Aggregation Pheromones of
Sap Beetles (Nitidulidae: Coleoptera)
R. Jason BishopColorado State UniversityFort Collins, Colorado 80523rjasonb@lamar.colostate.edu Abstract Several species within the genus Carpophilus areа known pests on a wide variety of hosts.а This paper reviews the elements involved in the attraction of theseа sepcies to their hosts, including host-related volatiles, aggregation pheromones , and the interaction between the two.а Kairomonal activity between species is also discussed.а This review is presented inа context of the value of knowledge of attraction of sap beetles to possible control strategies for beetle populations. Introduction ааааааа In the animal world, humans are not unique in the need to communicate.а Communication between and among species takes on many forms, from a simple visual cue to a complex set of chemical cues.а Among insects, communication often involves mechanisms which prove too difficult to understand from a human perspective without further investigation.а Humans tend to take for granted the chemical processes which occur in nature, as well as in our own bodies at any given time.а Chemical reactions are not always readily visible, and therefore tend to pass unnoticed. ааааааа Chemical communication among insects presents a diversity of forms and functions.а Among the primitively eusocial paper wasps (Polistes, Hymenoptera: Vespidae), cuticular hydrocarbons are the primary means by which nestmates recognize one another (Gamboa, 1987).а Lepidopteran species are known to use chemicals from a wide range of chemical classes for mating purposes.а Females release copious amounts of sex pheromones into the air to attract males, while males use pheromones to induce the females into mating once they have found their target (Jacobson, 1970).а Several species of arthropods are known to use chemicals for defense (Berenbaum, 1995).аа Chemicals are also known to be used by insects to insight mass aggregations.а The bark beetle Dendroctonus ponderosae (Scolytidae)а is a well documented example of such behavior (Ali, 1990).аа ааааааа Commonly referred to as sap beetles, members of the genus Carpophilus (Coleoptera: Nitidulidae) are cosmopolitan pests which occur on a wide variety of hosts, including fruits and grains, both before and after harvest (Hinton, 1945).а This paper reviews the attraction ofа sap beetles to different sources.а Sap beetles are known to respond to male-produced aggregation pheromones as well as host-related volatiles (Bartelt, 1993a; Blumberg, 1993).а Several species of sap beetles are known agricultural pests, occurring on a wide variety of host plants (Bartelt, 1995).а The presence of large numbers of sap beetles on a host plant can prove economic in terms of crop damage caused by theа feeding beetles, but impact on crop value is primarily due to the presence of beetles in products ready for sale (Bartelt, 1992).а In addition to damage caused by feeding, sap beetles are also known to transmit fruit -degrading microorganisms, such as brown rot [Monolinia fructicola (Wint.) Honey] in stored fruits (Tate, 1975). Damage of this nature to high value crops, such as stone fruits presents a need to understand the chemistry and biology of pheromones in nitidulid beetles so that tools for monitoring and controlling sap beetle populations may be developed (Bartelt, 1995). Chemical Communication ааааааа Before moving further, it is important to review some basic terminology used in chemical ecology.а The term semiochemical is used to describe any chemical which conveys information between organisms (Law & Regnier, 1970).а Semiochemicals, from the Greek semeion (= signal), can be further divided into two subcategories (Nordlund & Lewis, 1976):а (i) allelochemicals, and (ii) pheromones.а The division is important in that it distinguishes the receiver of the chemical message from its sender.а ааааааа From the Greek allelon (= of one another), allomones are chemicals which convey interspecific information. The receiver of the chemical message is of a species different from its sender (Ali, 1990).а Allomones can be further categorized according to the advantage ofа the behavioral response elicited by the released compounds (Ali, 1990).а If the behavioral response is advantageous to the receiver ofа the message, as with predators locating prey, the compound is termed aа kairomone (Brown et al., 1970).а If the advantage goes to the originator of the compound (i.e. defensive secretions) the allelochemical is referred to as an allomone (Brown et al., 1970).а аFor cases in which both the originator and receiver benefit, the allelochemical is appropriately called a synonome (Nordlund & Lewis, 1976).а Synonomes are exemplified by pollinating insects (e.g. honey bees) responding to floral scents (Ali, 1990).ааааааа Chemicals released by organisms for intraspecific communication are called pheromones.а Derived from the Greek, (pherein = to transfer; horman = to excite) pheromones are defined as substances which, when secreted to the outside by one individual and received by a second individual of the same species, will elicit a specific response (Karlson & Butenandt, 1959).а The categories of pheromones are numerous.