Herbivore Damage to Three Tropical Ferns (1983)

Herbivore damage to fronds of Polybotrya cervina, Thelypteris tuirrialbae, and Adiant,uin obliqutun from Finca La Selva, Costa Rica, was measured, and types of damage were distinguished. Herbivores removed 9.9, 5.5, and 7.3 percent of the leaf tissue, respectively. The amount removed did not differ significantly among the species, although the proportion of fronds with different types of damage was different. In this study the levels of damage to ferns are similar to those reported for tropical angiosperms, suggesting that the vegetative parts of tropical ferns and angiosperms are utilized equally by herbivores. The variety of damage types suggests that tropical ferns host a varied insect fauna. These results support the hypothesis that underutilization of ferns by herbivores is largely an artifact of inadequate sampling. MANY INVESTIGATORS HAVE COMMENTED on the paucity of insects associated with ferns (Schneider 1892, Brues 1920, Dethier 1947, Ehrlich and Raven 1964), although quantitative evidence for this is generally lacking. One study (Hendrix 1980) which compares the insect fauna associated with ferns and angiosperms indicates that far fewer insects than expected use ferns as a food source. However, some of this underutilization may be attributed to inadequate sampling (Balick et al. 1978, Hendrix 1980, Auerbach and Hendrix 1980). The one fern which has been extensively sampled (Pteridium aquilinum (L.) Kuhn) supports an insect fauna equivalent to that of angiosperms of similar geographic range (Lawton 1976, Strong and Levin 1979). Also, Auerbach and Hendrix (1980), using a statistical test, found no difference in the mean number of leaf-feeding Macrolepidoptera supported by ferns and herbaceous angiosperms. On the basis of the plant architecture hypothesis (Lawton and Schroder 1977, Lawton 1978, Strong and Levin 1979), Auerbach and Hendrix suggest that for insects utilizing a tissue common to ferns and angiosperms (such as leaf or root tissue), the ferns should support as many insect herbivores as angiosperms of similar growth form. If ferns support as many foliar-feeding insects as angiosperms, then we hypothesize that ferns and angiosperms should suffer equivalent amounts of damage to vegetative plant parts. Unquantified field observations indicate that ferns suffer only minor amounts of damage (Schneider 1892, Lowe 1898, Weiczorek 1973), but in the only quantified study, Balick et al. (1978) found that damage to three Mexican ferns was greater than damage to seven angiosperms. This study was undertaken to extend the limited data base on the amount of herbivore damage to ferns in order to compare this data to previous studies on herbivore damage to angiosperms. Here we report on an analysis of insect damage to individual fronds of three neotropical ferns. STUDY AREA AND FERN SPECIES Samples of fern fronds were collected at Finca La Selva in September 1979. Finca La Selva is located in the Altantic coastal lowlands of Costa Rica (see Holdridge et al. 1971 for site description). The three fern species sampled were Polybotrya cervina (L.) Ktze., Thelypteris turrialbae (Rosenst.) Morton, and Adiantum obliquum Willd., all members of the Polypodiaceae. Voucher specimens of these species were deposited in the University of Iowa herbarium. Polybotrya cervina forms clusters of 5 to 20 fronds on short, creeping rhizomes. Fronds of P. cervina are pinnate to bipinnate and can reach heights of 0.5 m; pinnae are 10-25 cm long and 2.5-7.0 cm wide. Thelypteris turrialbae forms clusters of 5 to 10 fronds at the end of long, creeping rhizomes. Fronds of T. turrialbae are pinnate and up to 0.8 m tall with pinnae 12-40 cm long and 3.0-9.9 cm wide. Adiantum obliquum fronds are scattered along the length of a long, creeping rhizome and are up to 0.2 m tall; pinnules are 3.0-6.5 cm long and seldom more than 1.5 cm wide. All three species occur in small populations of 5 to 15 clusters of fronds widely distributed in the understory at La Selva. Polybotrya cervina is almost entirely restricted to the boles of fallen trees, while the other two species grow directly on the forest floor. MATERIALS AND METHODS We randomly sampled one frond from each of 50 clusters for each fern species. Mature fronds were collected and immediately pressed for later analysis. No attempt was made to assess damage to underground parts or quantitatively to assess frond damage at different stages of development. Damage to P. cervina and T. turrialbae was assessed by photocopying individual pinnae, cutting out and weighing the intact outline to the nearest mg (A = total 108 BIOTROPICA 15(2): 108-111 1983 This content downloaded from 207.46.13.11 on Mon, 17 Oct 2016 05:39:50 UTC All use subject to http://about.jstor.org/terms weight) and then removing and weighing the white area (B = weight of tissue eaten). Percent damage to each frond was then calculated as I B/I A X 100. A different technique was used to determine damage to A. obliquum because its fronds have many small pinnules which are difficult to measure by the above method. The length of undamaged pinnae from the base of the first pair of pinnules to the base of the terminal pinnule was measured to the nearest mm, and all pinnules were then removed and weighed to the nearest mg. The regression of log weight on log length yielded a regression coefficient of 0.97 (N = 14, P 0.40). For T. turrialbae mean damage by the photocopy method was 6.37 ? 1.57 percent; by the second method mean damage was 5.37 ? 1.49 percent (P > 0.20). Before statistical analysis, all percentages were arc-sin transformed. A nested analysis of variance (ANOVA) was used to test for effects of fern species on percent damage per frond. The Duncan multiple range test was used to compare mean damage between fern species. Types of damage to the fronds were distinguished using a modified version of the categories developed by Balick et al. (1978). These categories and the insect order(s) most likely responsible for the damage are as follows: skeletonization (Lepidoptera and/or Coleoptera), pinholes less than 3 mm in diameter (Hemiptera and/or Homoptera), shotholes equal to or greater than 3 mm in diameter (Lepidoptera), irregular chewing (Lepidoptera, Coleoptera, and/or Orthoptera), and crescent-shaped notching at the edge of the pinnules (Hymenoptera: Formicidae (Atta spp.) and/or Megachilidae). In addition, damage to young fronds could be distinguished by the smooth edges and lack of dead cells in the area of dam-