Manipulating Insect Behavior, Biology tutorial

Introduction:

An insect is considered as a pest when it threatens a resource valued through humans, comprising human health. Protection of a resource from the pest is generally accomplished by poisoning the pest by a toxic pesticide; however it can as well be accomplished by manipulating a behavior of the pest. The manipulation of a pest's behavior to save a resource is not a fresh concept. The practice of trap cropping, that is, employing a sacrificial resource for the pest to attack, in order to protect the valued resource, has been recognized for centuries. Though, in the last 30 years or so, mostly due to enhancements in analytical methods and an increased desire to decrease the reliance on broad-spectrum insecticides, there has been raised interest in the behavioral manipulation for the pest management.

Behavioral Manipulation Methods

There are three principal elements of the behavioral manipulation process: 

  • Behavior of the pest
  • Means through which the behavior is manipulated suitably
  • Method which employs the behavioral manipulation for protection of a resource from the pest. 

In theory, any behavior of any phase of the pest can be selected, given that its manipulation protects the resource. Intuitively, one might anticipate that manipulation of the pestilential behavior (example: feeding on the resource) or a behavior closely associated to the pestilential behavior (example: determining the resource) is more probable to be helpful for pest management than manipulation of behaviors unrelated to the resource (example: mating). Successful manipulation of the pestilential behavior will make sure protection of the resource; successful manipulation of an unrelated behavior might decrease the local population however still not protect the resource as immigration of outside populations into area being protected, as can take place in the mating disruption processes for moths.  In practice, though, the criterion through which a behavior is generally selected for manipulation is not its relationship to the pestilential behavior however instead the availability of a suitable means for its manipulation. 

The behavior of an insect yields from the integration by its central nervous system of a diversity of inputs which derive from the stimuli acting on exteroceptors (that sense events external to the insect), enteroceptors (that sense the internal physiological state of the insect), and proprioceptors (that sense the relative positions of portion of the body). To manipulate a behavior one should change either the inputs or the processing of such inputs through the central nervous system. At present the latter approach is usually inaccessible for carrying purposeful manipulation of a behavior, however it might take place in certain instances as a sub-lethal effect of a toxic insecticide. Therefore insect behavior usually is manipulated via inputs to the behavior and more particularly via the stimuli which generate such inputs.

The selection of a stimulus to employ for behavioral manipulation is generally based on a number of attributes comprising the following:

1) Accessibility: The stimulus should be appropriate for presentation in a form which the insect can perceive.

2) Definability and reproducibility: The more accurately that the stimulus can be stated, the more accurately it can be reproduced artificially.

3) Controllability: The capability to control different parameters of a stimulus, comprising intensity and longevity, will provide greater control in the behavioral manipulation.

4) Specificity: The more specific a stimulus is to a specific behavior of a pest, the more possible it can be employed to manipulate that behavior. On the contrary, a stimulus which is ubiquitous in the environment is unlikely to be helpful for manipulating particular behaviors unless its intensity or quality can be perceived through the insect above the background level. For illustration, 'supernormal' visual targets (that is, objects which reflect high ratios of stimulatory to inhibitory wavelengths of light) are employed to out compete the natural visual background reflecting a lower ratio of stimulatory to the inhibitory wavelengths.

5) Practicability: Ecological hazards and cost of protecting a resource should be in practical limits. For illustration, chemicals which are persistent and encompass high mammalian toxicities might protect an edible resource however render it futile for the human consumption.

Examples of Behavioral Manipulation Methods:

In this part behavioral processes which have been employed for the control of insect pests are reviewed. The methods have been categorized into:

  • Those which act over a long distance (that is, finding-type behaviors)
  • Those which act at short-distance (that is, acceptance-type behaviors) 

To explain the kinds of stimuli, particularly chemicals, the common words of attractant and repellent for long-distance stimuli and stimulant and deterrent for short-distance stimuli are employed.

Stimuli that Act Over Long Distances:

1) Attract-Annihilate:

The attract-annihilate process is by far the most broadly employed behavioral manipulation for pest management. The policy of the method is simple: attract the pests to a site where as many of them as possible can be eliminated from the environment. The behavioral manipulation comprises a long-distance attractant and the method comprises of a device whereby the attracted pests are trapped or killed. This process has been employed for many pests whose behavior comprises long distance orientation, generally flight however as well walking, as in certain species of cockroaches. The most generally used attractants are volatile chemicals, however visual stimuli are as well used (intentionally or incidentally) and auditory stimuli could as well be employed.

2) Disrupting Behavior using Attractants and Repellents:

Disrupting behavior having stimuli which either elicit or inhibit orientation is as well efficient at long distances. In practice, only attractant and repellent chemicals have been employed. The difference between repellents and attractants is less clear than the names propose. Most of the stimuli which attract will repel at higher concentrations. Stimuli which repel a pest might elicit orientation in the similar or other species. For instance, the insect repellent N, N-diethyl-m-toluamide (deet) will attract the mosquito Aedes aegypti at adequately low concentrations.

