Crop loss and reference yield

A grower takes a particular pest control action to prevent an anticipated crop loss. Generally one cannot recoup losses that have already occurred, and pest control measures are not usually taken for vengeance! Prevention implies a certain predictive capability, and most often the prediction is based solely on the experience of the farmers–they know that if they see a particular pest and do not do something about it, they will soon suffer a loss. Unfortunately, growers’ predictive capabilities are often limited–they may overreact, or they may fail to react in time or with sufficient force.

To predict a crop loss, a reference yield is necessary. (Yield is used in the broad sense here to include both amount harvested and quality–perhaps yield in dollars.) What can one reasonably expect the yield to be in the absence of constraints?

The difference between the attainable yield and the theoretical yield is an unpreventable loss, whereas the difference between the actual yield and the attainable yield is a preventable loss. One can usually determine attainable yield experimentally. We must grow the crop free of constraints (e.g., pests). We attempt to push the actual yield up to the attainable yield (preventable loss). Making the actual yield the same as the attainable yield is not always economically rational. Ideally the actual yield should be the same as the economic yield.

Direct and indirect loss

It is useful to conceptualize crop loss as either direct or indirect. Direct loss occurs when the saleable commodity itself is affected by the pest. The loss can be one of quality, quantity, or both. Apple scab is an example of a direct pest that reduces the quality of the marketed product, and the bean weevil is an example of a direct pest that reduces both quantity and quality of dry beans and peas in storage. Direct loss is generally proportional to the pest population density at low densities and approaches an upper limit (often 100%) as the pest population increases.

Indirect loss occurs when the pest attacks plant parts other than the marketable product but reduces the yield and/or quality of the product. An indirect pest may have one or more of the following effects on the plant:

• Reduction of photosynthetic area
• Diversion of photosynthate
• Reduction in photosynthetic rate
• Reduction in net assimilation
• Increase in respiration rate
• Upset of water balance

Crop loss–that is, the quantitative nature of the yield response to pest population density–can vary widely from one crop species to another and from pest to pest within the same crop. At very low pest population densities, there is often no measurable yield loss, but when the pest density gets high enough, the yield begins to decline steeply. At very high pest densities, the yield may drop to zero, or it may level out at a low level.

Sometimes low levels of pest damage can slightly enhance yield . This can occur, for example, in crops where the lower leaves are heavily shaded by the top of the canopy, and a slight pruning of the upper leaves by a foliage feeder allows greater light penetration into the lower canopy.

Multiple pests can interact synergistically to reduce yields, or they can interfere with one another so that the yields with both pests present can actually be higher than would be obtained with either pest alone at the same density.

The crop loss or yield functions can change at different stages of crop development, and the final yield (quality and quantity) at harvest can be affected both by brief episodes of pest damage and by the cumulative effects over the season. The crop loss and yield functions can also be affected by factors in the physical environment (temperature, rainfall, cultural practices, etc.).

Pest-resistant cultivars

The crop loss or yield functions also vary with cultivars that have different levels of pest resistance. It is not uncommon to find primitive cultivars or landraces that may not be as high yielding as so-called "improved" cultivars if pest populations are kept low, but which can outproduce an improved cultivar when the pest pressure is high.

Control actions and yield

Crop loss and yield functions can be used to evaluate the effects of pest control measures that abruptly change pest populations (pesticides, sanitation measures, etc.) One first must have a model that predicts the pest population dynamics with and without the control measure.

Using this prediction of the controlled and uncontrolled pest population densities combined with the yield response functions, we can then estimate the yield loss that would be prevented by the control action.

Modeling yield with computers

Taken together, all the possible interactions of pest population dynamics, crop development, environmental variables, and management options to predict crop loss or yield is complex. However, this problem can be broken into its components, which are relatively straightforward quantitative relationships. The set of equations generated by this approach can then be solved numerically in an appropriately designed computer simulation. With a model that simulates pest and crop development with reasonable fidelity, "field experiments" can be run on the computer and used as an analytical tool to develop management models.