Page 10, The PALMETTO, Fall 1991

by Thomas Eisner, et al.

Analyses of leaf extracts of Oicerandra frutescens, a highlyaromatic mint plant from central Florida listed as an en-dangered species, revealed presence of twelve closely relatedmonoterpenes. The principal of these, (+)-trans-pulegol, isa new natu ral product, the synthesis of which is described.The terpenes are produced in glandular capsules that releasetheir contents upon injury of the leaf. Data from bioassayswith ants and cockroaches indicate that the terpenes servefor defense against insects. Oicerandra does, however, havea leaf-eating enemy, the caterpillar of a pyralid moth, Pyrausta

panopealis. The discovery of a new natural product from an

endangered species raises questions about the chemical im-plications of species extinction.

IntroductionDicerandra frutescens, or scrub

balm, is a rare mint plant (familyLabiatae) (Huck 1987; Kral 1982)

restricted in distribution to probably

no more than a few hundred acres ofa single county (Highlands County) of

central Florida. A low, multi-branched

herb, it is an endemic of the sandy

scrub, a chief habitat of the region. It

was only discovered in 1962, and is nowlisted as an endangered species.

Like many mint plants, D. frutescens

is highly aromatic. Manually macerated

leaves emit an intense terpenoid odor,

which may be apparent also from

meters downwind when a person

walks through a patch of the plant orbrushes across its branches by hand.Close examination of the plant revealedit to be remarkably free of insect-inflicted injury. This suggested thattheodorants might be anti-insectan and

therefore worth characterizing chem-

ically, particularly given the endang-ered status of the plant. Chemical workdone so far on plants of the genusDicerandra must be considered ten-

tative (Huck et al. 1989). We here dealwith the glandular source and chemi-

cal nature of the insect repellents of D.frutescens, and present notes on aexceptional pyralid caterpillar (Pyraustapanopealis) that is undeterred by thechemicals and feeds on the plant.

Biological aspects of the study, in-cluding observations of the caterpillar,were carried out mostly at the Arch-

bold Biological Station in Lake Placid,

whereD. frutescens occurs in patches.All leaves used for experimental pur-

poses (chemical extraction, morpho-

logical work, and bioassays) stemmed

Scrub Balm

transferred to CH2CI2 for extraction(at -20 C). .

Caterpillars of Pyrausta panopealis(family Pyralidae) were field collectedon the plant over a period of days in lateOctober 1989. They live singly, in loosesilken enclosures that they constructon branches of the plant, bindingtogether numbers of leaves, includingsome upon which they feed. They arequickly mobile, moving back and forthwith great agility when disturbed,and leaving the enclosure only whenforced to do so by persistent prodding.When grasped with forceps, theywrithe and anoi nt themselves and thei rimmediate surroundings with regurgi-tated fluid. Scanning electron micro-scopic examination of leaves that werein the process of being fed upon by thelarvae showed the glandular capsulesat the margins of chewed portions ofthe leaf to be in various stages of rup-ture, indicating that they themselves,as well as in all likelihood their con-

tents, were being ingested.A number of larvae, which pupated

within silken enclosures, gave riseto adults, providing specimens fordefinitive identification.

Mint plants are a rich source of"essential oils'~ terpenoid mixtures thataccou nt for their aroma, and for whichthey are often cultivated to providechemicals or chemical blends for thescent and flavor industries. Essentialoils are doubtless the basis of defensein these plants, and the specific ter-penoid mixture that we isolated from

D. frutescens is probably no exception.... Neither [ants] nor [cockroaches] areactual enemies of plants. However,their demonstrated response to the

from plants from the site.

I ndirect visualization of the glandu-lar capsules of the plant was effected

by pressing fresh leaves on thin-layerchromatography plates soaked inTollen reagent. The technique, whichprovides a quick general means forrevelation of plant glandular struc-tures, has been described (Eisner etal. 1987).

In preparation for scanning electronmicroscopy, leaves were frozen byquick immersion in liquid Freon 22*

(jacketed by liquid nitrogen) and im-

mediately freeze-dried. To provide

cross sectional views of the leaves,

some were broken transversely whileimmersed in Freon.

