ALARM PHEROMONES AND THE INFLUENCE OF PUPAL ODOR ON THE AGGRESSIVENESS OF Polybia paulista (IHERING) (HYMENOPTERA: VESPIDAE)

Original paper

ALARM PHEROMONES AND THE INFLUENCE OF PUPAL ODOR ON THE AGGRESSIVENESS OF Polybia paulista (IHERING) (HYMENOPTERA: VESPIDAE)

M.F. MANZOLI-PALMA

, N. GOBBI , M.S. PALMA

1 Department of Ecology, Institute of Biosciences of Rio Claro, UNESP, State of São Paulo, Brazil; 2 Center for the Study of Venoms and Venomous Animals - CEVAP, UNESP, State of São Paulo, Brazil; 3 Department of Biology, Institute of Biosciences of Rio Claro, UNESP, State of São Paulo, Brazil.

ABSTRACT. Field bioassays were used to demonstrate that aggressive behavior of Polybiapaulista (Ihering) workers is elicited by alarm pheromones present in the venom reservoirs of nest defenders and that the brood care pheromone (pupal odor) produced by the young inside the nest also plays an important defensive role. Pupal odor was extracted from the surface of pupa bodies with methanol. When bioassayed alone, the pupal odor elicited only attractiveness of workers towards the odor source, but no stinging attacks were observed. However, in the presence of alarm pheromones, the brood care pheromone potentiated the effect caused by the pupal odors, increasing the number of stinging attacks during an action of colony defense. Thus, the presence of pupae within the nest evidently not only releases brood care but also enhances the aggressiveness of workers in P. paulista colonies.

KEY WORDS: insecta, wasps, brood, defense behavior, venom reservoirs.

INTRODUCTION

Alarm pheromones are very common in social Hymenoptera which use these substances to warn the colony of a dangerous situation, thereby eliciting an alarm response and recruiting nest defenders(4,10,11,14). Among the social wasps, chemically mediated alarm behavior has been described in the subfamilies Vespinae(1,2,7,10,11,15) and Polistinae(5,6,13).

Some species of social Hymenoptera have developed autotomy of the sting apparatus as part of the mechanism in defending their colonies(3,12). One such species is Polybia paulista (Ihering) for which the number of stings found on targets employed in field bioassays can work as an indicator of the wasp's aggressiveness(8). Manzoli-Palma and Gobbi(9) reported that the aggressiveness of a colony of P. paulista depends on the number of workers and pupae present in the colony.

In the present work, we examined some of the variables that may influence the aggressiveness of P. paulista, by bioassaying with extracts of body parts and pupal washings applied to artificial targets.

MATERIALS AND METHODS

The nests of P. paulista used in the present work were located in Rio Claro, in the state of São Paulo, southeastern Brazil.

Tests to quantify the aggressiveness of nest defenders were carried out as described by Stort(16) and Overal et al.(12). Balls (5 cm in diameter) of black suede leather were hung from a nylon line held 15 cm from the nest entrance for periods of 1 min. In these experiments, visitors were considered as those wasps that landed on the target, but did not sting it and aggressors were those that did sting and left their sting apparatus on the target surface. The intensity of the wasps aggressiveness was quantified by counting the number of stings left on the targets. The number of visitors was quantified with a mechanical counter. The above scoring procedure was applied to all tests performed with a given nest (population = ± 2900 individuals). Two protocols were employed during the field bioassays. The protocols, which lasted 30 min each and were applied on consecutive days, are described below:

BIOASSAY OF EXTRACTS FROM BODY PARTS: Extracts were prepared by separately grinding (with a pestle mortar) the heads, thoraxes and abdomens from 15 workers in 2 ml of distilled water. These homogenates were then applied to the surface of a target with Pasteur pipettes. In some cases, the abdomens were dissected and extracts from their parts were prepared and bioassayed.

EFFECT OF ALARM PHEROMONES AND PUPAL ODOR ON THE AGGRESSIVENESS OF P. paulista WORKERS: These trials were run using two targets placed 10 cm apart and presented simultaneously 15 cm from the nest entrance. One of the extracts to be tested was applied to the surface of a target, while the other target was coated with either distilled water or another extract. Thus, the effects of alarm pheromones and pupal odor could be investigated, either alone or simultaneously with another extract. This arrangement allowed the study of possible interactions between different chemical stimuli. The targets were numbered 1 and 2 for later identification.

Fifteen black-eyed pupae were washed by shaking for 60 min either with 3 ml of distilled water, 3 ml of methanol or 3 ml of chloroform. Half a milliliter of each washing was applied to the surface of the targets using Pasteur pipettes. The solvents used in the foregoing extractions were tested alone as control targets. An extract of venom reservoirs (as source of alarm pheromones) from 15 workers was also applied to 2 ml of distilled water as described above and bioassayed (0.5 ml / target).

RESULTS AND DISCUSSION

All extracts tested were highly attractive to Polybia paulista workers and, with the exception of venom reservoir extracts, no stinging attacks were observed. At least part of this attractiveness may be due to the presence of alarm pheromones which are apparently mixed with another odorous substance that inhibits or confounds the aggressive behavior of nest defenders. The unknown odorous substance may be derived from the kin recognition pheromone present in the extract.

