Abstract

    The insect world is a unique and diverse world in many ways.  The insects in the order Hymenoptera (which means membrane wings) are no exception of this.  These insects, which include the ants, wasps, bees, and hornets,  are known for living in colonies together.  The members of these colonies all work together to survive in the predator eat prey world of insects.  There are many different components and behaviors that these colonies or hives have adopted over the course of their evolution.  Defense, foraging, and swarming are just a few of these behavioral patterns, not all of which are understood fully even to this day.  Some members of Hymenoptera; however, are solitary insects which also construct nests for the rearing of their offspring.  These insects may not be communal, but they also possess defensive tactics of their own.  This review article is going to explore some of the various ways that some of the insects of the order Hymenoptera defend themselves from predators and parasites.  The reasons for these behaviors are actually relatively straight forward.  A colony of wasps or bees must have some kind of defense to prevent the destruction of their home or future offspring.  Predators range from a vertebrate animal such as a lizard to even a neighboring colony of Hymenoptera insects.  These behavioral defense patterns range from the outright aggressive attacks of the “Africanized” bees, the chemical signaling of Apis dorsata using “defense waving” behavior and the Nasonov gland, to the venom spraying of the wasp Parachartergus colobopterus.  These are just a few of the many different examples that will be covered in this review article about defensive behavior patterns of Hymenoptera insects.

Introduction

    The Hymenopteran insects tend to make elaborate nests from which their workers emerge.  These insects are known as eusocial insects, or caste forming insects.  (Alcock 1998)  A single female would have a rough time raising her brood in the hostile environments that most insects live in.  If, however, she enlists the help of others in the care of her brood, her young would still be raised if she were to perish.  (1998)  This nest in turn becomes the investment of the female, or queen in some cases, of her genes for the next generation.  Because of this, several intricate defensive patterns have arose and are used by these insects.  Of the eusocial Hymenopterans, this paper will take a look at how some of the honeybees and wasps defend their nests from the likes of some predators.

Defense of Honeybees

Aggression:  The Direct Approach to Defense

    Apis mellifera scutellata or the “Africanized bee,” uses outright aggression in their defense.  This behavior has given them the name of  “killer bees.”  Several different experiments have been conducted on what causes this aggressive behavior that is defensive in nature.  To start, A. m. scutellata is more aggressive than its cousins the European honey bee Apis mellifera.  Early experiments were conducted to show this difference in aggression between the two species.  To quantify this aggressiveness, researchers developed a “sting recorder,” which consists of a black leather ball which is jiggled at various distances from a hive for sixty seconds.  At twelve inches away from the hive, the “sting recorder” averaged about two stings from A. mellifera and about fourteen stings from A. m. scutellata.  It was also observed that A. m. scutellata would pursue the ball an average of about 500 feet, compared to 70 feet for the A. mellifera.  (Evans 1985)  When bees sting, an alarm pheromone is released, stimulating more bees to attack the target.  Africanized bees tend to be more responsive to this pheromone than the European bees.  This alarm pheromone was also used in another experiment involving the aggressiveness of the Africanized bee.  The next experiment also deals with the aggressive nature of the Africanized bees compared to European bees.  The alarm pheromone was sprayed above nests of both European and Africanized bees.  This experiment was designed to test the number of bees at the hive entrance which reacted to the “attack,” reaction time of emergence of bees from the hive, and the number of stings received in the suede “sting recorder.”  The experiment was conducted using 150 large colonies from Baton Rouge, Louisiana and 147 similarly sized colonies from Monagas, Venezuela, the latter being bees allowed to breed with the feral Africanized bees in the area.  The experimental conditions; temperature, relative humidity, and hive composition were constant.  The results of this experiment are as follows:  There were more Africanized bees present at the hive entrance at all times.  The European bees required the stimulus for orientation and attack before becoming airborne.  Quite opposite are the Africanized bees which tended to go airborne immediately after the pheromone spraying took place.  The Africanized bees also reacted faster both the alarm pheromone and the moving target.  The Africanized bees averaged about five seconds of reaction time to the alarm pheromone opposed to about thirteen seconds for the European bees.  For the moving suede sting recorder, the Africanized bees averaged about one second compared to about ten seconds for the European bees.  Once again, the Africanized bees had more stings, eighty five stings within thirty seconds compared to about fifteen stings for the European bees.  (Collins, 1982)  For the Africanized bees, aggression seems to be the key defensive measure taken against predators.

