Aggregation pheromones: behavior modifying for insect management

Dora Carmona

Abstract.- Pheromones are chemical signals from one organism that stimulate a response in another individual of the same species. Insects of different orders responds to some pheromones with aggregation behavior. Male-produced sex attractant have been referred to as aggregation pheromones, because they typically result in the arrival of both sexes at a calling site leading to an increase in density of conspecifics in the vicinity of the pheromone source. Aggregation pheromones have been reported for members of the Coleoptera, Dictyoptera, Hemiptera, Homoptera and Orthoptera and have been identified for hundreds of species. During the last two decades the attention has turned to economically important species especially members of the Coleoptera, Curculionidae. The value of applying aggregation pheromones in the management of the boll weevil (Anthonomus grandis (Boheman), pea and bean weevil (Sitona lineatus (L.)) and stored product weevils (Sitophilus zeamais (L.), Sitophilus granarius (L.) and Sitophilus oryzae (L.)) has been demonstrated. These are some examples in which the communication code of the insects leads to improved methods of monitoring and control. There is a wide range of tactics or methods of pest management and aggregation pheromones are among the most ecologically selective pest suppression agents. Unlike the conventional insecticides , they are no toxic and they are effective at very low concentrations.

Key words Aggregation pheromones, Curculionidae, boll weevil, pea and bean weevil, stored product weevils.

Introduction

Insects communicate sharing information coded in the form of signals, measurable in the language physics or chemistry, that alters the response patterns of another individual (Matthews & Matthews 1978). The most outstanding characteristic about insect odor sources has been their variety and complexity. Chemical come not only from the abdomen but often from head and thorax. They have a variety of exocrine glands, clusters of secretory cells whose products are discharged to the outside of the body. Insects receivers are no less complex; chemoreceptor cells are surrounded by characteristics cuticular specialization called sensilla which comprise the most obvious external parts of the chemosensory organs. They may occur in the antennae which are by far the most common organ of olfactory reception (Matthews & Matthews 1978). One important function of intraspecific communication systems is species and sex recognition.

Pheromones are chemical signals from one organism that stimulate a response in another organism of the same specie. Generally, this behavior is either attraction to the opposite sex or part of courtship interaction and are referred as sex attractants pheromones (Landolt 1997). Male-produced sex attractants often are referred to as aggregation pheromones because they typically result in the arrival of both sexes at a calling site (Matthews & Matthews 1978, Jutsum & Gordon 1989; Landolt 1997). Aggregation pheromones have been reported for members of the Coleoptera, Dictyoptera, Hemiptera, Homoptera and Orthoptera and have been identified for hundred of species. During the last two decades the attention has turned to economically important species especially members of the Coleoptera, Curculionidae. The production of aggregation pheromones by several weevils, including Anthonomus grandis, three Sitophilus spp., and Sitona lineatus is well documented (Hardee et al 1969; Tumlinson et al. 1969; Phillips et al. 1981; Blight et al 1987). According to Jutsum & Gordonn (1989), the management of these and other major pest insects using aggregation pheromones has been examined. Overall the use of pheromones has a number of potential advantages. The compounds are naturally occurring, generally non-toxic and should not pollute the environment. Pheromones are insect-specific and their safety to beneficials makes them ideal components of integrated pest management systems.

Insect olfactory orientation and chemical communication. Communication is vital, in establishing an insect’s credentials within individuals of a same species and in recognizing the credentials of other insects as being like or unlike itself. One important function of intraspecific communication system is species and sex recognition. Of all the various shemes that have been developed to classify communication, perhaps the most widely accepted and utilized has been the one based upon the receptor involved. In terms of the transmission and reception channels, communication may be visual, acoustical, chemical, tactile or electrical (Matthews & Matthews 1978). Chemical communication is the most thoroughly studied because of is the arguably primary mode of information transfer in members of the class insecta. In insects, a distinction is often made between two types of chemoreceptor; the one receptive to vapors at relatively low concentration, normally referred to as olfactory, and the other mediating a response to substances in solution are relatively high concentration , usually called a gustatory , or contact, chemoreceptor. These facts might indicate that the chemosensory system of insects seem to relay far more heavily upon smell than on taste (Matthews & Matthews 1978; Bursel 1970).

