Testing an interference competition hypothesis to explain native lady beetle decline.

Project Investigators: Chelsea Smith and Mary Gardiner

A significant decline in the abundance of the native lady beetle (Coccinellidae), Hippodamia convergens, coincides with the establishment and population increase of exotic lady beetles in Ohio. This pattern has lead to the hypothesis that intraguild predation by exotic lady beetles explains this decline. Intraguild predation (IGP) is the act of two species, which share a limiting resource, preying on each other. Several laboratory experiments demonstrating the propensity of exotic lady beetles to act as predators of native coccinellid eggs provide support for this hypothesis.

The goal of this research was to determine the extent that exotic lady beetles predate on native lady beetle egg masses in the field. Two studies were conducted over the course of three field seasons (2009-11), the first examines the extent of predation on coccinellid egg masses, and the role of coccinellids as egg predators. The objectives were to 1) compare the extent of egg predation experienced by three coccinellid species: H. convergens, Coleomegilla maculata, and Harmonia axyridis; 2) examine the levels of egg predation occurring across three habitats; and 3) determine the guild of predators responsible for coccinellid egg predation. To address these objectives, egg predation experiments were conducted in habitats where coccinellids are commonly found foraging: grassland, alfalfa, and soybean. Eggs of each focal coccinellid species were placed in the fields for 48 hours. The proportion of eggs remaining was compared among lady beetle species and the habitats. Egg masses from the increasingly-rare native lady beetle, H. convergens, incurred significantly greater predation than eggs from the common exotic lady beetle, H. axyridis. Predation of the egg masses from the three species varied across habitats with the greatest amount of predation occurring in grasslands and the least in the alfalfa. These egg predation experiments provided supporting evidence for the IGP hypothesis. Video surveillance systems were placed in the field and focused on coccinellid egg masses. These preliminary video experiments provided evidence that exotic coccinellids were not common predators of the egg masses.

The second study consisted of video experiments conducted to determine patterns among the guild of predators contributing to coccinellid egg predation. The objectives of this study were to 1) measure the relative abundance and activity density of coccinellid egg predators present within grasslands, alfalfa and soybean fields across Ohio; 2) document the contribution of predator species to native and exotic lady beetle egg predation within each foraging habitat; and 3) determine if the relative abundance of aphids affects the intensity of egg predation experienced by lady beetle egg masses. To address these objectives, video surveillance systems were used to observe predation of H. convergens and H. axyridis egg masses in soybean, alfalfa, and grassland habitats. The relative abundance and activity density of aphids and egg predators was also determined using quadrat sampling, sweep samples, and pitfall traps. From the video observations, the guild of egg predators detected included Stylommatophora, Opiliones, Oniscidea, Coccinellidae, Gryllidae, Neuroptera, Tettigoniidae, Acrididae, Formicidae, Nabidae, Thripidae, Syrphidae, Araneae, Staphylinidae, and Diplopoda. This guild varied in diversity across the habitats, with the greatest diversity found within grassland habitats. Redundancy analysis revealed two egg predators that maintained a constant pattern of predation across both 2010 and 2011: Formicidae and Oniscidea. These findings indicate that exotic lady beetles are not a significant predator of native coccinellid egg masses within the systems studied.

Completed: 2012