BIOLOGICALLY ACTIVE NATURAL PRODUCTS: AGROCHEMICALS - CHAPTER 12 pps

12
Arthropod Semiochemicals as Multifunctional
Natural Products
Murray S. Blum
CONTENTS
12.1 Introduction
12.2 Multifunctional Semiochemicals
12.2.1 Cantharidin
12.2.2 Multifunctional Queen Pheromones
12.2.3 Venomous Alkaloids
12.2.4 2-Alkyl-6-Methylpiperidines
12.2.5 Pheromones as Interspecific Inhibitors
References
12.1 Introduction
The number of arthropod species (insects, millipedes, spiders, mites, crabs, and related
groups) is truly fulsome, conservatively numbering between 1,000,000 and 5,000,000 spe-
cies, and constituting about 80% of all kinds of animals.
1
The beetles alone (~500,000 spe-
cies) dominate the insects
1
and the vertebrates as well; there are at least 10 times more
beetle species than vertebrate species,
2
and new species of beetles are being described daily.
Significantly, a large variety of unique compounds has already been identified as arthro-
pod natural products,
3
notwithstanding the fact that relatively few species have been sub-
ject to analytical scrutiny. Clearly, for arthropod natural products the best is yet to come and
considering what has already been established, the future would appear to be remarkably

bright both in terms of structural chemistry and functionality.
In this brief review, the ability of arthropods to biosynthesize a variety of novel com-
pounds and to utilize these glandular products multifunctionally will be emphasized as
critical developments that have enhanced the competitiveness of the arthropods. Particular
emphasis will be placed on semiochemicals, signaling agents utilized in intra- and inter-
specific contexts.
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12.2 Multifunctional Semiochemicals
Arthropods are the paramount producers of semiochemicals in the animal kingdom and
these compounds have been adapted to subserve a variety of important functions for
selected groups of these invertebrates. For example, the virtuosity of arthropods as produc-
ers of alkaloids chiefly reflects the biosynthetic prowess of ants which generate most of
these compounds as poison gland products that often possess diverse functions
4
both
inside and outside of the colonial milieu. In short, the success of many arthropod species is
clearly identified with both the synthesis and adaptive utilization of semiochemical natural
products. In the discussion that follows, representative semiochemicals, each of which has
been demonstrated to possess a considerable diversity of important biological roles, will be
examined as examples of what has been referred to as pheromonal parsimony.
5
12.2.1 Cantharidin
Cantharidin, the anhydride of cantharidic acid (I), is produced by bee-
tles in the family Meloidae and a few related families and has been
referred to as Spanish fly for hundreds of years.
6
These insects are
sometimes described as blister beetles because, when disturbed, they
discharge cantharidin-fortified blood from their legs and this exudate
can cause severe dermal lesions in vertebrates and also can repel

invertebrates.
7
As we shall see later, the powerful vesicatory proper-
ties of cantharidin have been indirectly responsible for its widespread
use as a putative sexual stimulant.
Cantharidin possesses potent antifungal activities; the female beetle
is reported to coat her eggs with this compound, a potent growth inhibitor for Microsporum
and Trichophyton species.
8
Since the eggs of the beetles are incubated in warm and moist
environments which favor the growth of invasive fungi, the main function of this terpenoid
anhydride is probably to protect developing blister beetle embryos from insect-attacking
fungi.
9
Consistent with this suggestion is the fact that during copulation male beetles actu-
ally transfer large amounts of cantharidin to the females as a copulatory “bonus.”
10
The
transferred cantharidin is used to coat the eggs in what is almost certainly an antifungal
strategy.
In addition to its function as both a vesicant and a fungicide, cantharidin has been tested
as a powerful anthropogenic agent in a wide variety of human systems, dedicated to either
treating pathological conditions or attempting to enhance sexual ambitions.
Complete remission of epidermal cancer in pigs has been effected by topical treatment
with cantharidin.
11
In addition, this vesicatory anhydride when applied in low doses,
resulted in the total remission of dermal cancer in human beings.
12
Cantharidin, which is