The role of odors in communication is very important and the variety and complexity of their sources has been one the most outstanding characteristics. Exocrine glands , and clusters of secretory cells whose products are discharged to the outside of the secretory cells, send forth single liquids or blends, releasing them as streams, droplets, thin films or gases.

Insect chemoreceptor cells are surrounded by characteristics cuticular specialization called sensilla, which comprise the most obvious external parts of the chemosensory organ. These occur in at least four chemical sensitive forms: bristles or hairs, pegs, plates and pits. They may occur in the antennae, mouthparts, ovipositor, cercy and other structures. Insect antennae are by far the most common organ of olfactory reception. A single antennae may have in more of 8000 sensilla of different classes which respond to a wide range of chemical stimuli present in the environment. Because the principal insect olfactory organs are located upon a pair of antennae jutting out from the head, some theories stated that use of two such identical sets of receptors may serve to maximize sensitivity and efficiency of the receptor system (Matthews & Matthews 1978; Bursel 1970).

Chemical communication in insects is highly developed. They can perceive and, under the correct physiological conditions, respond to remarkably small quantities of chemicals. Insects generally perceive odors and the peripheral part of any sensorial cell reacts to these adequate stimulus with a temporary change in the electric charge of its membrane, in a measurable response of receptor potential. Cork (1990) mentioned that by inserting microelectrodes into the base and tip of an insect’s antenna it was possible to record a slow depolarization across the antenna in response to stimulation by volatile compounds. This technique was termed electroantennography (EAG), and the EAG response is considered to represent a summation of despolarizations of all the stimulated sensillae on the antennae. Electroantennogram (EAG), gives a measurement of the summed receptor potentials of a number of olfactory receptors responding to a stimulus (Cork et al 1990).

Many insect species are attracted from long distances to their host plants, host animals or other food sources; many hosts and other substrates produce odor that stimulate egg laying; certain chemicals in the insects’ food stimulate feeding response; and insect themselves emit odors known as pheromones for intraspecific communication (Klassen et al. 1982).

What do flying insects do when they orient to distance source of chemical?. Jutsum & Gordon (1989) stated that most of work on answering this question has been done in wind tunnels in the laboratory, although some methods have been devised for studying larger species under more natural conditions in the field. The basic strategy which leads insects to arrive at the source of an attractant chemical is to fly up-wind (as long as there is any wind) when stimulated by the chemical, and to land or to fly cross-wind when they lose the chemical. Most insects can orient in relation to the wind whilst airborne. This means that they can determine the particular value of airspeed and heading which will overcome the speed and direction of the wind. An airborne insect cannot determine these value through mechanoreceptive reactions to the direction of the air flow past itself because this flow depends on its own flight direction and airspeed, and not on the direction or speed of movement of the air over the ground. Instead, insects rely on visual reactions to the relative movement or fixed features on the ground to orient in relation to the wind (Jutsum & Gordon 1989).

 

Chemical signals: pheromones. Chemical signal are compounds employed for both intraspecific and interspecific communication and are termed semiochemicals. Within semiochemicals, allelochemics are those that have interspecific effect, and is subdivided into kairomones, which attract exploiters, and allomones, which are advantageous to the odor-releasing individuals. On the other hand, compounds which convey information between members of the same species are known as pheromones (Jutsum & Gordon 1989).

Pheromonal communication is present in insects, pheromones being employed to mediate a wide variety of behaviour. For example, sex pheromones, alarm pheromones, epideictic pheromones, trail pheromones and aggregation pheromones are the most widely documented types (Matthews & Matthews 1979; Jutsum & Gordon 1989).