In order to identify the origin of the alarm pheromones, aqueous extracts of the body parts from P. paulista adults were bioassayed for alarm-inducing activity. Only abdominal extracts demonstrated attractiveness and elicited stinging attacks (Figure 1). This result agrees with the observations of Jeanne(5), Landolt and Heath(7) and Heath and Landolt(2) that alarm pheromones appear to be stored in the venom reservoirs of social wasps. To confirm the source of alarm pheromone, the venom reservoirs were removed from P. paulista workers and homogenized in water. When applied to the surface of targets, this extract showed attractiveness and elicited stinging attacks (Figure 1).

Recently, Manzoli-Palma and Gobbi(9) suggested that P. paulista pupae produce an odor that influences the aggressiveness of P. paulista workers. To examine this possibility, pupae were washed with methanol, water or chloroform in order to extract the substance responsible. Methanol and water alone had no influence on the number of visitors and/or target aggressors (Figure 2). However, the pungent odor of chloroform strongly repelled P. paulista workers. This repellency meant that evaluation of chloroform washes was not possible.

Water was not a suitable solvent for the extraction of pupal odor since the attractiveness and aggressiveness elicited by the pupal washes were very similar to those observed with water alone. When a methanolic extract of pupae was bioassayed, a strong attraction of nest defenders towards the odor source was observed. During this period, marked palpation with antennae and licking of the target were noted. The control target (methanol alone) was also visited, although none of the foregoing behavior was seen. There were only a few stinging attacks against the targets, but these were not elicited by pupal odor. Thus, methanol appears to be a suitable solvent for the extraction of pupal odor since it is highly attractive and does not elicit aggressive behavior in P. paulista workers. This was confirmed by a bioassay in which both the pupal odor (methanolic wash) and alarm pheromone sources were simultaneously applied to the same target. The result was a high attractiveness of P. paulista workers which flew directly to and landed on the target surface, which was then antennaed, but not stung (Figure 2). Thus, a mixture of both odors at the same site elicited some alarm behavior, but the aggressiveness of the nest defenders was inhibited. These results suggest that pupal odor works as a brood care pheromone.

In order to further examine the effects of pupal odor and alarm pheromones, bioassays were performed using two targets simultaneously (Figure 3). During these bioassays, the effect of solvents applied to the surface of target 1 (control) was the same as that described above for bioassays with a single target.

When the venom extract was applied to target 2, all the aggressiveness was directed towards this target. All workers attracted to this target stung it, but showed no interest in target 1. This result suggests that P. paulista workers are able to identify the origin of alarm pheromones. It should be noted that the high aggressiveness observed against target 2 may have been enhanced by a cooperative effect resulting from the release of alarm pheromones immediately after each stinging attack. The methanol extract of pupae was highly attractive to P. paulista workers, both towards target 2 over which the extract was applied and towards target 1 which received only methanol. The latter target probably received visits as a consequence of pupal odor diffusion from target 2.

The interaction between alarm pheromones and pupal odor was also investigated using two targets in which the alarm pheromone source was applied to target 1 and the source of pupal odor was applied to target 2 (Figure 4). The attractiveness elicited by pupal odor was similar to that found when this extract was bioassayed alone (Figure 3). However, there was an increase in attractiveness towards the target that received the alarm pheromone, as already noted when the pupal odor extract was bioassayed alone. Almost all workers attracted to target 1 became its aggressors, probably due to the presence of alarm pheromones. Thus, pupal odor seems to act as a chemical signal to indicate the presence of pupae in the nest, attracting the workers which would care for and defend the pupae against predators. Although pupal odor does not elicit alarm behavior, it appears to interact with alarm pheromones by potentiating their effects. This interaction may be a feedback strategy to improve the efficiency of colony defense.

FIGURE 1. Effect of extracts from the body parts of adult Polybia paulista workers on the attractiveness and aggressiveness of nest mates. Each bar represents the mean frequency ± SD.

FIGURE 2. Bioassays of pupal washings using one target to assess the attractiveness and aggressiveness of Polybia paulista workers. Each bar represents the mean frequency ± SD.

FIGURE 3.
Bioassays with two targets showing the effect of alarm pheromone source and/or the pupal washings on the attractiveness and aggressiveness of Polybia paulista workers. The application of odor source was carried out as detailed below.

FIGURE 4. Interaction between alarm pheromones applied to the first target and pupal odor (methanol washing) applied to the second target on the attractiveness and aggressiveness of Polybia paulista workers. Each bar represents the mean frequency ± SD.

REFERENCES

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Received 11 November 1996

Accepted 24 March 1997

CORRESPONDENCE TO:

M.F. MANZOLI-PALMA, Caixa Postal 199, CEP 13.506-900, Rio Claro, São Paulo, Brazil.

Publication Dates

• Publication in this collection
  26 Nov 1998
• Date of issue
  1998

History

• Received
  11 Nov 1996
• Accepted
  24 Mar 1997