Paul C. Marino

Effect of Landscape Structure on Parasitoid Diversity and Parasitism in Agroecosystems

The armyworm (Pseudaletia unipuncta) was used as a model host insect to explore the influence of agricultural landscape structure at two spatial scales on larval parasitoid species richness and rates of larval parasitism in southern Michigan. First, within fields, we compared parasitoid communities in maize fields near, and distant from, a hedge- row edge. Second, we replicated these studies within a complex landscape (agricultural fields of small size embedded in a landscape with abundant hedgerows and woodlots) vs. a simple landscape (agricultural fields of large size embedded in a landscape with few hedgerows and woodlots). The structural differences between the simple and complex agricultural landscapes were characterized by analysis of aerial photographs and digital land-use data. After landscape analysis, three maize fields from each area were selected for the experimental studies. The complex landscape contained fields that were 75% smaller, had 63% more perimeter of wooded field edge per hectare of field area, and had 81% more field edge in wide hedgerow than fields in the simple landscape. Fields in the simple landscape had 74% and 53% more field edge in herbaceous roadside and crop-to-crop interfaces, respectively, than did fields in the complex landscape. In the six selected maize fields, third and fifth instar P. unipuncta were released indi- vidually onto maize plants 5 m and 90 m from a hedgerow edge. Larvae were recovered after 4-5 d and reared in the laboratory to record parasitoid emergence. Parasitoid species diversity was similar in both landscape types (simple landscape: four species; complex landscape: five species). Mean percentage parasitism was significantly higher in the complex landscape than in the simple landscape (13. 1% vs. 2.4%) but was not affected by the location within fields (near hedgerows vs. distant from hedgerows) in either landscape type.

Spatial scale mediates the influence of habitat fragmentation on dispersal success: Implications for conservation

Abstract Increasingly, conservationists are seeking insights from ecological theory to choose strategies of habitat management that will best maintain threatened species. Often, these questions revolve around ways of mitigating the dangers posed by habitat fragmentation. Problems involving the scale of both animal movement and spatial heterogeneity inexorably arise when assessing the effects of fragmentation. We present results from a simple spatial model that simulates the dispersal of animals in a landscape of stochastically clustered habitat fragments. Varying the number of clusters and the spatial scale at which clustering occurs illustrates that heterogeneity has different and conflicting effects on animal movement when it occurs at different scales. Indeed, the scale of clustering is the most important feature in determining disperser performance in our model. Seeking to compare our modeling results with actual data, we review empirical studies of fragmented populations and habitats. Surprisingly, we conclude that very few studies have addressed the mechanisms by which fragmentation will influence population dynamics or, in particular, the ways in which spatial scale mediate these effects. We conclude that the explicit consideration of scale is essential in discussions of habitat fragmentation and of optimal conservation strategies.

DOES AGRICULTURAL LANDSCAPE STRUCTURE AFFECT PARASITISM AND PARASITOID DIVERSITY

This study evaluates whether previous observations of a higher percentage of parasitism and parasitoid diversity in a complex agricultural landscape, relative to a simple landscape, represent a general phenomenon. Rates of parasitism and parasitoid diversity of the armyworm (Pseudaletia unipuncta) were assessed in three replicate (On- ondaga, Ingham, and Benton) regions in southern Michigan. Within each region, a simple landscape (primarily cropland) and a complex landscape (cropland intermixed with mid and late successional noncrop habitats) were identified through analysis of aerial photo- graphs. In each landscape, three maize fields were selected, and second to fourth instar P. unipuncta were released into three replicate plots of maize plants. Larvae were recovered after 6 d and reared in the laboratory to record parasitoid emergence. A principal component analysis conducted on landscape variables indicated that simple and complex landscapes were true replicates. Extra-field vegetation was similar among the three simple landscapes but differed among complex sites. Parasitoid species diversity differed among regions, with six species recovered in Onondaga and two species from both Benton and Ingham. Rates of parasitism were only partially consistent with previous experimental results. In Ingham, results were similar to those found previously in this region; rates of parasitism and par- asitoid diversity were higher in the complex landscape. However, in Onondaga and Benton, no difference in rates of parasitism or parasitoid diversity were found. Thus, the hypothesis that landscape complexity increases parasitoid diversity and rates of parasitism was not supported. Possible reasons for the observed differences in rates of parasitism among regions are discussed.

