3.1 Introduction
The amount of useful product that is obtained from crop plants or livestock is commonly referred to as ‘yield’. Estimates of yield may be: (i) quantitative, e.g. weight of grain, fruit or tubers per unit of land; or (ii) qualitative, e.g. the percentage of a product meeting certain cosmetic stan- dards which will vary for a given crop or livestock system according to weather, the levels and types of input and pest inci- dence. If all factors are optimal then the highest attainable yield is obtainable.
However, since conditions are rarely opti- mal, actual yields are normally well below those that are theoretically obtainable. The best a farmer can usually hope to achieve is a yield that provides the highest possible return on his investment of inputs.
a stream of benefits over a longer term (e.g.
investment in a plant breeding programme at a national level which requires a stream of outlays and benefits to the farmer accrued over a period of time).
The last two methods require the skills and knowledge of economists to implement and hence, are generally less applicable.
The other methods, with the exception of (1) the expenditure method, rely on some quantitative assessment of a yield/damage function. The generation of the data neces- sary to define the yield/damage function, especially in economic terms, underpins the majority of experimental approaches dealing with yield loss. This is especially true of the economic threshold concept (Stern et al., 1959) which has formed the basis of many pest management systems and is considered by some a central tenant of IPM. In addition to experimental approaches, surveys may be undertaken in order to understand and quantify farmers’
perceptions of yield loss due to pests.
The scale on which yield loss assess- ments are undertaken represents a further dimension to take into account. An assess- ment of crop losses on a regional level may be required to allow policy decisions to be made, perhaps concerning priorities for research (which pests and crops to study), to assess the need for control and to iden- tify the regions, farmers and communities most in need of assistance. Regional evalu- ation of crop loss can be carried out by use of either survey or experimental methods or a combination of both (Rai, 1977;
Nwanze, 1989).
On individual farms, yield loss assess- ments are normally carried out to establish criteria on which to base crop and pest management decisions. Crops are normally considered in isolation and the effect of a single major pest on yield is evaluated.
Detailed observation and experimentation are required in order to assess the impact of the pest on crop yield under a variety of conditions, such as the timing of infesta- tion in relation to crop growth stage, the weather and the use of different crop pro- duction practices, e.g. fertilizer and pesti-
cide use. In the latter situation the impact of pesticide inputs on the yield of a crop may be used to determine the value of such a practice to the farmer.
3.3 Measurement of Yield Loss
The primary aim of a yield loss assessment is to determine the type of relationship that may exist between pest infestation and yield. Initially experiments or surveys may simply attempt to establish that losses per se occur but more detailed information is usually required in order to determine the way in which pest infestation influences yield loss.
3.3.1 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 clas- sification of field samples by a visual rat- ing. 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 con- tributes 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 popu- lation structure of the pest can be deter- mined through the use of a more refined count procedure, so that the individual insects are classified according to develop- mental 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 deter- mined, e.g. the larvae of the green clover worm (Plathypena scabra) consume on average 54 cm2of 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 com- ponent 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 equiva- lents) plotted against time (e.g. Smelser and Pedigo, 1992; Annan et al., 1996).
3.3.2 Types of pest damage
The presence of an insect pest in a crop is usually characterized by a particular type of damage. The damage may take the form of injuries caused by insect feeding, the presence of contaminants, such as frass, that reduce the market quality of the har- vestable product or indirect insect damage caused by the presence of bacteria or viruses transmitted by the insect. The type
of pest damage will in turn influence both the likelihood and the extent of yield loss.
Insects feed and consume plant tissue or plant sap by chewing, sucking or boring.
Chewing insects that consume leaf tissue will reduce the area of photosynthetic material available to the plant. Although the damage caused may be obvious, the loss of leaf area does not necessarily result in a concomitant loss in plant yield simply because plants can often compensate for damaged tissue by enhanced growth (Section 3.9.1).
However, where chewing insects feed directly on flowering or fruiting structures then substantial yield loss can occur. For instance, a single adult bean leaf beetle (Ceratoma trifurcata) will feed on average on 0.494 soybean pods per day in a ‘nor- mal’ outbreak year (Smelser and Pedigo, 1992).
Insects that bore into plant tissue include leaf miners, shootflies, stem borers and those insects that bore into fruits and grains. The last group may cause direct yield loss due to consumption of crop grains, while others may reduce the value of the product by causing a decrease in quality. Stem borers can be particularly destructive because their larvae bore into the developing stems, often killing them and causing a yield loss by reducing the number of grain bearing shoots or by weak- ening stems to the extent that they lodge and cannot be harvested.
Insects that imbibe plant sap use their mouth parts to pierce and probe within the plant tissue until they locate a phloem ves- sel from which they take up the sap. The presence of sucking insects acts as a sink for the phloem, redirecting a large part of it away from the tissue for which it was intended and into the insect gut. In this way an infestation of phloem feeding insects may interfere with the normal parti- tion of photosynthates between plant organs (Bardner and Fletcher, 1974).
The extent of the yield loss will often depend on the feeding sites of the sucking insects. For example, there are marked dif- ferences in the feeding sites of cereal aphid
species in the UK and these are important in relation to the amount of damage which they cause (Vickerman and Wratten, 1979).
The aphid Metopolophium dirhodum is mainly a leaf feeder on wheat, where it intercepts the nitrogen and carbohydrates in the flag leaf that are allocated to the developing ear. M. dirhodum has been shown to reduce overall grain weight by as much as 7%. The grain aphid, Sitobion avenae, however, feeds at the glume bases and hence directly reduces the supply of assimilates in the developing grain reduc- ing the yield by 14% (Wratten, 1975). The differing effects of the two aphid species resulted from the degree of nutrient drain imposed at the particular feeding sites, combined with a reduction in the leaf area duration of the flag leaf.
The contamination of the harvested product with frass, exuviae or the insect itself, while not directly affecting yield, can be considered as damaging to the crop since it can reduce the market value of the product. This is a factor that greatly affects the market value of food products in devel- oped countries where extensive grading systems for food quality exist. For instance, in California in the USA, processing toma- toes are rejected if 2% or more of the toma- toes by weight have a larva or excreta of insects in the flesh of the tomato. Open
holes that are clean and contain no larvae are not subject to the 2% tolerance.
However, if a hole penetrates into the tomato so that the seed pocket is visible, the tomato is scored ‘as limited use’ and may be subject to a quality deduction by the processor. Between 1988 and 1990 an average 62% of loads were scored as hav- ing a trace or more of damage but rarely were loads graded as exceeding the 2% tol- erance (0.5%) (Zalom and Jones, 1994).
Insect vectors rarely cause direct losses to a crop, rather it is the diseases they transmit that cause the major problem.
However, often the most appropriate means of managing the disease is to control the insect vector. Virus yellows disease of sugar beet is one of the most important dis- eases to affect the crop in Europe causing reduction in sugar yields by up to 50% in some years (Smith and Hallsworth 1990;
Dewar, 1992b). Virus yellows diseases are caused by two viruses, the beet mild yel- lowing luteovirus (BMYV) and the beet yel- lows closterovirus (BYV). Their control relies on insecticides either applied at drilling or as a foliar application to prevent build up of their aphid vectors, the peach- potato aphid Myzus persicae and the potato aphid Macrosiphum euphorbiae (Stevenset al., 1994).