SPECIES COMPOSITION AND ACTIVITY OF THE NATURAL ENEMIES OF SUGARCANE APHID (Melanaphis sacchari) ON SORGHUM A Thesis by ERIN LYNETTE MAXSON Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Chair of Committee, James B. Woolley Co-Chair of Committee, Michael Brewer Committee Member, William Rooney Head of Department, David Ragsdale December 2017 Major Subject: Entomology Copyright 2017 Erin Maxson ABSTRACT The sugarcane aphid (Melanaphis sacchari) is an emergent sorghum pest in the southern United States. The objectives of this study were to identify the natural enemy species that are feeding on the aphid in grain sorghum in Texas, track seasonal changes in aphid and natural enemy populations across sorghum hybrids that have differing levels of susceptibility to the aphid, and measure aphid suppression by natural enemies of different size classes. Aphids and natural enemies were sampled on multiple sorghum hybrids at two field locations in south and central Texas over two years. Additionally, aphid suppression by natural enemies of two size classes was evaluated using exclusion cages. Aphids and natural enemies in both locations showed a trend of greater peak abundance on relatively more aphid-susceptible hybrids. At least 19 natural enemy species were present, consisting of parasitoids (Aphelinus sp. and Lysiphlebus testaceipes), lady beetles (Coccinellidae), hoverflies (Syrphidae), lacewings (Chrysopidae and Hemerobiidae), and minute pirate bugs (Anthocoridae). Aphelinus was heavily hyperparasitized by Syrphophagus aphidivorus. Aphelinus and Coccinellidae, the numerically dominant taxa, maintained high activity on resistant sorghum for a longer period than on susceptible sorghum. Natural enemy densities were similarly proportionate to aphid densities on both aphid-susceptible and aphid- resistant plants. In the cage experiment, small lady beetles (Coccinellidae: Scymninae) were the only natural enemies to differ significantly in density between treatments. Scymninae density and Scymninae per aphid were greatest in closed standard mesh ii cages. Scymninae density was greater on closed and open standard mesh than on fine mesh. Scymninae per aphid was higher on the resistant x susceptible hybrid. However, aphid density did not differ between cages or hybrids; therefore, aphid suppression was not detected. Problems and potential solutions involving cage integrity are discussed. Overall, the existing community of aphidophagous insects in Texas has responded rapidly to the sugarcane aphid’s introduction. Continuity in species composition and population trends across both counties and years indicates that the observed response is not an isolated event. The natural enemy complex was observed on both susceptible and resistant hybrids, suggesting that local natural enemies of the sugarcane aphid can complement host plant resistance. iii DEDICATION To my parents. iv ACKNOWLEDGEMENTS I thank Professor James Woolley, Jewel Coffey, Xanthe Shirley, and the other current and past members of the Woolley lab of the Department of Entomology for supporting my research from start to finish by sharing with me their skills, knowledge, and advice. I also wish to thank Rebecca Hapes of the Department of Entomology for always being willing and able to answer my questions about administrative forms and procedures. I am grateful to Darwin Anderson, Robert Bowling, Cesar Valencia, Gregory Sword, and everyone else from Texas A&M University and Texas A&M AgriLife Research who assisted with sorghum acquisition, cultivation, and management. Special thanks are due to William Rooney, who provided all germplasm of sorghum hybrid lines. I am indebted to the experienced workers at the Texas A&M University Farm, Somerville, TX, who miraculously retrieved my vehicle from the mud after a very rainy day of field sampling. v CONTRIBUTORS AND FUNDING SOURCES The thesis committee that supervised this research was composed of James Woolley (advisor and committee chair) and Michael Brewer (co-advisor and co-chair) of the Department of Entomology and William Rooney of the Department of Crop and Soil Sciences. Michael Brewer of the Department of Entomology ran the statistical analyses discussed in sections 2 and 3 using the field sampling data the student provided. Jewel Coffey, the technician of the Woolley lab, Department of Entomology, assisted with imaging and critical point drying of voucher specimens. Species identifications were provided in part by John Oswald and Ed Riley of the Department of Entomology, and Stephen Gaimari and Martin Hauser (California Department of Food and Agriculture). Research conducted at the Nueces County field site was enabled by Michael Brewer, Darwin Anderson, Robert