16.1.1. Assessment of pest status
The pest status of an insect population depends on the abundance of individuals as well as the type of nuisance or injury that the insects inflict. Injury is the usually deleterious effect of insect activities (mostly feeding) on host physiology, whereas damage is the measurable loss of host usefulness, such as yield quality or quantity or aesthetics. Host injury (or insect number used as an injury estimate) does not necessarily inflict detectable damage and even if damage occurs it may not result in appreciable economic loss. Sometimes, however, the damage caused by even a few individual insects is unacceptable, as in fruit infested by codling moth or fruit fly. Other insects must reach high or plague densities before becoming pests, as in locusts feeding on pastures. Most plants tolerate considerable leaf or root injury without significant loss of vigor. Unless these plant parts are harvested (e.g. leaf or root vegetables) or are the reason for sale (e.g. indoor plants), certain levels of insect feeding on these parts should be more tolerable than for fruit, which “sophisticated” consumers wish to be blemish-free. Often the effects of insect feeding may be merely cosmetic (such as small marks on the fruit surface) and consumer education is more desirable than expensive controls. As market competition demands high standards of appearance for food and other commodities, assessments of pest status often require socio-economic as much as biological judgments.
Pre-emptive measures to counter the threat of arrival of particular novel insect pests are sometimes taken. Generally, however, control becomes economic only when insect density or abundance cause (or are expected to cause if uncontrolled) financial loss of productivity or marketability greater than the costs of control. Quantitative measures of insect density (section 13.4) allow assessment of the pest status of different insect species associated with particular agricultural crops. In each case, an economic injury level (EIL) is determined as the pest density at which the loss caused by the pest equals in value the cost of available control measures or, in other words, the lowest population density that will cause economic damage.
The formula for calculating the EIL includes four factors:
- costs of control;
- market value of the crop;
- yield loss attributable to a unit number of insects;
- effectiveness of the control; and is as follows:
EIL = C/VDK
in which EIL is pest number per production unit (e.g. insects ha-1), C is cost of control measure(s) per pro- duction unit (e.g. $ ha-1), V is market value per unit of product (e.g. $ kg-1), D is yield loss per unit number of insects (e.g. kg reduction of crop per n insects), and K is proportionate reduction of insect population caused by control measures.
The calculated EIL will not be the same for different pest species on the same crop or for a particular insect pest on different crops. The EIL also may vary depending on environmental conditions, such as soil type or rainfall, as these can affect plant vigor and compensatory growth. Control measures normally are instigated before the pest density reaches the EIL, as there may be a time lag before the measures become effective. The density at which control measures should be applied to prevent an increasing pest population from attaining the EIL is referred to as the economic threshold (ET) (or an “action threshold”). Although the ET is defined in terms of population density, it actually represents the time for instigation of control measures. It is set explicitly at a different level from the EIL and thus is predictive, with pest numbers being used as an index of the time when economic damage will occur.
Insect pests may be described as being one of the following:
- Non-economic, if their populations are never above the EIL (Fig. 16.1a).
- Occasional pests, if their population densities exceed the EIL only under special circumstances (Fig. 16.1b), such as atypical weather or inappropriate use of insecticides.
- Perennial pests, if the general equilibrium population of the pest is close to the ET so that pest population density reaches the EIL frequently (Fig. 16.1c).
- Severe or key pests, if their numbers (in the absence of controls) always are higher than the EIL (Fig. 16.1d). Severe pests must be controlled if the crop is to be grown profitably.
The EIL fails to consider the influence of variable external factors, including the role of natural enemies, resistance to insecticides, and the effects of control measures in adjoining fields or plots. Nevertheless, the virtue of the EIL is its simplicity, with management depending on the availability of decision rules that can be comprehended and implemented with relative ease. The concept of the EIL was developed primarily as a means for more sensible use of insecticides, and its application is confined largely to situations in which control measures are discrete and curative, i.e. chemical or microbial insecticides. Often EILs and ETs are difficult or impossible to apply due to the complexity of many agroecosystems and the geographic variability of pest problems. More complex models and dynamic thresholds are needed but these require years of field research.
The discussion above applies principally to insects that directly damage an agricultural crop. For forest pests, estimation of almost all of the components of the EIL is difficult or impossible, and EILs are relevant only to short-term forest products such as Christmas trees. Furthermore, if insects are pests because they can transmit (vector) disease of plants or animals, then the ET may be their first appearance. The threat of a virus affecting crops or livestock and spreading via an insect vector requires constant vigilance for the appearance of the vector and the presence of the virus. With the first occurrence of either vector or disease symptoms, precautions may need to be taken. For economically very serious disease, and often in human health, precautions are taken before any ET is reached, and insect vector and virus population monitoring and modeling is used to estimate when pre-emptive control is required. Calculations such as the vectorial capacity, referred to in Chapter 15, are important in allowing decisions concerning the need and appropriate timing for pre-emptive control measures. However, in human insect- borne disease, such rationales often are replaced by socio-economic ones, in which levels of vector insects that are tolerated in less developed countries or rural areas are perceived as requiring action in developed countries or in urban communities.
A limitation of the EIL is its unsuitability for multiple pests, as calculations become complicated. However, if injuries from different pests produce the same type of damage, or if effects of different injuries are additive rather than interactive, then the EIL and ET may still apply. The ability to make management decisions for a pest complex (many pests in one crop) is an important part of integrated pest management (section 16.3).
From comparison of the general equilibrium density with the ET and EIL, insect populations can be classified as: (a) non-economic pests if population densities never exceed the ET or EIL; (b) occasional pests if population densities exceed the ET and EIL only under special circumstances; (c) perennial pest s if the general equilibrium population is close to the ET so that the ET and EIL are exceeded frequently; or (d) severe or key pest s if population densities always are higher than the ET and EIL. In practice, as indicated here, control measures are instigated bef ore the EIL is reached. (After Stern et al. 1959)