а For the purposes of this paper, only those pheromones which cause conspecifics to increase their density in the vicinity of the pheromone source will be discussed.а Such chemicals are known as aggregation pheromones (Ali, 1990).аа Aggregations likely serve to draw together the sexes, whereby increasing the probability ofа mating and exchanging genetic material (Ali, 1990).аа Exceptions can be found.а Among parthenogenic aphids (Homoptera), for example, aggregating serves a "safety in numbers" role. Bark beetles (Scolytidae), congregate in large numbers as a means to facilitate mating, but also to overcome the chemical defenses of their host plants (Birch, 1984).а This "mass attack" of host material allows beetles to colonize the host despite the trees' efforts to ward them off. Attraction of Sap Beetles to Pheromones and Host-Related Volatiles Host-related volatiles ааааааа As stated previously, sap beetles can be found on a wide variety of hosts.а Although many Carpophilus species are characterized by their affiliation with a specific host or group of related hosts, review of the literature indicates that the beetles are not restricted in their host range.а For example, Carpophilus davidsoni (Dobson), a small reddish brown sap beetle occurring in Australia, New Zealand, and Micronesia is known to infest Australian sweet corn, figs, and peaches (Bartelt, 1994).а Carpophilus obsoletus (Erichson), a small dark brown sap beetle, which occurs throughout tropical, subtropical, and milder temperate regions of the world, also attacks corn, but is a more serious pest of dried fruit commodities such as dates (Petroski, 1994).а The driedfruit beetle, Carpophilus hemipterus, a more prolific species, is a pest on fresh and dried fruit, many fresh and stored grains, spices, drugs, and seeds (Dowd, 1991; Hinton, 1945).а The corn sap beetle, Carpophilus dimidiatus (F).а can be found on corn, peanuts, cacao, and various spices, either fresh in the field or in storage (Connell, 1975).а The dusky sap beetle, Carpophilus lugubris (Murray) is a beetle pest that is seen to infest a wide variety of ripening fruits and vegetables, particularly sweet corn (Harrison, 1974).а Finally, the confused sap beetle, Carpophilus mutilatus (Erichson), feeds mostly on fallen and decomposing fruits, but can also be found on ripening fruit that has suffered damage (Bartelt, 1993a).а It is clear, then, that sap beetles occur on a wide range of hosts, with some specificity relative to each species, but with some overlap as well.а Host selection is not a simple proposition for sap beetles.а Selection of appropriate host material is based on many chemical components and their interactions (Bartelt, 1993b).а Sap beetles respond to a wide range of host-related volatiles as well as aggregation pheromones.а Furthermore, several species of sap beetles show increased response to interactions of the host-related volatiles and pheromones (Bartelt, 1992). ааааааа Before one can look at the aggregation pheromones responsible for drawing in large numbers of beetles, it is important to first discuss host-related attractants, as the nitidulid beetles seem to be involved in a variety of interactions that go beyond simple pheromonal attraction under field conditions (Bartelt, 1993b)..а For the most part, sap beetles are drawn to volatiles from fermenting fruit and grains (James, 1993).а As discussed with C. mutilatus, sap beetles will also infest those hosts that may be ripening, but have first taken on damage (Bartelt, 1993a). Suitable hosts will draw in large numbers of beetles, and in a given complex of beetles occurring on a host, the composition of species can be fairly uniform in terms of numbers present.аа High numbers of several species of beetlesа can occur simultaneously in others instances.аа The complex of beetles occurring on fermenting fruit exemplifies the point (James, 1993).а High numbers of several Carpophilus species can be found infesting fermenting fruit, particularly if said fruit is damaged (James, 1993).а Interestingly, numbers tend to favor some species over others in traps baited with host-related volatiles (Blumberg, 1993; Lin, 1992; Bartelt, 1992).а The notion that certain species can be attracted more readily than others in the presence of specified host-related volatiles has important implications for control options.ааааааа A suite of seven volatile compounds has been identified from the head space of fermenting whole-wheat bread dough as particularly attractive to C. lugubris (Lin, 1992).а The chemicals were isolated and identified using wind-tunnel bioassays as a means to determine the composition of the attractants as well as the proportions needed for each component (Lin, 1991).а These chemicals were then synthesized and incorporated into bait attractants used in sap beetle traps where C. lugubris was present in economic numbers (Lin, 1991).а Similar studies have also been done on the attractiveness of fermenting fig juice volatiles to C. hemipterus (Bartelt, 1992).а In both cases the synthetic volatiles proved attractive to the target beetles, however more potent attractantsа were desirable (Blumberg, 1993).