Insects aggregation pheromones: behavior modifiers for pest management. Aggregation pheromones function in many ways including mate selection, defense against predators, and overcoming host resistance by mass attack. A group of individuals (more than a pair) at one location may be referred as aggregation, whether comprised of one sex or both sexes. Male-produced sex attractants often are referred as aggregation pheromones because they typically result in the arrival of both sexes at the calling site. Although there is a considerable variation in how sex attractant pheromones function in the mate-finding strategies of insects, the norm, and often the expected, involves sex pheromones produced by the female that is attractive to males. A rather small percentage of sex attractants is produced by males(Jutsun & Gordon 1989; McCaffery & Wilson 1990; Landolt 1997). Most of the insects that use male-produced sex attractant are found within the Coleoptera and according to Landolt (1997), up to 1989 males pheromones attractive to females had been identified for 54 species. Forty of these species are coleopterans, 9 are dipterans, 2 are hemipterans, and 3 are lepidopterans. In addition, he found that since 1988 another 40 males pheromones attractive to females species were identified; 18 coleopterans, 12 dipterans, 7 hemipterans, 2 lepidopterans and 1 dictyopteran. This contrast with the identification of females-produced sex attractants, which are predominantly within the Lepidoptera.

In pest management survey and monitoring with pheromones and other attractants are practiced worldwide against a broad array of insect pest, and these techniques are integral parts of a growing number of control programs (Silverstein, 1990). According to Jutsun & Gordon (1989), many early studies concentrated on pheromones used by social insects, but subsequently attention has turned to non-social, economically important species. These include Coleoptera, Curculionidae species, such as the boll weevil (Anthonomus grandis (Boheman), pea and bean weevil (Sitona lineatus (L.)) and stored product weevils (Sitophilus zeamais (L.), Sitophilus granarius (L.) and Sitophilus oryzae (L.).

Cotton boll weevil (Anthonomus grandis (Boheman)). The boll weevil (Anthonomus grandis B.), indigenous to Mexico and Central America, is a narrowly oligophagous insect which feeds primarily on cotton, Gossypium hirsutum L. Since its introduction into U.S. about 1982 the boll weevil has been the most costly insect in the history of American agriculture (Hardee 1974, 1982). Attemp to solve the boll weevil problem resulted in considerable research in the past decades directed toward the development of ways to reduce the boll weevil problem and , as hoped by many, to eliminate it entirely from all infested cotton growing areas in the U.S. One approach that has received considerable emphasis has been the use of male-produced pheromone, grandlure, in conjunction with traps and/or trap crops.

Once male boll weevils locate their host plant, feeding ensues, and the weevils release in their frass an aggregation pheromones (Tumilson et al. 1969). In his study Dickens (1989), determined the mechanisms by which both pheromones and green plant volatiles, are detected by the boll weevil. He described the boll weevil aggregation pheromone, grandlure, composed of compounds I [racemic grandisol, ()-cis-2-isopropenyl- methylcyclobutaneetanol]; II) cis-3,3-dimethyl-A1.B-Cyclohexanoneetanol); and III + IV (a 50:50 cis: trans mixture of 3,3-dimethyl-A 1 -Cyclohenoacetaldeyde). Single cell recording and electroantennograms techniques used in the laboratory, clearly showed that boll weevil posses receptors to pheromones and green leaf volatiles. The brain must distinguish these compounds which activate the same receptor cells not on a response/no response basis, rather by patterning of impulses of several cells of different specificity, or across fiber-patterning.

In field studies the technique used is called mass trapping. Male or grandlure baited traps have been used to determine the feasibility of mass trapping the boll weevil for many years. Many trap designs have been evaluated from the 1960’s but, the called Leggett trap, and subsequent design based on it, had shown to be more effective and selective (Ridway et al. 1990). This trap has a conical body surmounted by a screen funnel and collecting container in which the lure was placed. The body of the trap was painted fluorescent yellow because findings that this color was attractive to the weevil. Boll weevils responded to the airborne pheromones and the color of the traps. On reaching the cone, they moved upward through the funnel into the collection container and were then ere unable to leave because they could not find the funnel opening.. The infield trap is smaller that Leggett trap but a wire mesh funnel is used to provide better ventilation. Cigarette filters are used as release substrate.

