Pest Damage Assessment and Crop Yields

There is need for continuous pest damage assessments to understand how best to manage pests to prevent them from causing losses to the farm since the ultimate aim of agriculture is to produce a sustained economic yield of crop production, so it becomes of prime importance to understand the effect of the insect pest population on the subsequent yield or harvest.

Obviously, if the pests are causing no crop loss their presence on the plants and the damage they cause may be ignored, and in the context of ecological stability, they should be left alone!

However, most pest populations produce some damage of significance, but the damage assessment in relation to possible or expected yield loss is difficult.

The total number of interacting factors responsible for determining crop yield is quite overwhelming, and any decision as to the probable effect of any single factor, such as the population of one insect pest species, is problematic.

However, the gradual accumulation of empirical data over many years has resulted in our being able to make various generalizations about some pest populations and their probable effect on crop yield.

These results are used to define economic injury levels (and economic thresholds) for some pests on some crops in different parts of the World. In general, much more data are required for many more pests on the more important crops, especially those in the tropics.

What is Pest Damage Assessment? Pest Damage Assessment refers to the systematic evaluation of the impact of pests on crops, taking into account the extent of the damage caused and the potential economic losses incurred.

This crucial process allows farmers and agricultural experts to quantify the effects of pest infestations, determine the severity of the damage, and make informed decisions regarding appropriate management strategies.

During the assessment, various factors are considered, including the type of pest, the stage of the crop’s growth when the infestation occurred, and the specific plant parts affected.

Assessors closely examine the visible symptoms, such as leaf discoloration, deformation, or the presence of lesions, as well as any observable changes in the plant’s growth rate and overall vitality.

Moreover, the assessment takes into account the potential yield losses caused by pest damage, as well as the subsequent reduction in the quality and market value of the affected produce.

By estimating the financial implications of the pest-related losses, farmers can better understand the economic impact and the need for timely and effective pest management interventions.

Pest damage assessment is not limited to quantifying immediate losses; it also helps predict potential long-term consequences, such as the increased susceptibility of crops to diseases or the persistence of pest populations across multiple growing seasons.

By understanding the broader implications of pest damage, farmers can proactively implement strategies to prevent future infestations and minimize the risk of recurring losses.

Through regular and meticulous pest damage assessment, farmers can make informed decisions about the most suitable and sustainable pest management practices to adopt, whether through the use of biological control methods, cultural practices, or the judicious application of pesticides.

This process plays a vital role in promoting efficient resource allocation, optimizing crop yield, and ensuring the overall resilience and sustainability of agricultural systems.

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Meaning of Crop Yields? Crop yields refer to the quantity of agricultural produce harvested from a particular area of land within a given time frame, typically measured in terms of weight or volume per unit area.

This essential metric serves as a key indicator of the efficiency and productivity of farming practices, providing valuable insights into the success of crop cultivation and the overall performance of agricultural systems.

A high crop yield signifies that a farmer has successfully maximized the production of crops from their land, achieving an abundant harvest relative to the resources invested, such as labor, water, fertilizers, and other inputs.

Factors contributing to favorable crop yields include favorable weather conditions, the use of high-quality seeds, effective pest and disease management, and the implementation of appropriate agricultural practices.

Conversely, low crop yields may result from various factors, including adverse weather events, poor soil fertility, inadequate irrigation, pest infestations, and suboptimal farming techniques.

Insufficient crop yields can significantly impact the livelihoods of farmers, leading to financial instability, food insecurity, and limited market competitiveness.

Measuring and analyzing crop yields play a crucial role in identifying areas for improvement and implementing targeted interventions to enhance agricultural productivity.

By understanding the factors influencing crop yields, farmers can make informed decisions about the selection of crop varieties, the implementation of sustainable farming practices, and the adoption of innovative technologies to optimize production and minimize resource wastage.

Moreover, monitoring crop yields over multiple seasons provides valuable data for assessing the long-term sustainability of agricultural practices and their impact on the environment.

Sustainable farming techniques that prioritize soil health, water conservation, and biodiversity preservation can contribute to improved crop yields while minimizing the ecological footprint of agricultural activities.

Ultimately, striving for optimal crop yields is essential for ensuring food security, supporting rural livelihoods, and meeting the growing demand for agricultural products globally.