Some Factors that may Influence Defensive Behavior of A. m. scutellata

    The high level of aggression seen in A. m. scutellata may be attributed to environmental factors such as weather, flower cycles, etc. and/or intracolony conditions. Stanley S. Schneider and Linda C. McNally conducted an experiment in 1992 to test these ideals.  This study was conducted in the Okavango River Delta in Botswana.  The eighty test nests were dissected and factors such as food storage, area of brood devotion, and empty space were determined.  These nests were monitored and dissected throughout the year to measure results.  Only seventy one of the eighty colonies were accounted for due to some colony differences.  For example two colonies were queenless and declining.  For these reasons, these colonies were discarded and only the seventy one remaining colonies were accounted in the study.  This study showed a greater aggression in October, a time when swarming is common.  This also coincided with the latter portion of the of the peak abundance of floral resources in the area.  The lowest time of aggression occurred in March, when there was a brief increase in the availability of blooming trees and shrubs observed.  There was also an increase in defensive behavior in May which was attributed to colony defense of food stores necessary to survive the death period.  Defensiveness of the hive was also related to the size of the colony.  Newly established nests were shown to exhibit the lowest levels of defensiveness in this study.  However, this all makes sense because when a honey bee worker stings, it will eventually die.  Smaller colonies would lose countless workers and be unable to replenish the work force if there were repeated attacks on the colony by predators.  After all, the workers keep the hive alive by performing their duties.  Larger colonies on the other hand, are more able to mount massive attacks to protect their larger investments.  The two main conclusions of this study are as follows:  First, the high levels of stinging may not be a characteristic of Apis mellifera scutellata.  There might in fact be different reasons for this aggression.  These reasons could be the founder effect and hybridization or selective forces that have operated during the rapid spread and colonialization of Central and Southern America.   Second, this study shows that the defensiveness of A. m. scutellata may change with seasonal changes in colony growth, development, and resource availability.  (Schneider 1992)  To elaborate on this point, another study by Anita Collins is examined.  This experiment was performed on both European and Africanized honeybees.  The first part of the experiment, performed on both species, examined the response of the colonies based on comb size.  The results of this experiment indicate that honeybee defensive behavior is indeed influenced by the amount of comb surface area in the colony.  When a moving target was used, the bees responded twice as fast as bees from the colony with less colony surface area.  (Collins and Rinderer 1984)  This trend was apparent in both the European and Africanized honeybees.  The second part of the experiment involved the removal of honey storage comb from the hives of European honeybees.  This elicited an interesting characteristic in the colonies, they became more aggressive.  When the honeybees were tested one week after the removal of the honey comb stores, more bees are present at the entrance of the hive and more stings were counted.  Rinderer attributes the increase of defensive behavior to the presence of volatiles in the empty comb.  These comb volatiles may function as primer pheromones, altering the physiology of worker bees making them more responsive to the primary stimuli eliciting colony defense.  (1984)  This appears to be a balance in the amount of stored honey in the hive.  Colonies with smaller honey stores, such as the feral Africanized honeybees, would be more aggressive in defending their food stores than an established colony with ample food.  (1984)