Post-dispersal weed seed predation in Michigan crop fields as a function of agricultural landscape structure.

Weed seed predation by invertebrates and vertebrates was compared between a simple (large crop fields embedded in a matrix of widely scattered woodlots and hedgerows) and a complex (small crop fields embedded in a matrix of numerous hedgerows and woodlots) agricultural landscape in southern Michigan. The structural differences between landscapes were evaluated by analysis of aerial photographs and digital land-use data. Seed predation experiments were conducted in four conventional tillage corn (Zea mays L.) fields within each landscape type. Trials included four common agricultural weed species, i.e., crabgrass (Digitaria sanguinalis), giant foxtail (Setaria faberii), pigweed (Amaranthus retroflexus), and velvetleaf (Abutilon theophrasti). Treatments to exclude vertebrates, invertebrates + vertebrates and no exclusion were established at 27 m from hedgerows. Fields in the complex landscape were 75% smaller, had 63% more wooded perimeter, and 81% more wide hedgerow perimeter than fields in the simple landscape. Fields in the simple landscape were surrounded mainly by herbaceous roadside and crops, whereas the complex landscape had fields surrounded primarily by wide hedgerows. In both the landscape types there was considerable post-dispersal weed seed removal with a tendency towards higher removal rates in the complex landscape. Although there were no differences in the rate of seed removal among the four weed species, seed predation showed a high degree of variability within and among fields.

The colonization of native phytophagous insects in North America by exotic parasitoids

Abstract Classical biological control could have a major environmental cost if introduced natural enemies colonize and disrupt native systems. Although quantifying these impacts is difficult for systems already colonized by natural enemies, the a priori condition for such impacts can be evaluated based on the extent to which exotics have acquired native hosts. We use native host records for exotic parasitoids introduced into North America for biological control to document the number of exotic species that have been recorded from at least one native insect species. We also evaluate the ability of six biological and ecological variables to predict whether or not a parasitoid will move onto natives. Sixteen percent of 313 parasitoid species introduced against holometabolous pests are known from natives. Further, the likelihood that a parasitoid had colonized native hosts was largely unpredictable with respect to the independent variables. We conclude that given the quality of the data available either now or in the foreseeable future, coupled with inherent stochasticity in host shifts by parasitoids, there are no rules of thumb to assist biological control workers in evaluating if an introduced parasitoid will colonize native insect communities.

Temporal variation in the response of parasitoids to agricultural landscape structure

Temporal variations in the relationship between agricultural landscape complexity and parasitoid abundance were assessed in Ingham County, MI. The study site consisted of a 3. 2k m× 13.9 km area including a complex and a simplified agricultural landscape. Landscape structure was quantified using black and white aerial photographs and digital land-use data. A heterogeneous mixture of crop and non-crop habitats characterized the complex landscape (40.6% of non-crop habitat), while the simple landscape was more uniform and contained less non-crop habitat (28.6%). In the complex landscape fields were 75% smaller, had 63% more perimeter of wooded field edge per hectare of field area, and 81% more field edge in wide hedgerow than in the simple landscape. In June–July 1993, 1996, 1998, 2000, and 2001, a total of 15,786 armyworm (Pseudaletia unipuncta) larvae were released into maize fields. A total of 3752 larvae were recovered and reared in the laboratory with 1410 individuals parasitized by 15 identified species of parasitoids. Two species, Glyptapanteles militaris and Meteorus communis, represented 96.3% of the parasitoids recovered. Agricultural landscape structure influenced the temporal dynamics of P. unipunctaparasitism. While G. militaris was equally present in the simple (48.8% of total recovery) and complex landscape (51.2%), M. communiswas found mostly in the complex landscape (78.9%). Overall, percentage parasitism differed between landscapes and among years with different trends in the simple and complex landscapes. These results indicate that effects of landscape structure on parasitism are not adequately characterized by short-term studies.