Such terms, specifically 'repellent', are frequently employed problematically in the literature, as the real effect of the chemical is surmised from the end outcome of the behavior, which could be affected by short-range stimuli like deterrents. Nonetheless, attractants or repellents are usually recognized as such in behavioral manipulation processes.

a) Attractants:  Most of the work on the use of attractants to interrupt a finding behavior consists of focused on mate location, specifically of moths, in the so-called mating disruption process. Big amounts of synthetic sex pheromone are distributed by the aim of preventing males from determining females. This process has been employed successfully for control of such herbivorous pests as the Pectinophora gossypiella, pink bollworm, on cotton; the Oriental fruit moth, Grapholita molesta, on stone fruits; the tomato pinworm, Keiferia lycopersicella, on tomatoes; and the current clearwing, Synanthedon tipuliformis, on blackcurrants. In virtually all situations where the process has been attempted, disruption of mating has been at least partly accomplished in the treated region. Though, the success of the process for management of a given pest based to a big extent on its biology and specifically on the potential for immigration of mated females from outside the treated region.

b) Repellents: The strategy for employing repellents is, usually, to stop a pest from determining a valued resource. Helpful repellents can be derived from the natural sources like insects (example:  defense secretions), or they might be just artificial as in the case of most insecticides. Most of the work on the practical use of volatile repellents has been to protect humans from the bites of insect, specifically from such insects like mosquitoes and black flies which are vectors of diseases.

Stimuli Acting at Close Distance:

After arriving at a resource, an insect is probable to contact additional (to those perceived at greater distances) stimuli. Such stimuli can either stimulate a behavior, keeping the insect at the resource, or restrain that behavior, resulting in the denial of and possibly movement away from the resource. Virtually all the short-range stimuli employed in behavioral manipulation are chemicals.

Stimulants

Most recognized stimulants are comprised in either feeding or oviposition, mainly the former. Benefits of stimulants comprise rising exposure to toxins which must be ingested and applicability to a broad range of pests, as in the case of sucrose. Feeding stimulants as well are frequently common carbohydrates, proteins and fats which are simply obtained and relatively cheap, while oviposition stimulants can be very specific, even  among pests that threaten similar resource and costly.

Deterrents:

A deterrent is a chemical which inhibits behavior, like feeding or oviposition, if applied to a site where such behavior generally takes place. In pest management, a deterrent is applied directly to the resource to prevent or decrease the consequences of a pestilential behavior like feeding. Efficiency based on the physiological state and behavioral responses of the pest throughout initial and subsequent encounters by the deterrent. When a pest stays on or returns to the resource, protection might break down in some manners, of which the most significant is desensitization. Many pest insects have been exhibited to lose their responsiveness to deterrents after repeated exposure and after rousingly long periods of deprivation of feeding or oviposition sites.

Protection as well might break down if the resource is not uniformly treated by the deterrent, and the pest can move to an untreated part. For illustrations, plants can grow after a deterrent has been applied and thus present the pest by unprotected surfaces over time, apart from in the case of systemic deterrents. Though, movement of a pest from a deterrent-treated site to a deterrent free site can be exploited for the pest management when the movement brings the pest to a non valued portion of the resource or a region which has been treated by a pesticide. This approach was employed against larvae of the mustard beetle, Phaedron cochliereae that were deterred from feeding by application of extracts of plants in the genus Ajuga to the young leaves at the top of mustard plants; beetles moving down to feed on older leaves were managed by an insect growth regulator.

Combining Deterrents and Attractants Stimulants:

Some authors have recommended that the efficiency of a deterrent-based process might be raised if employed in combination by the other process which fascinates the pest to a non-valued resource in a stimulo-deterrent diversion or push-pull strategy. The combination of process might overcome such difficulties having deterrents as desensitization and untreated regions of the resource.

The stimulo-deterrent strategy (SDD) was imagined for insect herbivores however are applicable to any pest and any resource kind. To date, it has only been tested against the small number of herbivores in small field or glasshouse trials and has yet to be employed commercially. The onion fly D. antiqua can be deterred from laying eggs on seedling onions through cinnamaldehyde and stimulated to lay eggs on valueless cull onion bulbs which are planted in the similar field.  In big-scale field trials utilizing microencapsulated cinnamaldehyde as the deterrent and cull onions as the stimulant, the SDD strategy appeared to provide good protection for the first few weeks following application. Control afterward broke down due to the short life-span of the deterrent formulation. Moths of the genus Heliothis were deterred from ovipositing on cotton through azadirachtin and stimulated to oviposit on pigeon pea or maize crops. However the two processes have not yet been joined, researchers in Britain have exhibited in separate field trials which the adult pea and bean weevil, Sitona lineatus, can be deterred from feeding on a leguminous crop through neem oil and fascinated into other crop fields employing aggregation pheromones. As appropriate deterrents and stimulants are recognized, it seems likely that such combined behavioral manipulation processes will be build up for a broad range of pests and resources.

Internal Stimuli:

One exception to the generalization which internal stimuli are inaccessible for manipulating the behavior is the sterile male method, in which big numbers of sterile males are discharged to mate by means of wild females. The mating induces numerous similar internal inputs in the females as in those mated to normal, fertile males. Such internal inputs induce a number of behavioral modifications in the female. Most helpfully, the female becomes refractory to further mating, and in species in which the female mates just once, will remain refractory permanently and therefore never be fertilized.  

The sterile male method has been employed successfully in eradication programs, generally in combination by other processes like poison baits, against isolated or incipient populations of economically severe pests where the risk of reintroduction is low, like the prime screwworm in the North and Central  America and different tephritid fruit flies. The process can be employed for control via ongoing population suppression, however the high cost of producing the big numbers of high-quality insects required to swamp or outcompete wild males usually makes this approach extravagant. Besides a big rearing program generating high-quality insects, the method needs a capability to sterilize big numbers of insects (generally by X-ray or γ-ray exposure) and an efficient monitoring system which can be employed to detect the pests and to approximate the size of the pest population.

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