The economics of pest control

Releases of natural enemies can often provide adequate pest control and result in a relatively undamaged crop. However, the cost of purchasing, monitoring,and managing natural enemies may be greater than the cost of using insecticides.

The operational expenses of producing the crop, including pest management costs, must be made up by the revenue from marketing the crop, as determined by the quantity and quality of the crop and the marketing approach used.The costs of pest management must be weighed against the potential economic impact of each type of pest and related factors.These may include (1) the amount of physical damage that each pest can cause to the crop—that is, the loss in yield or quality, (2) the actual number of pests present, (3) the cost of managing the pests either individually or, more appropriately, as an entire pest complex, (4) the effectiveness of the control methods used, and (5) the end use and market value of the crop.

The damage caused by each pest

Some pests cause much more damage than others. Caterpillars feeding directly on the flowers of ornamentals cause more damage than fungus gnats breeding in the potting medium of the same plants. Sweetpotato whiteflies remove more sap and produce more honeydew than greenhouse whiteflies. Western flower thrips transmit tomato spotted wilt virus and impatiens necrosis virus to ornamental plants, reducing their marketability, whereas greenhouse thrips only create feeding scars on the leaves. Each particular pest problem requires its own specific solution.

The level of the pest population

It is impractical,and usually impossible, to make your greenhouse completely pest free; the costs of control would far exceed the benefit of controlling the last few insects.Therefore, growers must monitor carefully and routinely for pest activity to determine the relative abundance of each pest throughout the growing season. Growers should also keep weekly records of pest activity and relate these to actual insect damage. Because of the relatively constant greenhouse environment, most greenhouse pests can occur at any time. During warm months pests may enter the greenhouse from outside, and those that are established in the greenhouse may develop rapidly. In northern states with cold winters, pest activity usually declines when the days are shorter and the temperatures lower, and when there is no pest invasion from outside.

The costs of managing the pests

The product used for controlling a pest, any equipment required for application, and the labor required for the control process are the primary costs of pest management.The costs of pest monitoring also can be significant.These include the costs of traps and other monitoring equipment, as well as the salary or consultant fee of the individual responsible for monitoring.

When considering costs, it is important to consider the entire pest complex. Typically, a particular biological control method is effective against only a single type of pest or a narrow range of pests. For example, some parasitoids only attack certain aphid species. If numerous types of pests are present, several biological control agents may be necessary. This could be much more costly than using a broad-spectrum insecticide.

Effectiveness of control methods

Label directions for traditional chemical insecticides can usually tell you how to achieve maximum control. Such precise recommendations may not be possible for many biological controls. Complex environmental factors can make it difficult to predict the ultimate degree of control that releasing natural enemies will provide. Furthermore, some commercially available natural enemies are much more effective than others. Although there are general guidelines for the release of natural enemies, growers must gain some experience in their own greenhouses to determine the effectiveness of the chosen biological control agents, and then must decide if the level of effectiveness is acceptable.

The end use and market value of the crop

The best pest control method for your crop may depend on the end use and market value of the crop.Whether a grower is cultivating chrysanthemums for cut flowers or for potted plants, for example, may determine the amount of damage that can be tolerated. Fungus gnats are unacceptable on potted chrysanthemums nearing harvest but are of no importance on cut chrysanthemums. Market value is determined by many factors, such as available supply, competition, quality, cultivar, and consumer demand.Values vary from season to season and even within the season.When supply is high and prices low, quality must often be the highest in order to sell the product at all. Quality becomes more flexible when supply is low and prices are high. Unfortunately, this market information is not usually available at the time pest control decisions are made. However, your marketing strategies can also influence the gross value of the crop. Biological controls can be more expensive than traditional chemical approaches, but if the additional expense is offset by a new marketing strategy—by selling greenhouse tomatoes to the organic market, for example—then this may still be a smart approach.

One of the most frequently asked questions is,"How many bugs will it take to cause economic damage to my crop?" There is no simple answer. However, you can consider the five factors listed above to develop action thresholds. These are actual pest population levels at which you will apply controls to avoid economic injury.There are two types of action thresholds.The economic injury level (EIL) is that pest population that will cause economic injury, all other factors considered (figure 9). At this point, the cost of applying controls exactly offsets the economic loss caused by the pest. If controls are exerted at population levels below the EIL, then the control costs exceed the benefits. Conversely, if controls are not exerted until the pest population surpasses the EIL, some economic loss to the crop will have occurred. It is usually impractical to apply controls exactly as the pest population reaches the EIL and still keep the pest from causing economic damage. The economic threshold (ET) is a population level somewhat below the EIL that allows sufficient lead time to implement control. By applying controls at the ET you improve the chances of achieving economic pest control.

The costs of different pest control strategies varies, and because crop susceptibility and market values vary, the EIL and the ET are not fixed values.There may be several values for each crop, depending on the end use, and for each pest. Chrysanthemums grown for cut flowers can tolerate some leafminers on lower leaves because only the top half of the plant is marketed. Potted chrysanthemums cannot have any significant leafminer damage.As a result, the thresholds for leafminers are higher on chrysanthemums grown for cut flowers than those for potted plants.

The above discussion assumes that the grower will purchase biological control agents. Some natural enemies invade greenhouses, but for the most part introductions are necessary to provide control. In a few situations, introduced natural enemies become established so that biological control is effective and permanent, requiring little if any ongoing economic input. However, regular releases are usually necessary to sustain biological control in greenhouses.

Figure 9. A hypothetical example of an uncontrolled pest population exceeding the economic injury level (EIL). Action should be taken at the economic threshold (ET) to avoid this problem.

Figure 9. A hypothetical example of an uncontrolled pest population exceeding the economic injury level (EIL). Action should be taken at the economic threshold (ET) to avoid this problem.

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