Thomas Eisner

Herbivore-Plant Interactions: Mixed-Function Oxidases and Secondary Plant Substances

The mixed-function oxidases of a polyphagous insect larva (the southern armyworm, Spodoptera eridania) were found to be induced by a diversity of secondary plant substances. The induction proceeds rapidly and in response to a small quantity of secondary substance. Following induction, the larva is less susceptible to dietary poisoning. It is argued that mixed-function oxidases play a major role in protecting herbivores against chemical stress from secondary plant substances.

Precopulatory sexual interaction in an arctiid moth (Utetheisa ornatrix): Role of a pheromone derived from dietary alkaloids

SummaryMales of Utetheisa ornatrix have a pair of brushlike glandular structures, the coremata, which they evert from the abdomen during close-range precopulatory interaction with the female. Males experimentally deprived of coremata are less acceptable to females. The principal chemical associated with the coremata, identified as a pyrrolizine (hydroxydanaidal), has a proven pheromonal role: males raised under conditions where they fail to produce hydroxydanaidal are also less likely to succeed in courtship, and the compound itself, as its (-)-isomer, is capable of inducing the principal receptive response (wing raising) of the female. Evidence is presented indicating that Utetheisa derive hydroxydanidal from defensive pyrrolizidine alkaloids that they sequester from their larval foodplants (Crotalaria spp.). It is proposed that in addition to signalling male presence to the female, hydroxydanaidal may provide the means whereby the female assesses the alkaloid content of the male and therefore his degree of chemical protectedness. The argument is made that such pheromonal assessment of defensive capacity may occur also in other insects, including danaid butterflies, many of which share with Utetheisa a dependence on pyrrolizidine alkaloids for sex-pheromone production.

Defensive Use by an Insect of a Plant Resin

Larvae of the sawfly Neodiprion sertifer (Hymenoptera: Diprionidae), when disturbed, discharge an oily oral effluent essentially identical chemically to the terpenoid resin of its host plant (Pinus sylvestris). The resin is sequestered by the larva upon feeding, and stored in two compressible diverticular pouches of the foregut. The fluid is effectively deterrent to predators. The defensive use by an insect of a plant resin provides an instance of secondary utilization by a herbivore of the protective chemical weaponry of its host.

Sex attractant of an arctiid moth (Utetheisa ornatrix): a pulsed chemical signal.

SummaryThe sex attractant pheromone produced by the female of the moth Utetheisa ornatrix was shown to contain Z, Z, Z-3, 6, 9-heneicosatriene. The compound, whose structure was confirmed by synthesis, proved active in electroantennogram and field bioassays. Pheromone emission occurs discontinuously, in the form of short pulses (pulse repetition rate=1.5±0.2 pulses/s). It is argued that such temporal patterning — which had not previously been demonstrated for an airborne chemical signal — can provide close-range orientation cues to the male moth as it seeks out the female.

Attractive and defensive functions of the ultraviolet pigments of a flower (Hypericum calycinum)

The flower of Hypericum calycinum, which appears uniformly yellow to humans, bears a UV pattern, presumably visible to insects. Two categories of pigments, flavonoids and dearomatized isoprenylated phloroglucinols (DIPs), are responsible for the UV demarcations of this flower. Flavonoids had been shown previously to function as floral UV pigments, but DIPs had not been demonstrated to serve in that capacity. We found the DIPs to be present in high concentration in the anthers and ovarian wall of the flower, suggesting that the compounds also serve in defense. Indeed, feeding tests done with one of the DIPs (hypercalin A) showed the compound to be deterrent and toxic to a caterpillar (Utetheisa ornatrix). The possibility that floral UV pigments fulfill both a visual and a defensive function had not previously been contemplated. DIPs may also serve for protection of female reproductive structures in other plants, for example in hops (Humulus lupulus). The DIPs of hops are put to human use as bitter flavoring agents and preservatives in beer.

CATNIP: ITS RAISON D' ETRE.

Catnip (nepetalactone) is closely related chemically to certain cyclopentanoid monoterpenes recently isolated from insects, and it shares with some of these terpenes an ability to repel insects. It is suggested that the adaptive function of catnip is to protect the plants that produce it against phytophagous insects.

Biochemistry at 100°C: Explosive Secretory Discharge of Bombardier Beetles (Brachinus)

The defensive chemical spray of bombardier beetles is ejected at 100�C, with a heat content of about 0.2 calorie per milligram.

Ultraviolet Video-Viewing: The Television Camera as an Insect Eve

A television camtiera, like the eyes of some insects, is sensitive to ultraviolet light. When equipped with an appropriate ultraviolet-transmitting lens, such a camera can be used for the direct examtiination of ultraviolet reflectiont patterns (for example, on flowers, butterflies) that are invisible to us, but visible to inisects.

Defensive behavior of a termite (Nasutitermes exitiosus)

Summary1.The defensive behavior of the termite Nasutitermes exitiosus (Termitidae, Nasutitermitinae) was studied in the field and laboratory. The termites were confronted with live arthropod enemies, mostly ants, and with animated mechanical dummies that served as surrogate enemies.2.Both soldier and worker termites take part in the defensive actions. The weapon of the soldier is the secretory spray from its frontal gland, ejected from the pointed nozzle, or rostrum, at the front of the head. The spray is a viscous entangling agent, capable of quickly hindering the mobility of ants. The secretion also acts as an irritant, inducing scratching and other preening reflexes that in turn cause the sticky contaminant to be further spread over the surface of the enemy. Bioassays with cockroaches and flies showed irritancy to be attributable to such monoterpenoid components of the secretion as α-pinene and β-pinene. Scanning electronmicrographs of sprayed ants revealed that the secretion can also cause such incidental topical effects as spiracular occlusion and blockage of sensilla. The worker termites have no special weapons, but they can effectively bite. Ants may be crushed by the bites, or they may be slowed down by workers clamped to them with their mandibles and thereby rendered more vulnerable to being sprayed by soldiers. None of the ants, spiders, and centipedes presented to our termites in laboratory tests survived the encounters.3.The soldiers secretion is an effective alarm pheromone. Once a target has been sprayed, other soldiers converge upon the site and deploy themselves around it. Attraction is effective up to a radius of 30 mm. Recruited soldiers add their own spray to target only if they are themselves assaulted upon arrival. Only direct contact stimulation causes soldiers to spray. The substance(s) in the secretion responsible for the alarm response remain(s) unknown. The workers are essentially unalarmed by fresh secretion.4.Recruited soldiers tend to remain longer beside a “lively” sprayed enemy than beside one that has already ceased moving. As was apparent from the tests with mechanical dummies, the soldiers seem to gauge liveliness directly by contact, and by monitoring the slight air motions engendered in the immediate vicinity of the enemy by its movements. Conventional sound and substrate vibration appear to be of minimal importance in the detection of liveliness.5.The attractiveness of discharged secretion to soldiers subsides with time, but more gradually than might be expected from an alarm pheromone. Even two days after discharge, secretion still elicits significant soldier approach rates. It is suggested that in nature, sprayed incapacitated enemies might be covered over by the workers with feces and soil, a behavior that could bring about a more timely blockage of the alarm signal.

Adhesiveness of Spider Silk

Moths, by virtue of the loose scales that cover their wings and bodies, are admirably adapted to elude capture by orb-weaving spiders. Rather than sticking to the web, they may simply lose some of their scales to the viscid threads, and then fly on. Other insects, covered with detachable hairs or waxy powder, are similarly protected against entrapment. Quantitative data are presented on the adhesiveness of spider thread to insect cuticles oq various kinds.