structurally similar to a well-known herbicide, is highly active as a selective herbicide.
13
Spanish fly has a long history in medicine, having been used by Hippocrates as a cure for
dropsy nearly 2000 years ago.
14
This compound was considered to be of great medicinal
value and was a major drug in the treatment of bladder and kidney infections and stones,
strangury, dropsy, and certain venereal diseases.
15
Painful conditions such as pleurisy and
sciatica were routinely treated with cantharidin as further testimony to the high esteem
with which this drug was held in medical circles. However, it is the long-standing reputa-
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tion of this compound as a putative aphrodisiac that has led to its use as a sexual stimulant
and in some cases, an abortifacient.
Although cantharidin in minute doses can cause bladder and kidney irritation and pain-
ful voiding of urine with a sensation of burning, these symptoms hardly justify its reputa-
tion as a powerful aphrodisiac. Many women have died of cantharidin poisoning. Indeed,
the Marquis de Sade was beheaded in absentia for feeding aniseed sweets treated with
cantharidin to two prostitutes who identified de Sade before they died.
16
The corrosive
action of the anhydride, first seen in skin lesions, is particularly devastating when it is
ingested, causing corrosion of tissues in the mouth, especially the palate. Curiously,
cantharidin has a completely unexpected effect on the human male.
In two instances French troops in North Africa ate the legs of frogs that had eaten copious
numbers of blister beetles. The frogs legs contained cantharidin that had been absorbed
from the ingested beetles. In both cases the soldiers experienced a potpourri of minor med-
ical problems but the troops were particularly inconvenienced by a painful priapism.
17-18

The erections incapacitated the Zouaves as effectively as any enemy action. “The Beetle of
Aphrodite” had struck again!
12.2.2 Multifunctional Queen Pheromones
Pheromonal parsimony may reach its highest expression in the honey bee, Apis mellifera, in
terms of social communication in the arthropods. The mandibular glands of the queen bee
biosynthesize a complex mixture of acids and esters which is dominated by a few novel C
10
compounds. The activities of one of these glandular constituents, the queen substance or
(E)-9-oxo-2-decenoic acid (9-ODA) (II), appears to be synergized by other compounds pro-
duced in the mandibular glands.
19
One of these,(E)-9-hydroxy-2-decenoic acid (9-HDA)
(III), also possesses several pheromonal roles of its own, further demonstrating the elegant
control of sociality achieved with the queen’s mandibular gland products. The utilization
of a limited series of very characteristic natural products as behavioral regulators for both
workers and drones has enabled queen bees to “fine tune” — and control — the social
structure of populous honey bee colonies.
9-ODA possesses two critical primer activities that are synergized by other compounds
such as 9-HDA; neither compound is active alone. Primer pheromones do not produce an
immediate response but rather, exhibit a delayed response which may occur after 24 hours
or more and frequently involves the reproductive system. For example, ovarian develop-
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ment of the nonreproductive workers is inhibited by 9-ODA, but the workers must have
physical contact with the queen in order for the the pheromonal activity to be expressed.
20
Another primer activity results in inhibiting worker activity leading to the construction of
queen cells, thus enabling the queen to pheromonally suppress the rearing of potential com-
petitors.
21
In addition to these primer functions that occur in the milieu of the hive, remark-

able multifunctionality characterizes the roles of 9-ODA (plus synergists) both in and out of
the hive. In the hive, workers are attracted to the queen by 9-ODA and form characteristic
retinues around her that can result in food and pheromonal exchange.
22
9-ODA also pos-
sesses a major releaser role for the workers, but this is expressed outside the hive. The
swarming of honey bees resulting in the selection of new nesting sites is controlled by a
medley of pheromones. The formation and movement of swarms is regulated by worker
(scout) pheromones that are secreted from an abdominal structure, the Nasanov gland. The
Nasanov secretion is dominated by a mixture of terpenes (e.g., geraniol, citral, farnesol)
that is ideally suited to attract other scout bees to a potential nesting cavity that has been
marked with the secretion.
23
Oriented swarm movement is realized if the queen secretes
9-ODA which acts as a short-range attractant for workers. In the absence of the Nasanov
terpenes, 9-ODA is unattractive to the swarming bees. 9-ODA has another releaser function,
but in this case it is with drone bees rather than with workers. This acid is a powerful sex
attractant for male bees, attracting large numbers of drones at altitudes of at least 200 m.
24
Only 9-ODA is required for powerful sex attractancy, in contrast to the need for glandular
synergists when this compound functions as a pheromone for workers. 9-ODA is a highly
specific sex pheromone, closely related compounds exhibiting no activity whatsoever.
25
12.2.3 Venomous Alkaloids
Many groups of ants in the subfamily Myrmicinae characteristically produce poison gland
secretions that are dominated by alkaloids, rather than the proteins typical of the ants’ rel-
atives, bees and wasps. A large variety of alkaloids are produced by ants and characteristic
compounds are identified with different genera.
26
However, while a variety of pharmaco-

