Stephen D. Hendrix

Compensatory Reproduction in a Biennial Herb following Insect Defloration

SummaryThe ability of the biennial herb, Pastinaca sativa L. (wild parsnip), to respond to and compensate for destruction of primary umbel seeds by the larvae of Depressaria pastinacella (Lepidoptera: Oecophoridae) was analyzed by comparing umbel and seed production of damaged and undamaged plants collected from five populations. Plants with a basal stem diameter smaller than 8 mm suffer a reduction in seed set of about 50% when the primary umbel is destroyed but larger plants are able to compensate for loss of primary umbel seeds by increased seed set of tertiary umbels. Depending on plant size, this is due to either an increase in the number of tertiary umbels that reach maturity or an increase in the number of seeds per tertiary umbel. Although seeds of tertiary umbels are significantly smaller than those of primary or secondary umbels, their viability is equivalent to that of secondary seeds and may be greater than that of primary seeds. Characteristics of P. sativas reproduction, such as the long flowering period and the initiation of more umbels than the plant is normally able to bring to maturity, are important to P. sativas ability to compensate for the effects of herbivore damage.

Bumble Bee (Hymenoptera: Apidae) Diversity and Abundance in Tallgrass Prairie Patches: Effects of Local and Landscape Floral Resources

Abstract Conservation of biodiversity recently has focused on the importance of pollinator services for the preservation of threatened plant species in fragmented landscapes such as the tallgrass prairie biome. In this study, we assessed the influence of local and landscape floral resource diversity and abundance on bumble bee (Bombus Latreille) diversity and abundance at eight tallgrass prairie sites in Iowa. We calculated a landscape floral resource index (LRI) for areas within 500- and 700-m radii of each site based on the density and diversity of flowering plant species in polygons of different landscape elements. LRI values, areas of each landscape habitat type, and the diversity and abundance of bumble bee–used flowering ramets at each site were compared with bumble bee diversity and abundance at the site. Bumble bee diversity was best predicted by a model including the LRI of grasslands surrounding sites at 500 m, which explained most of the variance, and the abundance of bumble bee–visited plants at the site. Bumble bee abundance was best predicted by a model including the percent of the landscape in grasslands at the 700-m radius, which explained most of the variance, and the abundance of bumble bee–visited plants at the site. Our results reveal that bumble bee diversity at tallgrass prairie patches is influenced by the availability of resources in the landscape, particularly in grasslands, and to a lesser extent by site characteristics, indicating the importance of spatial scale for creating successful pollinator management plans in fragmented landscapes.

An Evolutionary and Ecological Perspective of the Insect Fauna of Ferns

Analysis of the number and distribution of insects attacking ferns demonstrates that significantly fewer insects than expected utilize ferns as a food source. Also, insects which do attack ferns are not a representative sample of phytophagous insects in general. Analysis of the feeding habits of these insects indicates that few families or even genera of insects have specialized on ferns. Juxtaposition of the evolutionary histories of ferns and phytophagous insect groups fails to adequately explain the anomalies seen, since evolutionarily old groups are no better represented on ferns than younger groups. Differences in morphological complexity between angiosperms and ferns (primarily lack of flowers) probably accounts for a portion of the underutilization of ferns, but chemical defense mechanisms and ecological characteristics such as population size or phenology may be of importance. The present insect fauna of ferns appears to have been derived from insects utilizing woody plants via host-plant switching.

Reactions of Heracleum Lanatum to Floral Herbivory by Depressaria Pastinacella

Heracleum lanatum, an andromonoecious perennial plant, reacted to herbivorous de- struction of its primary (first) umbel by Depressaria pastinacella by an increase in the proportion of hermaphroditic flowers in late-developing umbels and a subsequent increase in seed set on these umbels. However, these reactions were not sufficient to offset completely the damage sustained; control plants produced 40% more seeds than damaged plants and seed biomass was 53% greater. Plant size (measured by basal stem diameter of the flowering stalk) was significantly correlated with total flower production, seed number, and seed biomass in both control and damaged plants. Plant size was also significantly correlated with the number of hermaphroditic flowers in the primary umbel of control plants and secondary umbels of damaged plants. Hermaphroditic flower production by the secondary umbels of control plants was negatively correlated with flower number in these umbels. Pastinaca sativa, another andromonoecious host plant of D. pastinacella, shows the same type of reactions as H. lanatum, but in P. sativa, compensation, at least in terms of seed number, is complete. The difference between the two species in replacement of lost offspring is interpreted in terms of frequency of reproduction. The iteroparous H. lanatum may maximize reproduction in later seasons by increasing vegetative growth rather than by replacing all lost offspring. In contrast, the semelparous P. sativa maximizes replacement of destroyed offspring during its single reproductive bout.

Plant-Herbivore Interactions: Insect Induced Changes in Host Plant Sex Expression and Fecundity

SummaryWe investigated changes in the reproduction of Pastinaea sativa which result in compensatory reproduction when herbivore feeding destroys the first infloresence produced. Late developing infloresences of damaged plants produce more flowers and a greater proportion of hermaphroditic flowers than late developing infloresences of control plants. In damaged plants the proportion of hermaphroditic flowers producing seeds in late developing infloresences also increases. These effects are due to the mechanical damage resulting from larval feeding. The herbivore acts as a mediator in a number of important plant processes affecting fecundity, including expression of sex.

