Is insect herbivory contributing to the threatened status of Agalinis auriculata (Orobanchaceae) in Illinois?1
Mulvaney, Christopher R
MULVANEY, C. R., (Department of Biological Sciences, Illinois State University, 4120 Biology, Normal, IL 61790), B. MOLANO-FLORES (Illinois Natural History Survey, Center for Wildlife and Plant Ecology, 1816 South Oak Street, Champaign, IL 61820), AND D. W. WHITMAN (Department of Biological Sciences, Illinois State University, 4120 Biology, Normal, IL 61790). Is Insect Herbivory Contributing to the Threatened Status of Agalinis auriculata (Orobanchaceae) in Illinois? J. Torrey Bot. Soc. 133: 560-565. 2006.-During August-November 1999-2002, we studied herbivory in two populations of the Illinois threatened prairie plant, Agalinis auriculata (Michx.) Raf. (Orobanchaceae). We collected and identified insect herbivores, and measured levels of folivory and granivory. We also tested whether observed levels of folivory influenced the reproductive success of the plant (i.e., fruit set, seed set and seed mass). Major herbivores included the blackhorned tree cricket (Oecanthus nigricornis), the verbena bud moth (Endothenia hebesana), and the buckeye butterfly (Junonia coenta). Significant differences were found between populations for percent leaf damage, although the percent of leaf area per plant removed by insects was relatively low, ranging from 0% to 40% in 2000 and 0% to 12% in 2001. A significant negative correlation between leaf damage and seed set was found, but not with fruit set or seed mass. In addition, E. hebesana was capable of causing up to 100% damage to seeds within individual fruits, although the infestation level of total yearly fruit samples was relatively low in 2000 (21%, N = 72) and 2001 (6%, N = 266), but high in 1999 (89%, N = 18). Insect herbivory, combined with habitat loss and other biotic constraints may hinder the recovery of Agalinis auriculata.
Key words: Agalinis auriculata, Endothenia hebesana, fiorivory, folivory, granivory, herbivory, Orobanchaceae, prairie.
Insects are among the primary herbivores of terrestrial plants (e.g., sap feeders, stem borers, seed predators, leaf feeders, root borers, etc.; Karban and Strauss 1993, Palmisano and Fox 1997, Fletcher et al. 2001, Groom 2001). These herbivores harm plants in many ways, including reducing area available for photosynthesis, lowering reproductive allocation, directly consuming reproductive tissue or seeds, and disrupting seedling establishment (Rockwood 1973, Bentley et al. 1980, Lee and Bazzaz 1980, Myers 1981, Louda 1982, Crawley 1983, Kinsman and Platt 1984, Wood and Andersen 1990). Herbivores can also transmit pathogens (e.g., Power 1987, 1991, Friedli and Bacher 2001) or make plants more susceptible to them (e.g., Kluth et al. 2001). All of these assaults can ultimately reduce population size (Verkaar 1987).
Even though insect herbivory can critically impact plant populations (Crawley 1983, Louda 1994), it is often ignored in recovery plans for endangered or threatened plants (Louda 1994). Given the importance of herbivorous insects, managers must be careful not to overlook them when preserving these plant species. In this study, we wanted to determine if insect herbivory is contributing to the threatened status of Agalinis auriculata (Orobanchaceae) in Illinois. Virtually nothing is known regarding herbivores of this plant, except two unpublished reports (McCarty 1987, Midewin National Tallgrass Prairie 1999) noting damage due to larvae of the buckeye butterfly (Junonia coenia), in addition to possible deer herbivory. The two main goals of this study were to: (1) Identify potential insect herbivores of A. auriculata and to characterize their damage; and (2) Determine whether natural levels of folivory or granivory limit the reproductive success (i.e., fruit set, seed set and seed mass) of A. auriculata individuals in natural populations. Such information could provide important clues about factors that may limit recovery of this threatened plant species.
Materials and Methods. SPECIES DESCRIPTION. Agalinis auriculata (Michx.) Raf. (Orobanchaceae) is an annual plant with opposite simple leaves and perfect flowers (Iverson et al. 1999). This plant usually possesses a single, vertical four-sided scabrous stem reaching 8 dm in height (Fernald 1950). Its lanceolate or lance-ovate shaped leaves are approximately 2,5-5.5 cm long and 0.8-2.0 cm wide (Fernald 1950). It produces purple, 1 to 2 cm diameter tube-shaped corollas 2 cm long with dark spots on the throat from late August to mid September (Fernald 1950). Each flower has four stamens and single pistil. It produces oval-shaped fruits 1-1.3 cm in length (Fernald 1950). The seeds are ellipsoid to ovoid in shape and about 1.3-1.6 mm long (Fernald 1950). This species is hemiparastic and may parasitize Rudbeckia fulgida Alton, Helianthus occidentalis Riddel), Silphium terebinthinaceum Jacq. and Solidago rigida L. (Cunningham and Parr 1990, Molano-Flores et al. 2003). Agalinis auriculata is threatened in Illinois and rare in other states (Illinois Natural Heritage Database 2003; USDA-NRCS 2002).
