Crop losses due to pests like armyworms, diamond back moths, and locusts were assessed for maize and cabbage crops. Armyworms and diamond back moths cause damage by feeding on leaves, while locusts damage leaves, stems, roots and seeds. Crop loss assessment techniques involve measuring pest infestation levels over time and comparing final yields between infected and uninfected crops. Locust damage is also modeled using "coupling points" that represent impacts on plant organs or growth processes.

1. Keywords: crop loss, insect damage
Introduction
It is well known that pests cause appreciable crop losses all over the world. However, reliable
and objective estimates of such losses are hardly available in any of the countries. Only guess
estimates are available, their credibility depending upon the agency making the estimates
(Pimentel, 2002).
Losses of crop have been defined in various terms, losses of food grains can occur during pre-
harvest, at harvest and post-harvest stages but in this discussion l will be confined to the pre-
harvest losses of maize and cabbage due to Spodoptera exempta (armyworm), locusts and
Plutella xylostella (diamond back-moth).
i) Armyworm (Spodoptera exempta) loss assessment in maize crop
Mechanism of damage
Loss of photosynthetic area due to foliar feeding. Under severe infestations entire young plants
can be consumed. Armyworms feed on leaf tips and along leaf margins. When they eat whole
leaves, they can remove them completely or leave only the midribs (Pedigo, 1996).

2. Figure 1| Armyworms feed on the maize foliage starting with the edge of the leaf. Feeding starts
on the lower leaves and progresses up the plant.
Crop loss assessment techniques for armyworm in maize
During the growth period, periodical observations, measuring the intensity and severity of the
incidence/ infestation due to armyworm should be recorded. Finally the crop yield should be
recorded at harvest. Since the crop grown in the cultivator’s field is exposed to various pests and
diseases, the observations on the incidence of all pests and disease should be recorded
simultaneously. The observations on these factors would make it possible to present the results in
the contingency tables for undertaking a deeper analysis to estimate the yield loss in relation to
armyworm or other individual pests of maize (Dent, 2000).
Various researchers have measured the crop loss intensity due to armyworm by taking the ratio
of infected maize plants to the total number of maize plants in the selected plot of the field at
various stages of growth.
Crop loss assessment due to armyworm is also measured by assigning the score of pest
infestation on the leaves with the help of standardized score charts. The number of grades of
such score charts may vary from pest to pest for different crops; usually it indicates the
percentage damaged area of the leaves (Dent, 2000).
Finally the yield of maize from the field is recorded as this is the indicator of the result of the
various inputs and other factors apart from incidence of armyworm. With the relationship of
severity of incidence or infestation of pests and diseases and the resultant yield determined by
taking into account the effect of other factors, the crop losses due to armyworm are determined
i) Diamond back-moth (Plutella xylostella) loss assessment on cabbage
Mechanism of damage
Damage is caused by larval feeding, the larvae are very small, and they can be quite numerous,
resulting in complete removal of foliar tissue except for the leaf veins. This is particularly
damaging to seedlings, and may disrupt head formation in cabbage. The presence of larvae in

3. florets can result in complete rejection of produce, even if the level of plant tissue removal is
insignificant.
Figure 2| Diamond-back moth (Plutella xylostella) severe damage to a cabbage plant
Crop loss assessment techniques for diamond back-moth in cabbage
Several methods, which are usually adopted at experimental stations, may be used to estimate
crop losses for cabbage in relation to Plutella xylostella. Such techniques can be categorized
broadly as follows;
Mechanical; the cabbage crop is grown under controlled conditions, or in an enclosure of wire-
gauze or cotton gauze, so as to keep diamond back-moth away and the yield of the crop under
such conditions is compared with that obtained from infected/infested cabbage grown under
similar conditions to estimate the degree of loss due to diamond back-moth (Best and Ruthven,
1995).
Chemical; under this technique the cabbage crop is protected from diamond back-moth by using
chemical plant protection measures. The yield of such a crop is compared with that of the
cabbage grown under similar conditions and which is exposed to natural infection/ infestation for
estimation of the degree of loss in yield. The loss in yield which can be avoided by application of
chemicals and by controlling the incidence of diamond back-moth is defined as avoidable loss.
In these techniques above, the yield per unit area in different fields having different degrees of
incidence of pests and other factors which affect yield are recorded and the relation between the

4. yield of the crop and the intensity and severity of disease is worked out to estimate the crop loss
(Hill, 2008).
This method can be adopted for the estimation of crop losses due to different pests and diseases
on larger areas, after selecting these at random. Estimation of crop loss due to pests can be made
after using standardized measurements techniques and the methodology can be used for finding
the crop loss due to each of the pests and diseases separately and also the total loss due to all
pests and diseases simultaneously by relating such incidence to the yield (Pedigo, 1996).
ii) Locusts loss assessment in maize crop
Mechanism of damage
Crop loss assessment techniques for locusts in maize
To assess the potential impacts of locusts on crop production, some crop models include
coupling points, special model variables whose changing values can be used to represent locusts’
damage to crop organs or growth processes. Examples of coupling point variables include leaf
mass or area, stem mass, root mass and seed mass or number, all of which might be negatively
impacted by locusts (Matthews, 2002).
By identifying specific damage pathways and rates of damage using these variables (example, a
locust infestation might be defined as reducing leaf area by 10% by the 40th day after planting),
growth models can quantify how crop development would be affected and, ultimately, what
would be the impact on crop yield i.e. crop loss.
The coupling point concept was first introduced in 1983 (Boote et al.), and later formally
implemented in the DSSAT crop modeling platform (Hoogenboom et al., 2009) as a Pest Module
(Batchelor et al., 1993).
The Pest Module allows users to input field observations and scouting data on locusts’ damage,
and physical damage to plants or plant components (example, grains or leaves) and to simulate
the likely effects of these locusts on crop growth and economic yield (Jones, et at., 2003).
For damage to be simulated, information must be provided on the specific pathways by which
locusts impact growth processes. These pathways can define, for example, how much damage
occurs on which parts of a plant on a daily basis as either a relative (example, percentage leaf

5. area destruction per day) or an absolute (example, 10g seed destruction per day) value. The
DSSAT Pest Module supports definition of the following pathways of locusts’ impact on crop
growth:
 Leaf mass destruction (%/day or g/m²/day)
 Leaf area destruction (%/day or g/m²/day)
 Stem mass destruction (%/day or g/m²/day)
 Number of plants destroyed (#/m²/day)
 Share of plants destroyed (%/day)
 Reduction in assimilation of biomass (%/day)
 Seed destruction (%/day , #/m2/day, or g/m²/day)

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