Physical Control of Insect Pests

Physical control is one of the main approaches to crop protection against insects, the others being chemical, biological, and cultural (Table I). From theoretical and technical points of view, all of these approaches have limits that make them more or less suitable for control of a given pest. In practice, the relative merits of each approach are also weighed against numerous factors before an actual decision is made regarding the most appropriate methods to be implemented. A majority of agricultural commodities are protected using chemical control but ideally, all components and technologies should be blended optimally and harmoniously into an integrated pest management (IPM) program.

PHYSICAL CONTROL METHODS

Definition, Context, and Literature

Physical control methods in crop protection comprise techniques that limit pest access to the crop/commodity, induce behavioral changes, or cause direct pest damage/death. The primary action may have a direct impact, for example, when insects are killed immediately by mechanical shock. In other instances, the desired effect is attained through stress responses.
With the rapid advances that have occurred in the physical, chemical, and biological sciences since the late 19th century, agriculture has been transformed from a strictly empirical activity, largely based on tradition and aimed primarily at staying off famine, to a quantitative form of agriculture focused on producing large quantities of food. During this transition, which has been sustained at an increasing rate over the past 50 years, physical control methods have been set aside because of the tremendous success of chemical control. It is only natural that some people should view the use of physical control methods as a step backward to those distant, ancestral practices. Thanks to technological advances and greater precision in the implementation of such methods, physical control now has all the necessary attributes for incorporation into IPM strategies.

Use in Agricultural Production

The different methods of physical control used against crop pests have some common characteristics. Passive physical control measures (Table II) have long-lasting effects although they may require periodic renewal (trap replacement) or maintenance (physical barriers, mulching). One of the characteristics that differentiate active physical control tactics from chemical control methods is the absence of persistence;the effect of a treatment is limited to the period of application. When treatment stops, the stressor disappears immediately or dissipates quickly. From the standpoint of exercising control over the treatment and its secondary effects, the absence of a residual action is an advantage. However, this characteristic can be a drawback, because the treatment may have to be repeated every few days to control crop pests that emerge and are active for a few days or a few weeks. In such cases, persistent chemicals constitute a much more convenient approach, although they are often undesirable from an environmental standpoint.
In addition to being restricted to the time of application, the impact of a physical control method is limited spatially. Mechanical, pneumatic, electrical, and thermal energies are dissipated locally over a distance of up to a few meters from the site of application. Electromagnetic radiation, which covers considerable distances (and is subject to numerous restrictions: reserved frequency bands, maximum power, and absence of interference), is an exception. Some pesticides have the unfortunate characteristic of dispersing over considerable distances. Similarly, many biological control agents can disperse or become dispersed beyond the treatment area.
Compared with traditional chemical control, present methods of physical control are more labor intensive and often time consuming (Table I). This drawback is one of the main reasons physical control techniques have had little success in penetrating the field-crop market. Given these circumstances, crops with a high profit margin per hectare represent an obvious market for physical control methods. From the viewpoint of implementation, physical methods compare favorably with biological methods, which often entail labor-intensive field observations and may be difficult to apply in a field-crop setting.

Modes of Action and Classification

Passive methods ( Table II) cause changes in the immediate environment and have a more lasting effect; although with no residual action after they are removed. For example, there are a wide variety of physical barriers in use which can be applied in the field, screen-house or food storage area. Barriers used in the field can take several forms (trenches, exclusion nets, etc.) and can be deployed on a range of scales to protect a complete field, a crop row, or a group of plants. Passive methods should be used whenever possible, because they extend the length of the treatment. For example, plastic-lined trenches along field boundaries trap Colorado potato beetles during the whole migration period; screening windows and doors in food storage prevent flying insects from entering. Physical barriers used to keep pests out, combined with suppression techniques, are the cornerstone of the approach adopted by industrialized countries to replace methyl bromide. Other techniques such as diatomaceous earth, hydrophilic particle films, sticky traps, and oils also are also passive techniques.
Active methods (Table II) are used to destroy, injure, or induce stress in crop pests or to remove them from the environment, and can be classified according to the mode of energy use: thermal shock (heat), electromagnetic radiation (microwaves, infrared, and radiof-requencies), mechanical shock, and pneumatic control (blowing or vacuuming tools).
Various applications that use thermal shock for crop protection in the field have been developed and research is in progress. This approach is based on the premise that the commodity or crop to be protected will be less sensitive than the target pest to an abrupt change in temperature. Research on thermal sensitivity thresholds and physiological reactions to short-duration thermal stresses is central to the development of control methods based on thermal shock.

Source: Reproduced with permission from , C., Panneton, B., and Fleurat-Lessard, F. (eds.) (2001). “Physical Control Methods in Plant Protection.” Springer/ INRA. Copyright Springer-Verlag.
Several avenues have been explored for applying the different forms of electromagnetic radiation as a tool for controlling insects. Non-ionizing electromagnetic radiation kills insects by raising their internal temperature. The utilization of radio, microwave, and infrared frequencies is based on a principle similar to that of thermal shock
methods except that, with applications involving electromagnetic radiation, the transfer of energy occurs without using a heat transfer fluid. Technologies that harness electromagnetic radiation are often too expensive for use in the field. Furthermore, existing regulations restrict the available frequency bands, either for reasons of