Consequences of a Mixed Reproductive System in the Hog Peanut, Amphicarpaea bracteata (Fabaceae)

SummaryThe forest annual, Amphicarpaea bracteata L. can reproduce via aerial chasmogamous, aerial cleistogamous, and subterranean cleistogamous flowers. Both plant size and light intensity influenced the utilization of the three modes of reproduction. chasmogamous and aerial cleistogamous flower number and the ratio of chasmogamous flowers to the total number of aerial flowers increased with plant size. The latter demonstrated a shift to xenogamy and outbreeding in larger plants. Light intensity indirectly influenced reproductive modes through its infuence on plant size. Seed set by both types of aerial flowers was low and unrelated to plant size. Subterranean seed number and the total dry weight of subterranean seeds per plant increased with size. The subterranean seeds of Amphicarpaea bracteata are thirty-four times larger than the aerial seeds (fresh weight). Under field conditions, subterranean seeds had greater germination after one year than acrial seeds. The plants arising from subterranean seeds were significantly larger and more fecund than those from aerial seeds. Seeds produced by aerial cleistogamous, hand selfpollinated chasmogamous, and naturally pollinated chasmogamous flowers had equivalent germination rates and produced plants of equal size and fecundity. This suggests that the outbred progeny from chasmogamous flowers have no advantage over the inbred progeny from aerial cleistogamous flowers.

Butterfly, bee and forb community composition and cross-taxon incongruence in tallgrass prairie fragments

Pollinators provide an important class of ecological services for crop plants and native species in many ecosystems, including the tallgrass prairie, and their conservation is essential to sustaining prairie remnants. In Iowa these remnants are typically either block-shaped or long, linear strips along transportation routes. In this study we examined differences in the butterfly, bee, and forb community composition in linear and block prairie remnants, determined correlations between species diversity among butterflies, bees and forbs in the 20 prairie remnants sampled, and examined correlations of community similarity among butterflies, bees and forbs. Correspondence analysis showed that distinct communities exist for butterflies and forbs in block versus linear sites and discriminant analysis showed that the bee and forb communities in block and linear sites can be distinguished on the basis of a few species. Diversity of one group was a poor predictor of diversity in another, except for a significant inverse relationship between bees and butterflies. These two pollinator taxa may be responding very differently to microhabitat components within fragmented ecosystems. Our studies show that there need to be differences in conservation strategies for bees and butterflies to maintain both pollinator communities.

Interacting effects of pollination, water and nutrients on fruit tree performance

Pollination is critical to fruit production, but the interactions of pollination with plant resources on a plants reproductive and vegetative features are largely overlooked. We examined the influences of pollination, irrigation and fertilisation on the performance of almond, Prunus dulcis, in northern California. We used a full-factorial design to test for the effects of pollination limitation on fruit production and foliage variables of whole trees experiencing four resource treatments: (i) normal water and nutrients, (ii) reduced water, (iii) no nutrients, and (iv) reduced water and no nutrients. In each of these combinations, we applied three pollination treatments: hand-cross pollination, open-pollination and pollinator exclusion. Pollination strongly affected yield even under reduced water and no nutrient applications. Hand-cross pollination resulted in over 50% fruit set with small kernels, while open-pollinated flowers showed over 30% fruit set with moderate-sized kernels. Pollinator-excluded flowers had a maximum fruit set of 5%, with big and heavy kernels. Reduced water interacted with the open- and hand-cross pollination treatments, reducing yield more than in the pollinator exclusion treatment. The number of kernels negatively influenced the number of leaves, and reduced water and no nutrient applications interacted with the pollination treatments. Overall, our results indicate that the influences of pollination on fruit tree yield interact with the plant availability of nutrients and water and that excess pollination can reduce fruit quality and the production of leaves for photosynthesis. Such information is critical to understand how pollination influences fruit tree performance.

Insect–fern interactions: macrolepidopteran utilization and species–area association

Abstract. 1 The generalization that ferns are under‐utilized by phytophagous insects in comparison to angiosperms may be invalid because of biases involving plant growth form, plant range, and unequal sampling efforts. 2 Comparison of nineteen fern species with 652 herb species, the angjosperm growth form most similar to the ferns, indicates no significant difference in the mean number of supported macrolepidopteran species. When the herbs are subdivided into annuals, biennials and perennials, only the annual herbs are significantly different than the ferns. 3 Comparisons of the occurrence distributions for ferns and the herb categories also demonstrate that only the annual herbs support more macrolepidopteran species than the ferns. The same results are obtained when random assemblages of herbs are created that are the same size as the fern assemblage. 4 Both the occurrence distributions and the species–area relationship for the ferns indicate that host records for insects feeding on ferns may be grossly incomplete. 5 The similarity of exploitation of ferns and perennial herbs by the Macro‐lepidoptera suggests that other foliage feeding insects may also use ferns at levels equivalent to angiosperms.

THE EFFECT OF POPULATION SIZE ON STIGMA POLLEN LOAD, FRUIT SET, AND SEED SET IN ALLIUM STELLATUM KER. (LILIACEAE)

In Allium stellatum, a xenogamous self‐compatible prairie species, we examined stigma pollen load, percentage of fruit set, and percentage of seed set in large and small populations over 2 yr. We tested for significant effects of population size on these characteristics, for relationships between stigma pollen loads and fruit and seed set, and for significant variation between years in reproductive characteristics. In 1994 stigma pollen loads were inversely related to population size and were significantly lower than in 1993. The lower stigma pollen loads in 1994 were associated with adverse weather conditions (high rainfall), which are known to affect pollinator activity and may have led to competition among plants for pollinators in large populations. However, population size was not correlated with percentage of fruit set or percentage of seed set in either year. The lack of fruit and seed set differences between large and small populations, particularly in 1994, likely resulted from the small amount of pollen needed to trigger fruit and seed production. Our results indicate that for some self‐compatible species that similarly require small pollen loads, the effects of population size on reproduction are minimal, even if differences in pollen‐movement exist between large and small populations.