logically active compounds are biosynthesized by these insects, many are not injected into
their assailants. Diverse alkaloid-producing ants do not have a functional sting, and the
externalized venom is smeared on adversaries often packing a topical wallop. On the other
hand, a variety of ant species attack their enemies by introducing venom by hypodermic
injection and the deterrent effects can be considerable. Nevertheless, notwithstanding their
method of introduction to an adversary, venomous alkaloids have been demonstrated to
possess great multifunctionality as defensive compounds.
12.2.4 2-Alkyl-6-Methylpiperidines
Mixtures of cis- and trans-2-alkyl-6-methylpiperidines have been identified as poison gland
products of workers and queens of ants in the genus Solenopsis, the trans-isomers generally
predominating in the venom of workers. The known dialkylpiperidines in the genus Sole-
nopsis possess long n-alkyl groups that are either saturated (IV) or contain a carbon–carbon
double bond at the ninth carbon from the terminal methyl group (V).
27
Members of the subgenus Solenopsis are known as fire ants because of the great pain that
is associated with the worker sting. A concatenation of toxicological and pharmacological
events occurs when Solenopsis venom is introduced subdermally into a human. An exami-
nation of the diverse activities of the venom alkaloids emphasizes their multifunctionality
as agents of deterrence against vertebrates.
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Stings of Solenopsis workers produce pronounced dermal necrosis followed by the for-
mation of pruritic and sterile pustules.
28
The alkaloids liberate histamine from mast cells
resulting in considerable algogenicity, a reaction that intensifies the deterrent effective-
ness of the alkaloids.
29
In addition to these reactions, the dialkylpiperidines possess
powerful lytic activity, instantly hemolyzing mammalian erythrocyes.
30

In addition to
these toxicological effects, the alkaloids demonstrate their not inconsiderable multifunc-
tionality by exhibiting a great range of pharmacological activities that are unleased
against a variety of biochemical systems. These compounds are strong inhibitors of
ATPases
31
and in addition, they reduce mitochondrial respiration and uncouple oxida-
tive phosphorylation.
32
The ability of the dialkylpiperidines to also block neuromuscular junctions
33
further
identifies these compounds as very versatile defensive agents. However, in addition to
their activities against pharmacological targets, these alkaloids exhibit considerable semi-
ochemical parsimony in a variety of ecological contexts. The poison gland products possess
a wide range of antimicrobial properties and, in addition, they are phytotoxic as well, fur-
ther identifying these compounds as versatile agents of semiochemical multifunctional-
ity.
34
The 2,6-dialkylpiperidines possess powerful antibacterial activity against a variety of
species
35
and in addition, these compounds are potent growth inhibitors of diverse fungal
species.
36
These alkaloids also exhibit considerable activity as insecticides which compares
to that of commercial insecticides.
37
Furthermore, the ability of these compounds to exhibit
insecticidal activity when applied either topically or by injection, enables fire ants to be

