Daniel C. Moon

Quality or quantity: the direct and indirect effects of host plants on herbivores and their natural enemies

Resource quality (plant nitrogen) and resource quantity (plant density) have often been argued to be among the most important factors influencing herbivore densities. A difficulty inherent in the studies that manipulate resource quality, by changing nutrient levels, is that resource quantity can be influenced simultaneously, i.e. fertilized plants grow more. In this study we disentangled the potentially confounding effects of plant quality and quantity on herbivore trophic dynamics by separately manipulating nutrients and plant density, while simultaneously reducing pressure from natural enemies (parasitoids) in a fully factorial design. Plant quality of the sea oxeye daisy, Borrichia frutescens, a common coastal species in Florida, was manipulated by adding nitrogen fertilizer to increase and sugar to decrease available nitrogen. Plant density was manipulated by pulling by hand 25 or 50% of Borrichia stems on each plot. Because our main focal herbivore was a gall making fly, Asphondylia borrichiae, which attacks only the apical meristems of plants, manipulating plant nitrogen levels was a convenient and reliable way to change plant quality without impacting quantity because fertilizer and sugar altered plant nitrogen content but not plant density. Our other focal herbivore was a sap-sucker, Pissonotus quadripustulatus, which taps the main veins of leaves. Parasitism of both herbivores was reduced via yellow sticky traps that caught hymenopteran parasitoids. Plant quality significantly affected the per stem density of both herbivores, with fertilization increasing, and sugar decreasing the densities of the two species but stem density manipulations had no significant effects. Parasitoid removal significantly increased the densities of both herbivores. Top-down manipulations resulted in a trophic cascade, as the density of Borrichia stems decreased significantly on parasitoid removal plots. This is because reduced parasitism increases gall density and galls can kill plant stems. In this system, plant quality and natural enemies impact per stem herbivore population densities but plant density does not.

THE EFFECTS OF SALINITY AND NUTRIENTS ON A TRITROPHIC SALT-MARSH SYSTEM

In recent years, ecologists have begun to develop a more fine-grained and integrative approach to examining the importance of top-down and bottom-up effects on herbivore populations by investigating how changes in abiotic heterogeneity affect the relative roles of these forces. We performed two factorial field experiments to determine how increasing salinity and nutrient supply affected the relative strengths of top-down and bottom-up forces among the gall-making midge, Asphondylia borrichiae, its host plant Borrichia frutescens, and a suite of parasitoids. Salinity was increased by the addition of salt pellets, and nutrient supply was increased by the addition of fertilizer. In both experiments, parasitism pressure was decreased by trapping hymenopteran parasitoids with yellow sticky traps. In both experiments, bottom-up manipulations had significant effects on gall density. Elevated salinity levels decreased the number of galls per 200 Borrichia stems, and fertilization increased the number of galls. In...

Relative importance of abiotically induced direct and indirect effects on a salt-marsh herbivore.

Few studies have attempted to determine experimentally the relative im- portance of direct and indirect effects of host plants on herbivorous insects in the field. This study identifies important direct and indirect effects of a coastal plant on its most common insect herbivore and assesses the relative importance of those effects. The direct effects of increased interstitial soil salinity and nitrogen on the abundance of Pissonotus quadripustulatus (Homoptera: Delphacidae), which feeds on Borrichia frutescens (Aster- aceae), are reported. Also reported are the indirect effects of these treatments on parasitism of P. quadripustulatus eggs laid in plant stems by the fairyfly parasitoid Anagrus sp. nr. armatus (Hymenoptera: Mymaridae). Soil salinity was experimentally elevated by the ad- dition of salt pellets, and plant foliar nitrogen was increased by the addition of fertilizer. Both salt and fertilizer increased the abundance of P. quadripustulatus. There was a significant salt X fertilizer interaction, suggesting that salt stress may be more important when plants are more nitrogen limited. Salt, by increasing the frequency of tough B. fru- tescens stems, decreased the rate at which Anagrus parasitized P. quadripustulatus eggs. Fertilizer, by increasing the frequency of B. frutescens stems that were softer and easier to penetrate, increased the rate of parasitism. Changes in parasitism, however, did not affect P. quadripustulatus density. Tests for density dependence showed that the results reported here were attributable to application of the treatments and not to spatial density dependence. This study suggests that, in a stressful salt-marsh system, the direct effects of plant quality on herbivores are more important than indirect effects of plant morphology mediated by natural enemies.

The effects of abiotically induced changes in host plant quality (and morphology) on a salt marsh planthopper and its parasitoid

1. In this study, the effects of shading and fertilisation on the density of the salt marsh planthopper Pissonotus quadripustulatus (Homoptera: Delphacidae), which is monophagous on Borrichia frutescens (Asteraceae), were assessed. The effects of these abiotic factors on the rate at which Pissonotus eggs were parasitised by Anagrus sp. nr armatus (Hymenoptera: Mymaridae) were also examined.