Bowling, and other researchers, staff, and students working at the Texas A&M AgriLife Station in Corpus Christi, TX. The AgriLife crew were responsible for acquiring sorghum germplasm, arranging field plots, seed planting, irrigation, and all other aspects of crop cultivation in Corpus Christi. With guidance from the author, the crew also assisted with some of the weekly sampling, especially in 2016. Research at the Burleson County field site was enabled by Gregory Sword and Cesar Valencia of the Department of Entomology and William Rooney of the vi Department of Crop and Soil Sciences, who were responsible for sorghum acquisition and cultivation. Construction, installation, infestation, and removal of exclusion cages (see section 3) was made possible with the help of James Woolley, Michael Brewer, Jewel Coffey, Xanthe Shirley, and Leo Deleon. All other work conducted for this thesis was completed independently by the student. This includes, but is not limited to, all insect photographs appearing in Figs. 3-7 that are not otherwise credited. This graduate research was supported by an Excellence Fellowship FY 2016 awarded by the Texas A&M College of Agriculture & Life Sciences. This work was also funded in part by a USDA-NIFA-CPPM-ARDP grant awarded to Michael Brewer and James Woolley under grant number 2014- 700622528. David Ragsdale and the Texas A&M Department of Entomology provided additional funding support for off-campus field research in 2015. The contents of this document are solely the responsibility of the author and do not necessarily represent the official views of the USDA, the Department of Entomology, the Department of Agriculture & Life Sciences, or Texas A&M University. vii NOMENCLATURE Aphid = all uses of this word in the text and figures of this document refer specifically to the sugarcane aphid, Melanaphis sacchari, except where stated otherwise Alatae = winged (alate) adult aphids Apterae = wingless (apterous) adult aphids Mummy = the dried, discolored husk of an aphid that has been killed by a parasitoid wasp larva, which the wasp uses as a pupation chamber R; Res = a sorghum hybrid with partial resistance or tolerance to the sugarcane aphid S; Sus = a sorghum hybrid susceptible to the sugarcane aphid RxS = a sorghum hybrid obtained by crossing a maternal partially sugarcane-aphid- resistant inbred line with a paternal sugarcane-aphid-susceptible inbred line RxR = a sorghum hybrid obtained by crossing two partially sugarcane aphid resistant or tolerant inbred lines SxR = a sorghum hybrid obtained by crossing a maternal sugarcane-aphid- susceptible inbred line with a paternal partially sugarcane-aphid-resistant inbred line SxS = a sorghum hybrid obtained by crossing two sugarcane aphid susceptible inbred lines viii TABLE OF CONTENTS Page ABSTRACT .......................................................................................................... ii DEDICATION ....................................................................................................... iv ACKNOWLEDGEMENTS ..................................................................................... v CONTRIBUTORS AND FUNDING SOURCES .................................................. vi NOMENCLATURE............................................................................................... viii TABLE OF CONTENTS....................................................................................... ix LIST OF FIGURES................................................................................................ .xi LIST OF TABLES.................................................................................................. .xiv 1. INTRODUCTION AND LITERATURE REVIEW............................................... 1 2. SPECIES COMPOSITION AND SEASONAL ABUNDANCE OF THE NATURAL ENEMIES OF THE SUGARCANE APHID (Melanaphis sacchari) ON SORGHUM ........................................................... 8 2.1. Overview.................................................................................................. 8... 2.2. Introduction.............................................................................................. 9... 2.3. Methods................................................................................................... 14.. 2.3.1. Field Locations..................................................................................14.. 2.3.2. Field Plot Design............................................................................. 15.. 2.3.3. Data Collection................................................................................ 16.. 2.3.4. Laboratory Rearing of Natural Enemies.......................................... 18.. 2.3.5. Statistical