The Role of Genetics in Honeybee Defense

    Dr. Warwick Kerr of Brazil was the first scientist to determine that the aggressiveness of Africanized bees is a genetic trait.  Responding to the public’s outcry to the escape of the African bees and their spread across Brazil, he began to supply the local beekeepers with European queens in hopes of weeding out the aggressive nature of the “killer bees.”  (Evans 1985)  More recently, Gloria DeGrandi-Hoffman, Anita Collins, and others have performed a mating experiment between Africanized and European honeybees to determine if the lineage of the honeybee is related to the defensive behavior it exhibits by determining the proportion of Africanized honeybees and European honeybees participating in colony defense in hives headed by either Africanized or European honeybee queens with both African and European subfamilies present.  (DeGrandi-Hoffman et al. 1997)  Their experiment involves the use of the Cordovan gene, which is a gene for body color.  The homozygous recessive alleles produce a distinctive yellow-brown cuticle colors in bees.  The bees which exhibited this feature were the European bees used in this experiment.  The African and Cordovan recessive European honeybees were crossed and workers were determined by these genes.  The African workers were either completely black or had a black thorax and a solid black stripe running down their abdomen.  The European workers were brown without any black on their first three body segments.  The colonies were then allowed two months to “mature.”  The human breath stimulus was used to test the bees.  Also a piece of dangling black Velcro was used to visually stimulate the bees.  The workers were sampled and the results obtained.  The colonies containing workers fathered by African honeybees were the most defensive of the colonies.  The next defensive colonies were those containing both African and European patrilines.  African honey bee queens inseminated with only European honeybee semen were the least defensive.  This study supports the hypothesis that the queen’s genotype has little effect on colony defense. (1997)  This could be the main reason that the Africanized bees have maintained their aggression despite massive attempts to weed out the aggression from the species.

Defense Waving and the Nasonov Gland in Apis dorsata

    The Asian giant honeybee, Apis dorsata, exhibits an interesting defensive behavior known as defense waving.  The hive of these honeybees consists of a single honeycomb which is covered by several layers of bees forming a curtain over the hive.  When these honeybees are disturbed, the bees shake their abdomens in a synchronous fashion which looks like a wave moving over the curtain of bees.  This defensive measure is said to repel predators effectively. (Kastberger et al 1998)  Gerald Kastberger and associates performed a study on A. dorsata to find out why this abdominal shaking takes place.  The test nests were filmed while being stimulated by a dummy wasp.  The results of this study show the involvement of the Nasonov gland in the defensive behavior of A. dorsata.  When the bees are stimulated, they thrust their abdomens ninety degrees in an upward direction.  This behavior was exhibited by the bees on the upper layer of the curtain of the colony.  Many bees stroked their wings and thrust their abdomens exposing the Nasonov gland.  The Nasonov gland resides in the intergitial membrane in between the sixth and seventh abdominal segments.  (1998)  Kastberger and associates have hypothesized three probable roles of the Nasonov gland, since it is still unknown for sure what the gland’s role is.  The first possible role for the Nasonov gland is it may function to act as a direct and defensive weapon similar to excreted allomones from arthropods.  This is unlikely because the glands structure is not like a defensive gland.  The second possible role is that of the transfer of information to a neighboring bee to induce the abdominal shaking behavior.  This is also unlikely due to the quick nature of the abdominal shaking.  The third role might be to maintain defensive stability in the bees during an attack.  This is the most probable role for the Nasonov gland.  This would deter individuals from performing individual acts.  The defensive waving would in fact maximize defense for the hive to protect it from predators.  (1998)

