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).
Case Study: The effects of feeding by larvae of Plutella xylostella
3.3.3 Measures of yield and yield loss Crop yield is usually measured in terms of kilograms of harvested product per hectare. As a standard measure this allows easy comparison between fields and trials.
However, on an individual plant level, yield will be influenced by the number of plant parts bearing the harvestable product (grains, fruits and tubers), the actual weight or size of these products and the number per plant part. For instance, in cereals yield loss due to aphids may result from a reduction in the number of ear- heads, the grain weight (usually measured as 1000-grain weight) or the number of grains per ear head (Vickerman and Wratten, 1979). These different yield com- ponents are not always stated, but they are
useful for indicating the ways in which yield may be improved. It is of little value to quote yield reduction as a percentage loss unless this figure is supported by an absolute measure of the yield or yield loss.
In the same way, a value for yield based on kilograms per unit length of row has little meaning unless the number of plants or the plant spacing is also given. Care must be exercised to ensure that yield loss data are quoted in a form that makes them meaningful to others.
The losses caused by insects in posthar- vest storage, such as Sitophilus spp. and Prostephanus truncatus, are usually mea- sured by reference to grain weight. The loss may be measured as a change in the weight of samples over a particular period, by This can be explained by the compensatory growth of the turnip after insect attack. The differing response of turnip to attack by the two insects can be explained by the way in which the two insects differ in both their method of feeding and where they feed on the plant. P. xylostella larvae feed indiscrimi- nately on all leaves, but only damage the area actually eaten, leaving the veins intact; yield was not affected because attacked plants retained their older leaves longer than unattacked plants. These older leaves grew large after P. xylostella had stopped feeding; the plants also grew side shoots and produced more leaves than unattacked plants. In contrast, P. cochleariaefed mainly on older leaves and caused greater damage because the larvae sever leaf veins and rasp the leaf sur- face thereby killing more tissue by desiccation than they actually consumed.
Table 3.1. The average leaf area defoliated by the larvae of Plutella xylostella and Phaedon cochleariae and its effect on root yield of radish and turnip plants (from Taylor and Bardner, 1968).
Plutella xylostella Phaedon cochleariae
Leaf area Root dry Leaf area Root dry
Larvae per plant (cm2) weight (g) (cm2) weight (g)
Turnip plants
0 500 1.85 355 2.13
5 414 1.99 282 2.26
10 426 1.91 205 1.68
15 429 1.71 217 1.55
20 409 2.83 163 1.19
Radish plants
0 283 2.96 287 3.83
5 191 1.85 217 2.09
10 125 0.73 155 1.45
15 100 0.80 94 0.74
20 56 0.39 93 0.98
comparative weights of damaged and undamaged kernels or by the determination of the percentage of insect damaged grain which is then converted into a measure of weight loss. In fruit tree crops losses may be measured by fruit damage, decreased yield or mortality. The losses incurred due to tree mortality may be assessed in terms of the costs of replanting and the delay in production, for example in young apple and pear trees attacked by the leopard moth, Zeuzera pyrina (Audemard, 1971).
Losses in forest trees caused by defoliating pests such as sawflies (e.g. Neodiprion sertifer), the jack pine budworm, Choristoneura pinus, and the spruce bud- worm, C. fumiferana,may be measured by incremental loss in terms of shoot elonga- tion, radial increment, area of annual rings, tree height, top and root mortality and ultimately by the lost volume of wood (Kulman, 1971; Day and Leather, 1997).
There are some circumstances where the effect of a pest on the quality of a product can be considered as a yield loss. In horti- cultural crops where grading and sorting are carried out, pest damage may render a certain percentage of the crop unfit for sale, in which case the percentage of discarded crop will provide a measure of yield loss (Southwood and Norton, 1973). Where a lower price will be obtained for a poorer quality product the loss can be expressed as the proportion of the crop falling within each grade/category.
Insect pests can also have a direct effect on the quality of the harvested product. Infestations of the aphid Sitobion avenaehave been shown to affect signifi- cantly the bread making quality of wheat by reducing the percentage flour extrac- tion, increasing the colour, nicotinic acid and thiamine (vitamin B12) content and reducing the baking value of the flour (Lee et al., 1981). However, different aspects of the grain quality do not change in parallel with one another, or with yield changes
and thus damage thresholds have to vary according to the yield/quality measure under consideration.
3.4 Crop Loss Surveys
Crop loss surveys may be undertaken sim- ply to determine the types of losses occur- ring and their main causes, to determine the distribution of losses in different areas or to actually evaluate losses with a view to forecasting crop production or justifying control measures (Walker, 1987). In addi- tion, surveys may be undertaken to meet more specific objectives such as defining farmers’ perception of yield losses (Mulaa, 1995) or to establish baseline data by which to evaluate area-wide control mea- sures (e.g. classical biocontrol introduc- tions; Farrell et al., 1996). Such surveys may be conducted as face-to-face inter- views with individual or groups of farmers (e.g. Mulaa, 1995) or by use of postal ques- tionnaires (e.g. French et al., 1992, 1995).
The question/interview approach can be supplemented by on-site evaluation of actual yield losses. In the study conducted by Mulaa (1995) the interviews were con- ducted and then the numbers of maize plants damaged by the stalkborer Busseola fuscawere counted on each farm from five areas (10 m 3 10 m) selected at random.
The percentage of damaged plants per hectare was then determined using a sim- ple formula, taking into account plant spacing, which allowed comparison of the distribution and extent of plants damaged by stalkborers in the Trans Nzoia District, Kenya. Surveys provide useful information on yield loss that it is often impossible to collect by any other means. However, to carry out surveys correctly requires a great deal of resources. In the B. fuscaexample above, 300 farmers were interviewed in five different agro-ecological zones (Mulaa, 1995).