STUDY SITE. We studied herbivory in two populations, separated by 6 km, of Agalinis auriculata located at the Midewin National Tallgrass Prairie in Will County, Illinois. A large population (666 individuals, 1999-2002 average) covered 3600 m^sup 2^, whereas a small population (132 individuals, 1999-2002 average) covered 1296 m^sup 2^. Both populations existed in unmanaged, mesic tallgrass prairie remnants. Several invasive, exotic plant species also occupied the sites, including black locust (Robina pseudoacacia L.), autumn olive (Elaeagnus umbellata Thunb.), multiflora rose (Rosa multiflora Thunb.), bush honeysuckles (Lonicera spp.), cut-leaf teasel (Dipsacus laciniatus L.), common teasel (Dipsacus sylvestris Huds.) and Canada thistle (Cirsium arvense L.) (Midewin National Tallgrass Prairie 1999).
HERBIVORE COLLECTION. During July-November of 2000, 2001 and 2002, we collected voucher specimens of non-pollinator insects observed feeding on A. auriculata, by moving from plant to plant, and quietly and carefully inspecting the entire plant. Our study included ~35 hrs of diurnal and 9 hrs of nocturnal observations. To properly identify insect larvae, we reared all collected specimens to the adult stage.
FOLIVORY. We randomly selected 15 plants in 2000 and 25 plants in 2001 from each population, post seed set, at the end of the growing season (in August). We traced all damaged leaves onto paper and scanned these drawings into a computer. Only damaged leaves were traced in 2000, whereas in 2001 all damaged leaves in addition to at least one leaf from every axial pair on the plant was drawn to obtain more accurate measurements of total plant leaf surface area. Leaves from the same axial pair are similar to each other in size (per. obs.). Using Scion Image Software (Scion Corp. 2000), we measured the amount of damaged and pre-damaged surface area per leaf. Pre-damaged surface area for leaves was estimated by digitally completing the original leaf boundary using the image analysis software. In cases where this process was not feasible due to extensive damage to the leaf margin, the pre-damaged surface area was estimated by measuring the surface area of the opposite leaf. In 2001, the pre-damaged area for each un-drawn leaf from an axial pair was estimated using its drawn counterpart. We calculated total original leaf surface area (mm^sup 2^) per plant as the total pre-damaged area (mm^sup 2^) of all leaves for each plant. Likewise, total damaged area per plant was the total damaged area from all leaves from an individual plant. Because only damaged leaves were drawn and measured in 2000, we estimated total pre-damaged area per plant using an area (mm^sup 2^) vs. total number of leaves regression equation (y = 472x – 6175, R^sup 2^ = 0.893) that was derived from the 2001 leaf area data. For both years, we calculated the total percent leaf damage per plant as: (total removed or necrotic leaf area/total pre-damaged leaf area) x 100%. We compared total percent leaf herbivory between populations and years using a two-way ANOVA in SAS (SAS Institute 1999-2001). Data were log square-root transformed to meet the necessary assumptions. Follow-up tests included limited pairwise comparisons of least squares means using the Bonferroni approach (Sokal and Rohlf 1995).
In early September and early October, we counted the number of flowers and number of mature fruits, respectively, on each plant that was used to determine leaf damage in August to calculate fruit set (i.e., total number of fruits/total number of flowers); and then we collected between one to three fruits per individual to determine seed set. Whenever feasible, we attempted to collect a fruit from the upper, middle and lower portions of the inflorescence from each plant to account for potential variation in the number of seeds due to position along the stem. In 2000, we collected 22 fruits in the large population and 15 fruits in the small population and 71 and 63 fruits respectively in 2001. For each plant, we determined seed set as the mean number of seeds per collected fruit. In 2001 we also estimated mean seed mass per plant as another indicator of reproductive success. We calculated the mean mass of individual seeds for each collected fruit as the total mass of all seeds in the fruit divided by the number of seeds. We averaged this value from all collected fruits on a plant to obtain the mean mass per seed for each plant. In 2000, plants from the large and small population experienced extensive damage from unidentified large mammal herbivores (likely white-tailed deer, Odocoileus virginianus). As a consequence, we could determine fruit and seed set from only five plants in the small population and nine plants in the large population during 2000. In 2001, we placed nylon bags of human hair throughout the study sites, and no plants were lost to large mammal herbivores. Because of the extensive damage in 2000, we combined the data from 2000 and 2001 in the correlation analysis. We analyzed for correlations between leaf damage, fruit set, seed set and seed mass (only in 2001) using a Spearman rank correlation analysis. All analyses were conducted using SAS (SAS Institute 1999-2001).