Colony defense of Wasps

Defense of Parachartergus wasps

    Wasps and bees, although from the same order of insects, exhibit different methods of colony defense.  Joan E. Strassmann, Colin R. Hughes, and David C. Queller investigated the various aspects of colony defense of the Social Wasp, Parachartergus colobopterus.  The aspects studied are nest camouflage, defense behavior, mimicry of adults, and the tastiness of larva.  A typical nest of a Parachartergus colobopterus is gray in appearance and is composed of several cells wrapped in an envelope.  These nests resemble a carton in shape.  The nests also have lichens and other parts of plants built into the envelope which adds to the camouflage of the nest.  When the nests of P. colobopterus are disturbed, the workers emerge from the nest and march up the carton, making a arrhythmic rustling sound from the vibration of their gasters against the envelope of the colony.  If these disturbances continue, the workers fly off the nest, circle the area, and eventually leave.  These wasps rarely try to sting their attackers.  The workers do sting; however, in self defense.  The individual will protect itself rather than the whole hive.  A possible explanation for this lies in the mimicry that P. colobopterus exhibits.  It’s body is yellow with brown bands, similar to more aggressive species in the area.  The larva of this species were once thought to be distasteful to predators, but after testing this was not found to be so.  The adult wasps do however mimic species of distasteful larva by adopting the posture of the related toxic larval species.  P. colobopterus may be more docile due to the lack of vertebrate predators on this species.  (Strassmann et al. 1990)  Robert L. Jeanne and Malcolm G. Keeping went further into the defensive behavior of P. colobopterus.  They found out that P. colobopterus does defend it’s nest, just in a more obscure manner.  The start of the alarm recruitment of this species is triggered by environmental stimuli.  Like the Strassmann study, the wasps emerged and spread out across the nest.  The carton tapping then ensued like before.  Jeanne and Keeping suggest that the tapping is an alarm signal which causes more workers to exit the nest and fan out across the nest.  If not visually stimulated, the wasps would slowly return to the inside of the nest.  However if the wasps are visually stimulated, they would sometimes turn their gaster to one side, pointing the tip forward.  The wasps would then spray a mist into the air in front of them.  The mist is sticky in texture and is thought to be venom.  The reasoning behind it being venom is enforced by two observations of Jeanne and Keeping.  The first is that the spray has the same odor as the stinging odor of a worker stinging in self defense.  The second observation is that the venom sac is the only exocrine gland at the tip of the gaster and that is capable of performing a spray.  Another interesting defensive behavior is exhibited in the defense of P. colobopterus from ants.  When a worker encounters an ant, it begins to rears the front of it’s body up and back, buzzing it’s wings in brief turns, attempting to blast air at the ant.  This is then sometimes followed by a “pecking” behavior.  The worker tries to grasp the ant and rebound off the nest, throwing the ant off the nest.  Venom spraying was not observed in defense against ants.  (Jeanne and Keeping 1995)  Parachartergus fraternus, a more aggressive species, was also studied by Jeanne and Keeping.  This species of wasps rapidly exited the nest upon disturbance.  There was no gaster tapping observed by this species of wasp.  These wasps took flight and attempted to land on and sting the disturbance source.  Venom spraying was also not observed in this species of wasps.  (1995)

Defense of Polistes wasps

    Polistes wasps are also social wasps which build colonial nests.  Polistes canadensis, a large Polistes wasp with a reddish brown body and brown wings, shows strong defensive aggressions among females of it’s species.  In fact, this wasp is most aggressive towards humans.  P. canadensis will also attack other Polistes’ nests, taking the larva for food.  It appears that unmated females of this species are not as aggressive as mated females.  (Ito 1993)  This would have to do with the existence or future existence of progeny.  Tim M. Judd has studied the defensiveness of wasps throughout of the colony cycle of Polistes fuscatus.  Judd tested this by eliciting attack on the wasp hive using a bird model.  The level of aggression of these defensive attacks were scaled on a scale of one to five, from least aggressive (wasp abandons nest) to most aggressive (wasp left the nest and stung the model).  These were recorded over the colony cycle.  The study concluded that as the reproductive cycle neared, aggression in the became more apparent.  (Judd 1998)  This aggression was not shown to correlate with the number of workers however.  In period four of Judd’s study, aggression decreased although the number of workers is high.  This is due to the fact that the reproductive gynes have left the nest to mate and start colonies of their own.  (1998)  Polistes fuscatus and Polistes exclamans were examined by David C. Post and associates for traces of venom spraying in this genus of wasps.  Bioassays were performed on these two species in search of an alarm pheromone present.  P. exclamans wasps reacted to the venom by rapid movement and wing fluttering.  The wasps though did not attempt to sting the models used for visual stimulation.  P. fuscatus responded with great alarm directing their responses upwind.  (Post et al. 1984)  However during this experiment, there was no evidence of venom being released by the wasps even though the wasps did open their sting chambers.  Post and associates have three possibilities on why the wasps open their sting chambers.  First, the sting chamber opening might be an inadvertent consequence of gaster bending.  Second, it may be a visual defensive display to predators or nestmates.  Third, it may be in preparation to stinging.  (1984)
 

References

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