An essential prerequisite for the most efficient operation of pheromone traps against boll weevils is the reduction of overwintered weevils population (Ridway et al. 1990). Diapausing (hibernating) adult weevils spend the winter in leaf litter and other protected sites close to the cotton fields. In the spring, emerging weevils move into the cotton fields in search of food. About 4 generation occur during the cotton growing- season and then at the end of the season , the weevil leave the fields to find shelter for overwintering. Therefore, in the spring, the most effective placement of traps is along the borders of the cotton fields, to intercept the emerging weevils as they search for cotton. As the cotton grows, traps placed in the fields become more efficient (Mitchell & Hardee 1974). Has been shown that in spring, trap baited attract both sexes of overwintered adults,, and again in the fall, weevils of box sexes respond, reducing the weevil population.(Mitchel et al. 1976). (Rummel et al. 1976) stated that the use of aggregation pheromones as suppression strategies in a boll weevil management program allows the use of lower doses of insecticides and increase the control effectiveness.

Pea and bean weevil (Sitonia lineatus (L.)). This weevil is a pest of leguminous crops and is widely distributed throughout Western Europe, the middle East and the north west region of North America. According with Bight et al (1987) pea and bean weevil produce aggregation pheromones and presumably does assist the aggregation of the weevils in legume fields for the purpose of both feeding and mating. The aggregation pheromone, 4-methyl-3,5-heptanoide, has been utilized in mass trap of these insects and to time insecticide application.

Adults weevil make characteristics notches on the leaf margin of host plants, and the larvae cause serious damage by feeding on nitrogen-fixing root nodules. In spring overwintered adult males invade newly emerging leguminous crops and produce an aggregation pheromone that attracts adults of both sexes. Mating occur and eggs are deposited on the soil around the host.

Studies with in-field trap baited shown that S. lineatus populations can be aggregated by pheromones application, indicating that an effective control strategy for the weevil could be developed using these techniques (Blight et al. 1987; Nielsen & Jensen 1993; Smart et al. 1994). Trap baited with aggregation pheromones, could be used to attract overwintered weevils during the spring dispersal, and accurately timed application of insecticides or other control agents (Smart et al. 1994).

Stored product weevils (Sitophilus zeamais (L.), Sitophilus granarius (L.) and Sitophilus oryzae (L.)). These three weevil species have long been recognized as a serious pests of stored grains in all the world. In some parts of the world, infestations may extend to field crops as well . Both adults and larvae can survive on a wide variety of food substances, but gain notoriety as a pest species primarily from their infestations of corn, wheat, rice, and sorghum (Walgenbach et al. 1983). Evidence for the existence of a male-produced aggregation pheromones by these weevils and interspecific attraction between weevils (that suggest that are very closely related) is well documented (Phillips 1981; Faustini 1982; Wangelbach 1983).

According to Burkholder (1990) for practical trapping effort, the aggregation pheromones are specially useful because both long-lived sexes respond. Besides, the strong cross (interspecific) attraction facilitates the control of the weevil using trap baited with aggregated pheromones.

Conclusions

There is a wide range of tactics or methods of pest management and aggregation pheromones are among the most ecologically selective pest suppression agents. Unlike the conventional insecticides, they are no toxic and they are effective at very low concentrations. From 1960’s has been shown the efficiency of aggregation pheromones in mass insect trapping. Suppression by mass trapping has been demonstrated in a number of experiments but few are reported to be successful in providing adequate control of the pest species. The most promising results are obtained with the combination with other techniques such as color attractants and the application of lower doses of insecticides.

Several conditions must be fulfilled if successful mass-trapping is to be achieved:

* The insect must respond effectively to an attractive pheromone.

* This pheromone must be fully identified, the synthetic components commercially available and formulated in appropriate dispensers.

In addition, the insect must be the main pest to avoid use of high doses of insecticides and the trapping devices must protect predators and parasites from being captured.

In spite of the potential value of aggregation pheromones, in the context of integrated pest management we have to take into account that we are incorporating an approach that requires a sophisticated understanding of the target insect, of the pheromones which demand precise timing and uses small dosages and high cost of synthesis compounds. Each strategy that integrate a control program, will modify the insect behavior in a specific way. The insect will respond differently to different stimuli applied at the same time. Further studies in the pheromones isolation, insect behavioral responses and their integration with different strategies of management are necessary.

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