GRANIVORY. We examined a total of 18 fruits in 1999, 72 fruits in 2000, and 266 fruits in 2001, for evidence of herbivory (e.g., dried larvae, damaged seeds, silk, frass, etc.). Fruits collected in 1999 were taken during preliminary observations of the populations prior to the larger study in 2000 and 2001. Fruits inspected in 2000 and 2001 included those collected as part of the previously described folivory study. We collected and identified live granivores found within the fruits, and quantified the number of damaged seeds (in 2001). In 1999 fruits were collected in November and in 2000 and 2001 fruits were collected in October.
Results. HERBIVORE COLLECTION. We collected examples of nine potential insect herbivore species from the leaves, stems, flowers and fruits of Agalinis auriculata (Table 1). We noted black-horned tree crickets, Oecanthus nigricornis F. Walker (Gryllidae) feeding on floral tissue each year. During the early morning hours prior to anthesis of A. auriculata flowers, O. nigricornis individuals typically rested on the stem below the buds. Once corollas opened, they moved onto the flower and began feeding on the petals, ovaries, pistils, and stamens. During 2002, we noted, for the first time, caterpillars of the Buckeye butterfly, Junonia coenia Hubner (Nymphalidae), feeding on leaves, buds and flowers, in some cases consuming nearly all of the bud or floral tissue. Adult soldier beetles, Chauliognathus sp. (Cantharidae), fed on pollen, but did not appear to consume floral parts. On several occasions, we also observed a species of Miridae (Heteroptera) on A. auriculata corollas, but we were unable to determine if they fed on tissue. In the small population during 2000, we observed a specimen of Pentatomidae (Heteroptera) within an old flower calyx as well as another individual of the same species on the stem and leaves of a different A. auriculata plant. In 2001, we noted approximately ten juveniles of the same species within the calyx surrounding an aborted bud. We collected two species of Chrysomelidae (Coleoptera) within the flowers of A. auriculata and another from the leaves and stems. Although this latter species was observed on numerous occasions elsewhere at the study site, this was the only time it was seen on A. auriculata.
FOLIVORY. Total percent leaf damage for individual plants ranged from 0% to 40% in 2000 and from 0% to 12% in 2001. Mean (± SE) percent leaf damage in 2000 was 7.1 ± 3.2 and 3.6 ± 0.9 in the large and small population, respectively. In 2001, the large and small populations had mean (± SE) percent leaf damage values of 1.3 ± 0.3 and 5.2 ± 0.7, respectively. A two-way ANOVA indicated significant differences between populations (df = 1, F = 12.16, P = 0.001) in addition to a significant interaction effect (df = 1, F = 6.96, P = .010). Population differences were only significant in 2001. Although overall leaf herbivory varied between years, the difference was not statistically significant (df = 1, F = 0.78, P = 0.379). The average number of seeds per fruit ranged from 61 to 190 in 2000 and from 42 to 374 in 2001; total number of fruits per plant ranged from one to 55 over the two years. A Spearman rank correlation analysis indicated a significant negative correlation between percent leaf damage and the number of seeds per fruit (R = -0.51, P
GRANIVORY. During 1999, 2000, and 2001, we noted larvae of the moth, Endothenia hebesana Walker (Tortricidae) feeding on seeds within A. auriculata fruits. Sixteen of 18 (89%) fruits collected in 1999 contained either a dead larva or evidence (e.g., silk, frass, damaged seeds, etc.) of prior infestation. Based on examination of larval head capsules and type of damage, we believe that all fructivory was due to E. hebesana. Of the 72 fruits examined in 2000, we found a total of 15 (21%) with evidence of E. hebesana presence, all in the large population. Likewise, in 2001, an examination of 266 fruits yielded 15 (6%) larva-infested fruits (2 [1%] in the small population; 13 [5%] in the large population). In all cases only one larva was observed inhabiting a single fruit. We found no evidence that any previously infested fruit ever contained more than one individual caterpillar. The percentage of damaged seeds in the herbivore-infested fruits in 2001 ranged from 0% to 100%.