Leaves were clipped in the field, orfrom potted plants transported to our

Cornell greenhouses, and directly

The PALMETTO, Fall 1991, Page 11

tion offindinga novel compound in anendangered plant. How many plants,

one wonders, are now threatened

with extinction worldwide as a conse-quence of habitat destruction, andhow many unknown chemicals of

potential interest are in danger of

vanishing forever as a result?

Dr: Thomas Eisner is a professor inthe Section of Neurobiology andBehavior, Cornell University, Ithaca,N.~ He spends several weeks each year

at Archbold Biological Station. This

article has been excerpted from a

longer article published in Chemoe-

cology 1 (1990). Used with permission

of the author:.

So-called "inducible" defenses ofplants are also sometimes circumven-table. In some plants the induction in-volves direct translocation of deterrentchemicals to the site of an attack, aresponse that some insects are able tocounter by chewing an encirclingtrench into a leaf prior to initiation offeeding at the center of the trench.

The defense of Dicerandra is subjectto no such cou nterploys. Neither vei n-cutting nor trenching can affect a de-fense based on permanently deployed,densely spaced glandular capsules. ForDicerandra, the chief encumbrancemay be the cost of its defense. To mai n-tain a chemical "mining" of the entireintegumental surface doubtless re-quires investment of substantialmetabolic energy.

Pyrausta evidently copes withDicerandra byvirtueof insensitivity tothe terpenes of the plant. It apparentlyingests these, indicating that it mightitself use the chemicals defensively aspart of the oral effluent it emits whendisturbed. If so, itwould not be uniquein employing such a strategy. Regurgi-tative use of dietary resins for defensehas been documented for sawfly larvae

and a caterpillar. Although R pano-

pea/is had not previously been re-ported from D. frutescens, it is knownto feed on other mint plants in nature.The apparent tolerance of terpenes bythis insect is in itself of interest, giventhe known toxicity of compounds suchas pulegone to other caterpillars.

A final point concerns the implica-

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Dicerandra terpenes may be indicativeof a more general insectan sensitivity,encompassing actual herbivores aswell. The fact that Pyrausta was virtuallythe only herbivorous insect noted to befeeding on D. frutescens, attests to thehigh degree of invulnerability of thisplant in nature. Interesting in this con-nection is that pennyroyal oil, with itsrich pulegone content, has long beenused as a flea repellent by humans.

While the demonstration that thechemical defenses of a plant are em-

bodied in its epidermal glandular cap-sules is in itself not novel, two pointsbrought out by our data are worthnoting. First, the Dicerandra capsulesappear to be hermetic enclosures. Ex-posure of the leaves to freeze-dryinginvolved confinement at high vacuum

for 12 to 20 hou rs. The fact that the cap-sules retained their volatile contentsunder these conditions signifies thattheir walls must be literally imperviousto outward diffusion of the terpenes.In the normal plant, evaporative loss ofdefensive material must thus be heldto a minimum.

The second point concerns thedemonstration that leaves of Dicer-andra proved repellent to ants onlyafter transection. This strongly impliesthat it is through capsule rupture that

repellency is initiated. While it is com-

mon knowledge that the scent of ami nt

plant is revealed to the human nosethrough leaf maceration, we know of

no other direct demonstration that theinsect repellency of the leaf is itself alsothus activated and therefore linked tothe scent. Clearly, the capsules aredesigned to externalize their defensive"message"when the plant is attacked.While insects may indeed be Dicer-

andra's major potential enemies, otherherbivores, including perhaps mol-lusks and vertebrates, may also find theplant unpalatable.

Plant defenses are diverse and varied,and differ in their adaptive meritsand shortcomings.laticiferous plants,for instance, which store latex underpressure in canals aligned with thevenational system of the plant, derive

protection from the latex that oozes

from sites of vein injury. Butthe systemis imperfect in that latex outflow in-

variably restricts the potential for fur-ther effluence in tissue distal to theinitial cut. Insect specialists that feedon laticiferous plants exploit this short-coming by cutting veins at intendedfeeding sites, thereby inducing local-ized latex depletion, and then feedingdistal to the cuts.

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