Biochemical and Evolutionary Aspects of Arthropod Predation on Ferns

Oecologia (Berl.) 35, 55-89 (1978) Biochemical and Evolutionary Aspects of Arthropod Predation on Ferns Michael J. Balickl, David G. Furthz, and Gillian Cooper-Driver3 * ' Botanical Museum of Harvard University, Oxford Street, Cambridge, MA 02138, USA ' Department of Biology. Yale University, New Haven, CT 06520, USA ' Jhpartment of Biological Sciences, Boston University, 2 Cummington St., Boston, MA 02215, USA Summary. The widely held assumption that very few arthropods feed on ferns was questioned following field observations of arthropod damage on ferns in the state of Veracruz, Mexico. The extent and type of damage was recorded and it was found that in a measured locality, ferns were no less attacked than the angiospermous flora. As chemistry and arthropod host relationships have been shown to be so closely intertwined, plants collected in the field were analysed for both condensed tannins and cyanogenic glycosides, compounds known to be effedtive deterrents in temperate climates. Although all ferns tested contained tannins these did not appear to inhibit predation. Cyanogenic glycosides were present in only 3% of the fern species analysed, and it is, therefork unlikely that they play a significant role as defensive compounds in the ferns examined. A literature search revealed a large number of ferns cited as being arthropod hosts. Approximately 420 named species of arthropods have been recorded, the majority of which are from the orders Coleoptera, Hymen- optera,. Lepidoptera, and Hemiptera. Both evolutionary primitive (sawflies) and advanced (moths) arthropods are reported to be present on ferns suggest- ing possible coevolution of arthropods and ferns both before and after the radiation of angiosperms. I. Introduction It is assumed that ferns generally are not eaten by herbivorous insects (Soo Hoo and Fraenkel, 1964; Eastopp 1973; Southwood, 1973). Indeed in their cl'assical paper on the co-evolution of bufferflies and plants Ehrlich and Raven (1964) state "In fact, very few insects feed on ferns at 'all, a most surprising and as yet unexplained fact with no evident chemical or mechanical basis ". 'Some ferns have toxic effects on both invertebrates (Carlisle and Ellis, 1968) * To whom offprint requests should be sent M.J. Balick et a]. as well as vertebrates (I.A. Evans, 1976; W.C. Evans, 1976), but as have generally managed to exploit most other toxic plants, why is it that they have been reported to avoid ferns? During field studies on the biology of ferns in the state of Veracruz, Mexico (March-April, 1976) it was noticed that many of the ferns showed a considerable amount of damage which was apparently due to arthropod feeding. It appeared, therefore, that entomologists and field ecologists may have either over-looked or ignored ferns as possible host plants for insect herbivores. Since earlier work had shown that two groups of secondary plant compounds, the tannins and cyanogenic glycosides (Cooper-Driver et al., 1977), are probably important in determining the extent of herbivore attack on a given fern species, the original observations were followed up with estimations of the amounts of these compounds in selected fern species. We also carried out an extensive literature search revealing a large number of references to ferns being used as host plants by arthropods, although it must be stressed that these records are not always clear as to whether the host plant was providing shelter or food. Few of these records relate to tropical flora and fauna, surprisingly in that insects are much more diverse in the tropics than elsewhere and doubtless constitute the major class of herbivorous animals (Janzen, 1975). From these preliminary observations on the degree and type of damage to Mexican ferns and from the records in the literature, it is concluded that the widely held assumption that very few arthropods feed on ferns, is not well founded. The chemical studies showed that while both tannins and cyanogenic glycosides were present in the ferns examined, their role as efficient feeding deterrents may not be as great as in other plant phyla (Swain, 1977), or as in temperate fern species (Cooper-Driver, 1976; Lawton, 1976). These findings are used to discuss the way in which ferns, during the course of evolution, have developed defensive strategies and in fact have co-evolved with their arthropod predators. 