а Synergism between host-related volatiles and aggregation pheromones turned out to be the element necessary to add potency to attractants used in traps.Synergism Between Host-related Volatiles and Pheromones ааааааа Having specific attractants available for trap-out strategies is a promising alternative to traditional chemical control of sap beetle populations, however, as previously mentioned, traps with higher efficacy are desirable (Blumberg, 1993).аа Synergistic effects of host-related volatiles used in combination with aggregation pheromones have been tested in the field for both C. lugubris and C. hemipterus (Lin, 1992; Bartelt, 1992).ааа In both investigations, significantly higher numbers of beetles were reported to have been present in traps containing both the aggregation pheromone of the target beetle, as well as host-related attractants, than in traps containing each constituent individually (Lin, 1992; Bartelt, 1992).аа One exception to this interaction was observed in C. antiguus (Bartelt, 1993b).а This beetle was unusual in that the pheromone was quite attractive in the field without a synergist (Bartelt, 1993b). ааааааа Using wind-tunnel bioassays, aggregation pheromones have been identified for a number of species of sap beetle (Bartelt, 1991; 1993a; 1994; 1995).аа Sap beetle pheromones consistently turned out to be characterized as polyene hydrocarbons.а Both beetle-derived pheromones as well as synthetic versions are attractive to conspecifics , howeverа kairomonal effects of the pheromones between species has even been investigated (Bartelt, 1993b).а Although a novel pheromone has been identified for C. antiguus, these beetles respond quite well to C. lugubris pheromone (Bartelt, 1993b).а Bartelt (1993b) suggests the kairomonal effect to be quite important.а Both C. lugubris and C. antiguus respond best to their own pheromones, however the responsiveness of C. antiguus to C. lugubris pheromone is advantageous (kairomonal) in that a new host source is made available for C. antiguus.а Without the presence of C. lugubris on corn ears, C. antiguus would not normally be able to feed on kernels as it is not big enough or strong enough to penetrate the husk (Bartelt, 1993b).а Being that corn is a suitable food source for C. antiguus, it appears that this species uses C. lugubris pheromone to find the ears (Bartelt, 1993b).а Using wind-tunnel bioassays, Bartelt et al. (1993b) were able to evidence the kairomonal action of C. lugubris pheromone.аа Tests showed significant response of C. antiguus to C. lugubris pheromone, but very weak interspecific responses to other pheromones (Bartelt, 1993b). Kairomonal effects add yet another dimension to the chemical interactions taking place in the attraction of sap beetles to host material.а All dimensions are now in place and control strategies can be developed.а Implications for Control Strategies ааааааа With the knowledge of the various elements involved in attracting sap beetles it now seems possible to tailor control strategies for specific beetles or complexes of beetles which may be occurring in economic numbers.а For example, in a date garden being used as a test site it was observed that the most abundant species, C. mutilatus, did respond significantly to the pheromone of C. hemipterus (Bartelt, 1992).а When a synergist was added, C. mutilatus still showed attraction to the baits, but not in as dramatic proportions as C. hemipterus (Bartelt, 1992).а Additionally, C. lugubris, C. obsoletus, and C. humeralis showed synergistic attraction to the C. hemipterus pheromone, with only C. obsoletus responding to the pheromone alone (Bartelt, 1992).а It was also shown that C. freemani responds significantly to the C. hemipterus pheromone, but not as well as to its own pheromone used synergistically with bread dough (Bartelt, 1992; Bartelt, 1990b).а By manipulating various elements of trap design, one could conceivably become quite specific.а ааааааа The kairomonal action discussed between C. lugubris pheromone and C. antiguus presents a similar situation in which possible control strategies could be developed using specific attractive interactions.а If both C. lugubris and C. antiguus are present in a stand of corn, it is conceivable that trapping of both speciesа could be achieved through use of C. lugubris pheromone and a synergist (Bartelt, 1993b).ааа ааааааа It is clear that by adjusting various elements of a trap design one might be able to easily target specific species of sap beetle or entire complexes.а With the knowledge of synergistic effects of pheromones and host-related volatiles as well as the kairomonal effects of aggregation pheromones the potential to do so is evident.а Investigation into the biologies of sap beetle species involved is also critical.а One must first understand the relationships between a given species and its own pheromone alone, and in combination with host-related volatiles.а Interactions with pheromones of other species must also be elucidated to further understand the ecology of sap beetles and their host selection. 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in California stone fruits. Phytopathology. 65: 977-983.аааааааааааааааааааааааааааааааааа