Date of Award

Summer 2019

Project Type

Thesis

Program or Major

Biological Sciences

Degree Name

Master of Science

First Advisor

Rebecca G Sideman

Second Advisor

Rebecca G Sidemand

Third Advisor

Iago Hale

Abstract

Growers across the globe and in Northeastern United States have reported economically damaging populations of cabbage aphid (Brevicoryne brassicae) in Brussels sprouts (Brassicacea). The pest affects the Brassicacea family, which includes economically important agronomic, forage, horticultural, and forage crops. However, cabbage aphid management in certified organic systems is very challenging because tools are limited and reports evaluating efficacy of insecticides used in organic systems are sparse. There are natural insect predators and parasitoids of aphids that may serve as biological control agents whose populations can be augmented using insectary plants. Use of alyssum insectary intercropping has been successful for control of this pest in California but use of this practice is untested in the Northeast. Our research aimed to find an integrated approach to managing cabbage aphid on Brussels sprouts by using chemical and biological pest management strategies in conjunction. Our overall goal was to explore the relationship between Brussels sprout and cabbage aphid in organic agroecosystems, with three specific objectives.

Our first objective was to evaluate three organic insecticides for their efficacy in controlling cabbage aphid. We compared Azera (azadirachtin and pyrethrins), AzaGuard (azadirachtin), and M-pede (Potassium salts of fatty acids) against an untreated control. In 2016, a rotation of M-Pede and Azera provided moderate control of cabbage aphid with significantly more marketable clean (aphid-free) sprouts as compared to the unsprayed control. In 2017, Azera and AzaGuard treatments had significantly less aphids on 28 Sept 2017 than the control and M-Pede treatments, but by final harvest there were no significant differences between treatments. Aphid numbers continued to rise in all treatments into September 2017 until a spontaneous epizootic resulted in a collapse of aphid numbers. In both 2017 and 2018 two different entomopathogenic (insect-attacking) fungi were identified on cabbage aphid. Based on these observations, we conclude that Azera and AzaGuard insecticides may provide moderate control of cabbage aphid and further investigation is needed on years without fungal epizootics.

Our second objective was to evaluate seven species of insectary plants in the field for their ability to attract predators and parasitoids of cabbage aphids. Insectary plants were observed over ten sample dates (July through October) for hoverfly visitation and sweepnet samples were taken for hoverfly species identification. Alyssum, buckwheat, cilantro, and dill had greater hoverfly densities than calendula, phacelia, and fennel. Alyssum was found to be a low maintenance plant that hosts the most prevalent aphid-eating hoverfly species (Toxomerus marginatus) from July until frost.

Our third objective was to determine whether parasitism of the cabbage aphid varied with proximity to insectary plants. We used sticky traps and visual observation on Brussels sprouts leaves to count aphids, predators, and parasites at distances ranging from four to 107 feet away. In 2017, we found that predation and parasitism rates were not significantly different at distances ranging from four to 107 feet from the insectary plants. In 2018, aphid populations were not high enough to repeat the experiment. From 2017 results, we concluded that proximity of insectary plants from the Brussels sprout crop did not correlate with predation or parasitism in distances observed in our study.

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