Discussion. Like many plant species, Agalinis auriculata is fed upon by an array of insects. We collected nine species of herbivorous insects from plants at our study sites; the most common included flower-feeding blackhorned tree crickets, Oecanthus nigricornis, seed-eating Endothenia hebesana larvae and, during 2002, the caterpillars of Junonia coenia, which consumed a variety of above-ground parts. These latter two herbivores have been found associated with Castilleja indivisa Engelm. (Orobanchaceae) as well (Adler 2000). Even though we did not directly observe plant bugs (Miridae) or stink bugs (Pentatomidae) feeding on plant tissue, it is likely that they do, given that many are generalist plant feeders (Schuh and Slater 1995) and individuals of these two groups were observed on several occasions. The soldier beetles (Cantharidae) observed in 2001 fed on pollen. The two species of Chrysomelidae found within flowers were likely feeding on pollen or floral tissue, as members of this family are typically phytophagous (Borror et al. 1992). The third species of Chrysomelidae was noted on A. auriculata only once and therefore it remains unclear whether its presence was accidental or for feeding purposes.
Although floral feeding by tree crickets was common in the large population, overall damage was limited and did not appear to have affected reproduction. Mulvaney et al. (2004) found high levels of fruit and seed set in this population in 2000 and 2001. Nonetheless, increases in florivory could lower reproductive output due to reduced pollinator visitation or consumption of reproductive organs (Strauss 1997, Krupnick and Weis 1999, MaIo et al. 2001). In the case of A. auriculata, the latter will be most likely because the species is highly autogamous (Mulvaney et al. 2004).
Overall, insect folivory was low and plants typically lost less than 10% of their total leaf area. In addition, leaf damage varied between populations. Even though levels of herbivory were relatively low, our study shows that this herbivory may have an impact on plant reproduction (i.e., seed set). We found a significant negative correlation between leaf damage and seed set, suggesting that even small amounts of tissue removal can have a negative impact. Several studies have found a similar correlation between seed set and leaf herbivory (Rockwood 1973, Bentley et al. 1980, Lee and Bazzaz 1980, Myers 1981). In contrast, neither seed mass or fruit set was affected by leaf damage, suggesting that although herbivory reduced seed production, it did not affect resource allocation to seeds nor affect fruit development.
Of the insect herbivores associated with A. auriculata at our two study sites, the moth, Endothenia hebesana, may be the most detrimental. In 1999, 89% of fruits examined showed evidence of seed herbivory. Infestation levels were relatively low at our sites in 2000 and 2001 (21% and 6% of the fruits, respectively). Regardless, E. hebesana was still capable of causing high levels of damage to the seeds within the fruits it occupied; some fruits experienced a complete loss of seeds. Stamp (1987) noted similar damage levels with E. hebesana-infested fruits of Chelone obliqua L. (Scrophulariaceae). Although the proportion of seeds damaged per fruit in 2001 typically fell below 20%, these measurements could largely underestimate the actual damage sustained in the field. These values represent damage levels from fruits collected in early October, the point at which many were maturing. Presumably, the amount of damage inflicted by these larvae increases with the length of time they remain in the fruit. If the 2000-2001 fruits had been collected at a later time, as were the 1999 fruits (i.e., November), they might have shown higher levels of seed damage. Although during 2000 and 2001 we discovered most larvae well after fruit collection, we stored fruits in a 5°C refrigerator prior to counting seeds and this likely slowed down larval feeding rates and inhibited further damage to the seeds.
Overall, leaf herbivory and seed predation by E. hebesana may have a negative impact on the reproduction of A. auriculata. This combined with habitat loss and other biotic constraints (Molano-Flores et al. 2003), may hinder recovery of this threatened plant. In addition, further research must be conducted on the impact of large mammal herbivores on this species, because at our study sites whitetailed deer (or some other mammalian herbivore) consumed A. auriculata.
1 This study was supported by an R.D. Weigel grant from the Beta Lambda Chapter of the Phi Sigma Biological Science Honors Society and a research grant from the Illinois State University Graduate Student Association. We also thank Roger Anderson and Edward Mockford for their valuable input. Terry Harrison of the University of Illinois identified the moth specimens. We are extremely grateful to the Illinois Department of Natural Resources and the staff at the Midewin National Tallgrass Prairie for granting us access to the sites. Finally, we thank Melissa Mulvaney for her assistance in the field.
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Christopher R. Mulvaney2,3
Department of Biological Sciences, Illinois State University, 4120 Biology, Normal, IL 61790
Illinois Natural History Survey, Center for Wildlife and Plant Ecology, 1816 South Oak Street, Champaign,
Douglas W. Whitman
Department of Biological Sciences, Illinois State University, 4120 Biology, Normal, IL 61790
2 Current address: Chicago Wilderness, 1000 Lake Cook Rd., Glencoe, IL 60022.
3 Author for correspondence: Email: cmulvaney@ chicagobotanic.org
Received for publication October 29, 2005, and in revised form July 31, 2006.
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