Effects of elevated CO2 on foliar quality and herbivore damage in a scrub oak ecosystem

Atmospheric CO2 concentrations have increased exponentially over the last century and continuing increases are expected to have significant effects on ecosystems. We investigated the interactions among atmospheric CO2, foliar quality, and herbivory within a scrub oak community at the Kennedy Space Center, Florida. Sixteen plots of open-top chambers were followed; eight of which were exposed to ambient levels of CO2 (350 ppm), and eight of which were exposed to elevated levels of CO2 (700 ppm). We focused on three oak species, Quercus geminata, Quercus myrtifolia, Quercus chapmanii, and one nitrogen fixing legume, Galactia elliottii. There were declines in overall nitrogen and increases in C:N ratios under elevated CO2. Total carbon, phenolics (condensed tannins, hydrolyzable tannins, total phenolics) and fiber (cellulose, hemicellulose, lignin) did not change under elevated CO2 across plant species. Plant species differed in their relative foliar chemistries over time, however, the only consistent differences were higher nitrogen concentrations and lower C:N ratios in the nitrogen fixer when compared to the oak species. Under elevated CO2, damage by herbivores decreased for four of the six insect groups investigated. The overall declines in both foliar quality and herbivory under elevated CO2 treatments suggest that damage to plants may decline as atmospheric CO2 levels continue to rise.

THE INFLUENCE OF A SALINITY AND NUTRIENT GRADIENT ON COASTAL VS. UPLAND TRITROPHIC COMPLEXES

We examined the relative strengths of top-down and bottom-up effects acting on insect herbivores feeding on five species of plants growing in coastal or upland regions of a Florida salt marsh. Three of the plant species, Borrichia frutescens, Coccoloba uvifera, and Baccharis halimifolia, grew close to the shore, while the two remaining species, Myrica cerifera and Persea borbonia, grew in upland areas with lower salinity and no tidal input. For each species, we added nitrogen fertilizer and reduced parasitism with sticky traps in a fully factorial design, and assessed the relative impacts of these bottom-up and top-down manipulations on the most common herbivores of each plant. We then used a within-study meta-analysis to integrate the results and quantitatively compare the strengths of top-down and bottom-up effects on the herbivores of these coastal and upland plants. Both fertilization and reduction of parasitism resulted in significant increases in herbivore densities, and the effects of these treatme...

PROTECTING RARE FLORIDA CACTI FROM ATTACK BY THE EXOTIC CACTUS MOTH, CACTOBLASTIS CACTORUM (LEPIDOPTERA: PYRALIDAE)

Cactoblastis cactorum (Berg) represents a threat to rare Opuntia cacti in the Florida Keys. Conservation of such species may depend upon successful outplantings of young cacti in places that minimize attack rates by Cactoblastis. This paper discusses how to maximize the success rates of such outplantings of the endangered semaphore cactus, O. corallicola Small. A 1998 outplanting of 180 cacti in the Lower Keys showed that planting close to Opuntia stricta (Haw.) Haworth infected with Cactoblastis results in heavy losses, as Cactoblastis bleed over from O. stricta to attack outplanted O. corallicola. Growth rates of outplanted O. corallicola are greatest in shade conditions, but attack rates by Cactoblastis are also greater in the shade. An outplanting of 240 O. corallicola cacti on six different Keys in 2000, all far away from O. stricta, had no Cactoblastis related mortality. The most successful conservation strategy for O. corallicola thus appears to be outplanting in the shade, possibly in tropical hammocks, far away from Opuntia cacti that might contain Cactoblastis.

Experimental tests of trophic dynamics: taking a closer look

Abstract The relative importance of bottom-up versus top-down forces, and the effect of productivity on community dynamics continue to be of much interest to ecologists. Trophic dynamic theories are difficult to test, as they require explicit knowledge of the many organisms involved, as well as the nature of the interactions between them. The Oksanen-Fretwell (OF) theory, which suggests that the relative roles of top-down and bottom-up factors vary with primary productivity, is well known in the literature, but is difficult to test rigorously. Recently, two experimental studies have tried to test OF theory. In this paper we discuss methodological problems associated with these studies that may weaken the conclusions drawn by the authors.

Non-random distribution among a guild of parasitoids: implications for community structure and host survival

Abstract 1. Immature stages of the gall midge, Asphondylia borrichiae, are attacked by four species of parasitoids, which vary in size and relative abundance within patches of the gall midge’s primary host plant, sea oxeye daisy (Borrichia frutescens).

Interactive effects of mycorrhizal fungi, salt stress, and competition on the herbivores of Baccharis halimifolia

1. Plant stress and association with mycorrhizal fungi have been shown to significantly influence plant quality, yet their roles in influencing plant–insect interactions remain unclear. Even less is known about how these factors might interact with or be modified by within‐trophic level interactions.