Pest intensity

The intensity of pest attack can be described as the product of three effects:

1. The numbers of the pest present.

2. Their development stage.

3. The duration of the pest attack.

It is the combination of these three factors in relation to the crop that influences crop yield.

Estimates of insect numbers or density are usually made through actual counts of the insect on the crop, or by measuring the proportion of those plants or plant parts that are infested. Occasionally, relative sampling methods may also be employed. Another sampling procedure that is used in yield loss assessment studies is that of a scale of damage or infestation and the classification of field samples by a visual rating. This is a technique commonly used in breeding trials that assess the effect of plant resistance on insect/pest numbers (Chapter 5).

When a count of insects is made and used as a measure of pest intensity, it is assumed that each individual insect contributes an equal amount to the total yield loss of the plant or crop. However, different insect developmental stages may have a differential effect on plant yield. Hence, in order to assess accurately the effect of insect intensity on yield loss some account should be taken of the population structure of the infesting pest population. The population structure of the pest can be determined through the use of a more refined count procedure, so that the individual insects are classified according to developmental stage or by the use of an index that reflects the developmental stage. The yield loss caused by each developmental stage or index level can be related to that caused by the most damaging developmental stage of the insect.

Developmental stages having a similar effect can be clumped together; for example, in the assessment of yield loss caused by aphids on cereals, adults and fourth instar nymphs were given an index of one and nymphs younger than this one-third, so that three nymphs were required to equal one adult (Wratten et al., 1979). Aphid counts are thus adjusted to 'adult equivalents' which are then used as the measure of pest intensity. Alternatively the damage or area consumed by the immature stages required to complete their development can be determined, e.g. the larvae of the green clover worm (Plathypena scabra) consume on average 54 cm2 of soybean leaves in order to complete development (Hammond et al., 1979; Browde et al., 1994a). The length of time for which a pest infestation is present on a plant or crop will also influence the extent of yield loss. Hence, any index of the size of an infestation needs a temporal component that can take this into account. The level of attack can then be expressed as insect days, which is the area beneath a graph of insect numbers (or adult equivalents) plotted against time (e.g. Smelser and Pedigo, 1992; Annan et al., 1996).

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