Year

2023

Degree Name

Bachelor of Science (Honours)

Department

School of Earth, Atmospheric and Life Sciences

Advisor(s)

Katarina Mikac

Abstract

Evolution of resistance within insects to pest control has frequently resulted in changes to the organism’s morphotype, including changes in wing shape. By measuring these changes, it is possible to distinguish resistant from non-resistant populations. Geometric morphometrics (GM) quantifies morphological variation within and among populations, it has been used in previous studies to identify changes in morphotype and distinguish between resistant and non-resistant populations of insects. Helicoverpa Zea (corn earworm) is one of the most economically damaging pests for crops across the United States of America. Infestations of H. zea annually migrate from southern USA to the north, causing damage to almost all agricultural crops in its path. Although corn (Zea mays) is the preferred host and thus the most affected by this insect. A common strategy to manage this pest is the use of Bt Corn; genetically modified corn that produces the Bt toxins. Cry proteins (Cry1AF and Cry1B) are the most common to be produced in Bt Corn, but they are considered low dose toxic to H. zea. There are suggestions to include Vip3A by gene pyramiding corn as the combination of Vip and Cry proteins are high dose toxic to H. zea. Overtime, H. zea has evolved resistance to the Cry proteins expressed by Bt Corn making them a no longer effective control mechanism. This thesis the first study to apply the use of GM methods to the species H. zea. The aims of this thesis are to investigate the effect that resistance to Bt Corn has on wing morphology of the H. zea moth, and to determine if these changes in wing shape can be detected using GM methods. A total of 145 H. zea were provided from two locations across the USA, one in South Carolina and another in North Carolina. Moths were from four different treatments of corn; a control containing corn with no Bt toxins (Treatment 1); crops of Bt Corn containing two toxins (Cry1AB and Cry1F) with a structured refuge present (Treatment 2); crops of Bt Corn containing three toxins (Cry1AB, Cry1F, and Vip3A) with a structured refuge present (Treatment 3); crops of seed blended Bt Corn with 80% containing three toxins (Cry1AB, Cry1F, and Vip3A) and 20% not having any toxins (Treatment 5). Left and right forewings of each moth were chemically bleached, and the scales removed. Once wings were slide mounted, photographs were then taken of each wing. Fifteen type I landmarks were identified in the venation pattern of the forewing for the use of GM analyses. Preliminary tests were conducted and determined that results observed were not confounded by measurement error, bilateral asymmetry, allometry, or sexual dimorphism. A Principal Component Analysis (PCA) was conducted to visualise the whole data set, and a Canonical Variate Analysis (CVA) was done to visualise the relationship between wing shape and treatment of corn. Lastly a Discriminant Function Analysis (DFA) was done to determine the ability of wing shape to act as an indicator and biomarker for the different treatments of corn. The results determined that there was significant difference between forewing shape of susceptible and resistant H. zea individuals. The CVA identified that wing shape of moths’ resistant to Vip3A protein is significantly different from the wing shape of moths’ resistant to just the Cry proteins, thereby identifying resistance to the Vip3A protein. Practical field resistance of H. zea populations to the Vip3A protein has only been recorded once before, therefore these findings present a serious concern. If resistance to the Vip3A protein can become established within H. zea populations, it may inhibit future pest management strategies and pose a serious economic threat to future corn production within the USA. The results of the DFA showed that the forewing shape of H. zea can effectively be used as a biomarker, and that GM can be used to monitor the development and spread of resistance to both Cry proteins, and Vip3A proteins. GM methods are much cheaper and requires less expert knowledge than genetic markers, and thus will allow greater access to the monitoring of resistance within the H. zea population to a broader range of professionals.

FoR codes (2020)

310403 Biological adaptation, 410202 Biosecurity science and invasive species ecology

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.