Unintended Effects of Potato Late Blight Fungicides over the Proliferation of Green Peach Aphid, Myzus persicae (Sulzer)

Abstract

Pesticides have brought multiple benefits to humanity, but their use has also caused negative effects to the environment through direct contamination and ecological disruption of many species of organisms. Intensive pesticide use has eliminated natural enemies of the most important agricultural pests. An example of this phenomenon can be found in the United States, specifically in Minnesota and North Dakota’s agricultural fields with an ever-increasing number of green peach aphid (GPA), Myzus persicae (Sulzer) over the last two decades, which correlates with an increased number of viral diseases on potato, Solanum tuberosum (Linnaeus). This is due to the intensive application of foliar fungicides on potato to control early and late blight diseases, Alternaria solani (Sorauer) and Phytophthora infestans (Mont.) DeBary, respectively. Such intensive application has altered the natural balance of a group of entomopathogenic fungi of the order Entomophthorales. At the end of the 1990s, this situation promoted the study on fungicide applications of different chemical composition (i.e., ethylene-bis-dithiocarbamates, EBDCs or dithiocarbamates and chlorothalonil-based fungicides), in different application regimens, and in different agricultural production systems (irrigated or non-irrigated). At the conclusion of this study, all fungicides evaluated, no matter what their chemical composition or application regimen, were detrimental to Entomophthorales. Without foliar fungicides, GPA populations reached 30% mycosis (disease or infection level) by the end of the first 4 weeks after the aphid had arrived to the plant. However, when foliar fungicides were applied to control Late Blight, three things were observed: a reduction of GPA infection rate, a lower aphid mortality rate, and an increase of aphid numbers throughout the growing season. Although all foliar fungicides exhibited a strong fungicidal effect, chlorothalonil-based fungicides were more effective than EBDC-based fungicides in suppressing GPA’s mycosis caused by Entomophthorales. Suppressed mycoses elicited lower aphid population mortality rates, which led to higher apterus (wingless) aphid densities. Higher densities of apterus aphids led to higher productions of winged (alate) aphids. These latter two variables were associated with differential dispersal patterns, where the effectiveness of the applied fungicide varied according to prevalent climatic conditions as it did with the predominant species of entomopathogenic fungi of each growing season. It is clear that the fungicidal effect on the population dynamics of GPA, although unintended since it has not been designed to affect the insect per se, does occur, but via suppression of some of the aphid’s most important parasites, a select group of entomopathogenic fungi.

Ceiba, 2012. Volumen 53(1):7-16