Influence of sunlight and soil nutrients on clonal growth and sexual reproduction of the understory perennial herb Sanguinaria canadensis L.1

ation. Also, half the plots in each site were fertilized to explore effects of soil fertility on vegetative growth and sexual reproduction. Plants from a third high-light site were used in a potting study to explore combined influences of elevated sunlight and fertilization on vegetative growth and sexual reproduction. Fertilization resulted in plants with proportionally fewer small and more large leaves in high-light permanent plots and had no effect on plants growing in low-light permanent plots. Fertilization increased leaf number and total leaf area, but had no influence on seed production in the potting and reciprocal transplant studies. Unshaded plants had significantly more leaves and greater total leaf area in the potting study. Also, in the high-light site of the reciprocal transplant study, both fertilized and unfertilized plants (especially fertilized plants originating from the high-light site) had more leaves and a higher total leaf area per plant than plants growing in the low-light site. Light intensity did not influence seed production in the potting study; however, more seeds were produced on plants growing in the high-light site than in the low-light site. Mortality was high for plants transplanted into the low-light site and for fertilized plants growing in shade in the common garden study. Clonal growth as measured by leaf size and leaf number in S. canadensis is highly plastic and responds vigorously to increased sunlight and to fertilization when under high-light conditions. In contrast, sexual reproduction exhibits relatively little plasticity. We suggest that local populations of S. canadensis rapidly respond to increases in light and nutrients associated with disturbance through increased clonal growth. This rapid response to environmental variability may partly explain the patchy spatial distribution of S. canadensis populations.

Competition between mosses (Splachnaceae) in patchy habitats

The boreal mosses Tetraplodon angustatus, T. mnioides, Splachnum ampullaceum and S. luteum (Splachnaceae) grow on the droppings of large mammals and have their spores dispersed by flies (Diptera). This study was conducted to determine if (i) the interactions between these mosses on droppings (within-patch interactions) and (ii) the influence of habitat and type of dropping on such interactions promote their coexistence. Comparisons of two-species competition experiments with single-species growth experiments in which the mosses were grown from spores on moose droppings in the laboratory and field indicated that the mosses compete for resources (...)

Influence of mound building and selective seed predation by the red imported fire ant (Solenopsis invicta) on an old-field plant assemblage

In seed predation experiments, ragweed seeds were selected less often than those of four other early successional species: pigweed (Amaranthus retroflexus), bluegrass (Poa annua), common lambs quarters (Chenopodium album), and goldenrod (Solidago altissima). Moreover, fire ants selected both larger and smaller seeds than those of ragweed indicating that selection was not driven by relative seed size. There was an increase in the number of ragweed seeds removed by fire ants as the summer progressed, suggesting that more ragweed seeds were incorporated into the diet of fire ants as other species of seeds became less abundant. Lastly, nearly all seeds of pigweed (a preferred species) placed 1, 2, 3 and 4 meters from ant mounds were consumed, which suggests that fire ants may effectively forage the entire field. Both fire ants and ragweed are early successional species that rapidly invade disturbed areas created by humans and natural events. Fire ants significantly contributed to the success of ragweed plants growing in an old field community.

Environmental and clonal influences on host choice and larval survival in a leafmining insect

The larvae of Phytomyza ilicicola (native holly leafminer) generally have higher survivorship and densities on Ilex opaca (American holly) grown in cultured suburban settings than on natural forest trees. Survival and density also vary among trees within these two habitats; natural enemies and host-plant factors are thought to generate between-habitat variation in Phytomyza densities, while mainly host factors generate within-habitat variation. We explored the latter by examining the effects of fertilization, clonal variation, exposure to sunlight and site of exposure to attack (forest or open field) on Phytomyza success using potted holly clones in a common garden setting