manipulations can be targeted to improve natural enemies, not just during the growing season when they are active, but during their winter dormancy. In addition, damaging levels of M. sacchari can be avoided if the crop is harvested prior to the damaging levels of M. sacchari. Here, I investigated the biological control of M. sacchari by trying to enhance natural enemies prior to M. sacchari outbreaks as well as testing the effects of sweet sorghum planting date to avoid the large and damaging populations of M. sacchari at the end of the season.

Overwintering habitat conditions can influence the health and abundance of beneficial insects in the spring, potentially increasing natural enemy abundance in the summer. Overwintering predacious lady beetles had a higher survival and spring reproduction when given access to prey, and to a lesser extent sugar in a two-year combined field and laboratory study. During the early part of the growing season, crop fields often have few resources to maintain natural enemy populations. Providing food in the form of flower nectar from buckwheat and lures emitting herbivore induced plant volatiles to attract natural enemies has increased natural enemy abundance in other crops. Three years of field trials found no effect of either treatment in sweet sorghum on natural enemy abundance or pest suppression. Buckwheat flowers were only attractive within the buckwheat border strips, the increase in natural enemies did not extend to the neighboring sweet sorghum. No evidence of parasitoid attacks was observed on M. sacchari in central Kentucky. Laboratory and greenhouse trials determined two parasitoid species, Aphidius colemani Viereck (Hymenoptera: Braconidae) and Aphidius ervi Haliday suppressed M. sacchari populations and produced viable offspring. Parasitoids can be an important part of aphid biological control and their addition could help suppress M. sacchari populations. Further research is needed to determine how these parasitoids interact with M. sacchari in the field.

In addition to the manipulation of natural enemies, altering planting date can also aid in pest management. Planting earlier in the growing season has been recommended as a means of avoiding the large M. sacchari populations that develop late in the growing season but has not been experimentally tested. Three planting dates, separated by 30 days, were tested over a three-year period. Planting early had a lower density of M. sacchari, it also reduced sweet sorghum yield. Early planting and faster maturing cultivars, however, may still be a viable option for growers who do not use the insecticide available for sweet sorghum. Multiple compatible tactics need to be developed to sustainably manage M. sacchari which act to reduce aphid densities. The diverse projects discussed in this dissertation highlight the necessity of treating crop fields as ecosystems by considering the entire life history of pests and connections to the surrounding environment.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.094

Funding Information

National Institute of Food and Agriculture, U.S. Department of Agriculture, Hatch project accession numbers 1008480 (2015-2020) and 1010996 (2015-2018).

Mercer Supplemental Files.zip (14 kB)
Dissertation Supplemental Files

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