both effective aggressors and predators.
The fire ant’s offensive arsenal is further expanded by the well-developed repellency
to ants exhibited by the 2,6-dialkylpiperidines,
38
which should allow these ants to suc-
cessfully compete with other ants for critical resources. In some cases repellency of dif-
ferent ant species is achieved by dispersing venom through the air (gaster flagging),
39
but
in the milieu of the nest dispersion of venom by workers is utilized to treat vulnerable
fire ant larvae with the antibiotic alkaloids.
40
Similarly, the queen fire ant treats her
freshly laid eggs with her poison gland contents, and the concentration of alkaloids is
high enough to inhibit the growth of entomopathogenic fungi.
41
The chemical ecology of
the fire ants vis-à-vis the 2,6-dialkylpiperidines constitutes a highly adaptive system for
both exploiting the acquisition of food and protecting the immatures from intrusive
microorganisms.
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12.2.5 Pheromones as Interspecific Inhibitors
Many species of bark beetles in the family Scolytidae have evolved a unique strategy for
attacking coniferous trees and utilizing these often formidable plants as sites for mating,
feeding, and reproduction. Optimal utilization of the tree as a resource requires that the bark
beetles develop a large enough population to overcome the mostly chemical defenses of the
target tree. In addition, maximum utilization of the trees’ not inconsiderable resources also
requires the establishment of large numbers of immature and adult beetles to convert the tree
into an attractive target for additional bark beetles of the same species. This scenario presup-
poses that successful colonization of a pine tree will be achieved by a single species of bark

beetle. Since bark beetles utilize aggregation pheromones to attract members of their own
species, these volatile information-bearing compounds would seem to constitute ideal agents
for guaranteeing the specific integrity of the “chosen” tree. And indeed it has been estab-
lished that the pheromonal attractants of selected species of bark beetles possess critical mul-
tifunctional value as inhibitors of the attraction of competitive beetle species.
42
Aggregative pheromones are frequently synthesized by bark beetles from monoterpenes
derived from the host’s oleoresin. Novel mixtures of oxygenated monoterpenes have been
identified as aggregative pheromones in species in the genera Ips and Dendroctonus and these
compounds have been demonstrated to function, under field conditions, as both intra- and
interspecific agents of mutual interruption.
42
For example, colonizing beetles of the species
Ips paraconfusus produce an aggregation pheromone that is dominated by the monoterpenes
ipsenol (VI), ipsdienol (VII), and cis-verbenol (VIII).
43
These compounds, which are mainly
produced by male beetles, constitute a true synergistic pheromone, all three compounds
being required for attraction to the host, ponderosa pine. On the other hand, the attraction of
another Ips species, I. latidens, which also attacks ponderosa pine, is inhibited by the ternary
mixture employed by I. paraconfusus as an aggregation pheromone.
44
Nevertheless, I. latidens
responds to both ipsenol alone and to ipsenol plus cis-verbenol. However, when the complete
I. paraconfusus aggregation pheromone containing ipsdienol, ipsenol, and cis-verbenol is pre-
sented to Ips latidens, the attraction of adults of I. latidens is completely inhibited. Whereas
adults of I. paraconfusus convert myrcene to ipsdienol,
45
an important aggregation phero-
mone for this species,

43
adults of I. latidens do not produce this oxygenated monoterpene and
it functions as a powerful inhibitor of aggregation when they encounter it.
Mutual interruption of attraction can occur when species in the same genus inhabit the
same host tree. For example, males of I. pini and I. paraconfusus, boring side by side in logs of
ponderosa pine, attract fewer beetles of both species than logs containing males of a single
species. Furthermore, the response of I. pini is reduced in the presence of either (–)-ipsenol or
(+)-ipsdienol,
46,47
the latter compound being the enantiomer of (–)-ipsdienol, the aggregation
pheromone of I. pini. Since both ipsdienol and ipsenol are part of the aggregation pheromone
of I. paraconfusus,
43
it is probable that these terpenes are primarily responsible for inhibiting
the attraction of I. pini.
Mutual interruption also can occur between species in different genera that inhabit the
same tree. The western pine beetle, Dendroctonus brevicomis, competes with Ips paraconfusus
for ponderosa pine, and these species frequently inhabit the same host tree. Verbenone
(IX),one of the aggregation pheromones produced by males and females of D. brevicomis,
effectively interrupts the attraction of I. paraconfusus to host trees.
48
However, mutual inter-
ruption between these species also has been observed,
49
and it would appear that individuals
of both species generate cross-specific attractant inhibitors that probably ensure that a
newly invaded host tree will not be catastrophically overwhelmed by beetles of either species.
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