11. Materials and Methods I. Field Collection All field studies and collections were made in the state of Veracruz, Mexico near Jalopa in areas around Puente National, Misantla, Las Vigas and Perote, during March-April 1976. A total of six ecologically diverse sites were visited and every species of fern in the area was examined for insects or visible indications of damage due to insect feeding. Many of the specimans were collected, pressed, dried and examined in the laboratory. To determine the relative amount of damage to ferns and other plants, an "ecological plot", 2 by 4 m was laid out in a Liquidambar forest site at 1350 m altitude near Las Vigas. The location of the individual plants was recorded and a survey for insect damage made. Representative samples of each plant were pressed and dried and a rough estimate of the amount of damage obtained by photocopying the plants, cutting out the outline on the copy and weighing it (A, "intact plant") then removing from the copy the (white) area plainly showing damage (B) and weighing this. The percentage damage was then B/A x 100. 2. Chemical In~stigations Chemical tests were carried out using both fresh and dried material. The presence of a cyanogenic glycoside was determined using the method of Eyjolfsson (1970). Fresh frond samples in the field Biochemical and Evolutionary Aspects of Arthropod Predation on Ferns were tested for the production of HCN by treating the material (ca. 1.5 g of terminal pinnae) with 2-3 drops of toluene in a sealed tube with a filter paper strip, which had been pre-treated with sodium picrate solution, suspended from the stopper and leaving the tube at room temperature for 24 h. Any change in the color of the papers from yellow to brown that was observed indicated that HCN had been released. Estimation for condensed tannins was carried out in the laboratory on dried specimans. The plant material was extracted twice with 80% boiling methanol and the extracts combined. Equal volumes of the extract and 5% conc. HCI in n-butanol were heated at 100" C for 40 min and the absorbtivity read at 530 nm (modification of Swain and Hillis, 1959). Quebracho tannin was used as the standard. 3. Literature Search This was carried out using abstract literature from Biological Abstracts, Review of Applied entomol- ogy and Bibliography of Agriculture from 1930 up to the present time. In addition, the entomological libraries of the Connecticut Agricultural Experimental Station, Yale University and Harvard (Museum Comparative Zoology) were xanned for host plant data in taxonomic monogaphs or faunal surveys. 1. Survey for Arthropod Damage Of the 137 fern species collected in the Veracruz area of Mexico, insect damage was noted in a total of 26 or approximately 19% of the total fern flora observed. Species for which damage was recorded are given in Table 1. The damage ranged from slight to that which appeared to be severe enough to have affected . the photosynthetic and reproductive capacity of the plant. Despite signs of considerable insect damage to the ferns in this tropical Mexican locality there : were very few arthropods actually found on the ferns at the time of collecting. This may be due to a predominance of nocturnal feeding, arthropod seasonality in the Mexican populations, collecting techniques, or some other factors. Subse- quent careful examination of pressed ferns from this locality revealed several different types of apparent arthropod damage. We have speculated, with each damage-type, which insect order might have caused the particular damage. These are as follows: a) small crescent notching of pinnules either at apex, base, or bases of several adjacent pinnules - Coleoptera (Curculionidae) (Fig. 1) ; b) large notching of pinnae-Lepidoptera, possibly Orthoptera (Figs. 2 and 5); c) skeletonization of upper surface of pinnule between the veins- Lepidoptera, Coleoptera (Fig. 3); d) shot-hole (upper surface), often concentrated but not always spherical -possibly Hemiptera (Homoptera) (Fig. 4); e) complete or par- tial elimination of several or many pinnules on one or both sides of the rachis- Lepidoptera (Fig. 6). Several fern species displayed fungal or viral damage and it may be that these pathogens are transmitted by arthropod vectors, i.e. aphids ' or leafhoppers, as is known to occur in other plants (Wood, 1967). The 8 square meter plot in the Liquidambar forest contained 3 species of ferns and 6 other flowering plant genera. The average percentage dafnage to the leaves of several selected individuals of these different taxa is recorded in Table 2. From these results it is apparent that ferns were preferred as a M.J. Balick et al. Table 1. Fern species collected in Veracruz, Mexico on which arthropod damage was recorded, most of which were subsequently analysed for condensed tannins Taxa % condensed Proposed type tannin of arthropod mg/g dry weight damage Adiantum fenerum Sw. 8.40 C Anemin adiantifolia (L.) Sw. 7.80 a Asplenium momnfhes L. 0.90 d Blechnum varians (Fourn.) C. Chr. 5.10 a, b Ctenifis subincisa (Willd.) Ching - a a Cyrtomium juglandfolium (H + B) Moore - a Dicksonia ghiesbreghtii Maxon - a Dryopteris parallelogramma (Kze.) Alston 5.40 a, b, c, e Eluphoglosum latifoium (Sw.) J. Sm. 12.30 a, b Hypolepis reprms (L.) Presl. 0.14 a, d, e Lygodium mexicanwn Presl. 0.30 a Polypodium aureum L. 0.60 a Polypodium Ianceolarum L. - b. Polypodum loricewn L. 0.60 a, b Polypodium pectimtum L.

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