user and environmental safety or because certain frequency bands have been set aside for specific applications that do not tolerate interference (e.g., microwave-based landing guidance systems for aircraft).
Pneumatic control consists in using an airstream to dislodge insect pests. Insects that are removed by vacuum pressure are killed when they pass through the moving parts of the blower (mechanical shock). After being dislodged by a blowing device, individuals of some insect species are injured and die because they are unable
to climb back onto the host plant. Other machines are equipped with a device for collecting the dislodged insects, which are subsequently killed. Sound knowledge of the target insect’s behavior is necessary to enhance the effectiveness of this type of approach as exemplified by the management of tarnished plant bug as a pest of strawberries.
Like any pest management approach, physical methods have strengths and weaknesses, and some of them are likely to have secondary effects on no target organisms, for example, pollinators of strawberry. In an IPM context, the decision to use a physical control tactic must therefore be made on a case-by-case basis according to the same criteria as in decision-making regarding the appropriateness of pesticide applications: efficacy, cost-effectiveness, and undesirable effects. In addition, no physical control technique is sufficient on its own for all pest control treatments in a given crop.

Postharvest Physical Control

Postharvest situations offer ideal opportunities to research and implement physical control methods. In long-term storage of non-perishable agricultural commodities (seeds and grain, dried fruits, byproducts, dried and dehydrated plants, spices, herbs, coffee, cocoa), the most serious losses are due to the action of insects and mites or the spoilage by certain microorganisms (e.g., fungi). Chemical control using persistent insecticides is the most commonly used approach for preventing damage to grain and seeds by insect pests. Some of the benefits of this strategy are low cost, ease of implementation, and protection that lasts for several months, until the quantity of active residues falls below the lethal threshold for the target species.
Active physical control methods (cleaning, cold, heat, ionizing radiation) are major postharvest phytosanitary treatments to prevent the spread of invasive species while permitting trade in fresh and stored commodities. Although the use of methyl bromide fumigation has been exempted for postharvest phytosanitary applications, physical methods are replacing methyl bromide in an increasing number of instances and research in this area is quite active.
Physical control applications developed for postharvest treatments in stocks of stored grain have focused on procedures for controlling physical conditions (temperature and water content), thermal or mechanical shock, the establishment of extreme conditions for insect pests (anaerobiosis, pressure, and modified atmospheres), the use of abrasive or dehydrating minerals, and the erection of physical barriers to keep insects out. Postharvest control approaches are essentially based on passive methods, with the exception of thermal and mechanical shock treatments. In postharvest situations, most physical methods are suitable solely for prevention against pests and hence cannot be compared with classical chemical control. A thorough knowledge of IPM strategies is required, since these techniques afford no protection following application, unlike persistent insecticides.
Practical use of physical control methods necessitates verification at each step, particularly in relation to secondary effects on the quality of treated products (e.g., the germinating power of malting barley or the baking quality of bread-making wheat). The use of persistent contact insecticides on processed food products (e.g., wheat, semolina, and dried fruit) is prohibited worldwide, and the prospect of registration reviews for non-persistent pesticides that are currently used in protection of foodstuffs in processing and storage facilities has revived interest in postharvest physical control. Physical methods hold promise as a complement to, and a means of moving away from, exclusive reliance on chemical control. Furthermore, the most accessible physical methods for this pest management sector, such as dry heat treatments or airtight storage using inert gases, should help to diminish the secondary risk of spoilage of raw foodstuffs by storage molds.

GENERAL CONSIDERATIONS

Although physical control methods are very diverse, this strategy of pest control deserves to be recognized as an area of expertise similar to the area of biological control. This recognition is bound to come in the future as the quest for alternatives to pesticides intensifies. Although physical control methods became far less important with the advent of chemically based pest control methods in the middle of the 20th century, the limitations of pesticide use, coupled with their adverse affect on implementing biological control, have created an impetus for the renewed development of physical control.
The regulatory framework for physical control differs markedly from that for agrochemical products (Table I). First, many physical techniques are subject to rules concerning their use (i.e., the registration process), which are designed to protect users and the general public. Sometimes, such as with the use of propane gas, specialized training is required. The use of electromagnetic radiation is constrained by telecommunications regulations, some of which stem from international agreements. In the case of microwave energy, for instance, only a handful of frequencies have been set aside for industrial, scientific, and medical applications. With regard to the regulatory framework for physical control technologies, it is completely defined a priori. In short, the equipment employed must meet the applicable standards (mostly related to user safety). With chemically or biologically based methods, the difficulty in anticipating secondary effects precludes the establishment of comprehensive specifications that would be known a priori. This explains the need for increasingly costly test protocols designed to evaluate pesticide safety from the standpoint of human health as well as ecotoxicology.
A number of factors tend to complicate the implementation of physical control methods, and these tactics cannot be readily compared with chemical crop protection systems. For agricultural operations the use of a tractor and agricultural sprayer to apply pesticides entails low variable costs and fixed costs. In contrast, the equipment used for physical control is often very specific: for example, microwaves and ionizing radiation for stored-product pests, vacuuming device for field pests, and so on (Table I). Very few physical control tools offer the operational versatility that would allow them to perform several types of crop protection operations. Integration efforts, such as designing multi-task burners for controlling Colorado potato beetles, killing weeds, and performing top-killing, are needed to enable physical control tools to penetrate the crop protection market.
Most physical methods of control can be used in a crop protection program incorporating chemical, biological, and cultural controls. Chemical and biological methods are sometimes incompatible; however, biological, cultural, and physical methods can be compatible. Organic produce represents a growing market segment. In short, public demand and economic and legislative incentives will provide the leverage for the use of physical control methods and secure their success as alternatives or supplements to pesticides.