By prioritizing sustainable and efficient farming practices, farmers can contribute to the resilience and long-term viability of agricultural systems, fostering a more stable and secure food supply for communities worldwide.

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Part of the Plant Body Damaged

Some types of damage are obviously more important than others, depending upon the part of the plant body damaged and the part harvested; if the two are the same then clearly the damage is more serious.

A single Codling Moth larva can effectively destroy a single apple or peach, and a relatively small number can ruin an entire crop. On the other hand, an apple tree can accommodate a large number of foliage-eating caterpillars, sap-sucking bugs, and root-eating beetle larvae, with no discernible loss of yield.

In general, root crops can stand considerable leaf damage without appreciable yield loss; pulses and most cereals can likewise tolerate leaf-eating, root-eating, and sap-sucking (with a few exceptions) at a moderate level.

Vegetables such as cabbage, lettuce, celery, etc., may have their outside leaves removed at harvest, prior to sale, so damage to the outer leaves is relatively unimportant.

It has long been known that many crop plants can tolerate partial defoliation without discernible loss of yield; two well-documented cases are cucumbers attacked by Red Spider Mite, where 30% of leaf cover has to be damaged before there is an effect on yield, and sugarbeet, where young crops suffer little loss of yield unless defoliation exceeds about 50%.

In summary, damage that can be ignored on one crop may be of considerable economic importance on another (even closely related) crop, so damage assessment is different for each crop grown.

For example, Fig. 1 shows five different Brassica crops together with a seedling; should the seedling be killed by Cabbage Root Fly, cutworm, or white grub, then the damage is usually serious for the entire future plant is lost and there is a large gap left in the field which encourages weed development (except rape).

With Broccoli, the flower heads are eaten, so damage to lower leaves and some root damage can be ignored. Brussels sprouts are lateral buds harvested over the winter period, so late caterpillar defoliation in the autumn may be of no consequence, but a single Cabbage Root Fly inside a button for freezing is serious damage.

The many types of Cabbage are grown for the ‘heart’, so all the outer leaves may be damaged without affecting the salability of the heart. Turnip is one of the cruciferous root crops and will tolerate considerable leaf damage, but even slight Cabbage Root Fly damage may spoil the appearance of the root.

Rape is becoming more and more important in many regions as a source of seed for oil extraction, and in this crop, it is the flower and pod pests that are important (as they are also for the other crops grown to seed); small numbers of plants destroyed have no effect on final yield because of the density of the crop.

Brassica seedling.

Broccoli (B. oleracea var. botrytis), grown for flower heads.

Brussels sprouts (B. oleracea var. gemmifera), grown for lateral buds.

Cabbage (B. oleracea var. capitata), grown for ‘heart’, i.e. telescoped the main shoot.

Turnip (B. rapa), grown for the swollen root.

Rape (B. napus), grown for seeds as a source of oil.

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Pest Infestations

There are two basic ways in which pest infestation (or damage) can be assessed. The incidence of the pest (or damage symptoms) is generally the proportion of plants in a sample that are host to the pest (or which show damage symptoms), and is usually expressed as a percentage.

The severity of the infestation is a measure of the size of the pest population on the plants, or the extent of the damage done, and is often measured as so many insects per plant, per bush, per 10 leaves, egg masses per plant, etc.

And of course, the total damage caused to a crop is a combination of the severity of infestation together with duration (time). In ecological studies of populations of both plants and animals, several methods of assessment are employed.

These are based upon the proportion of area covered within the habitat (for plants), the number of animals seen or sampled, in relation to the area, or the proportion of plants examined (sampled).

Botanists are able to use more precise systems for plant population assessment because of the immobility of the organisms, and the three most widely used methods employ between 4 (Raunkiaer, 1934), 6 (Braun-Blanquet, 1927) and 11 (Domin) abundance categories.

However, for small, highly mobile insects the latter level of precision is not feasible, especially when different recorders are being used.

It is advocated that the use of four abundance/frequency categories for population size assessment, without the use of lengthy or detailed sampling procedures, and this approach would seem to be appropriate for assessing field populations of insect pests on crops.

These categories of abundance are as follows:

Abundant (a) =          very common (VC)

Frequent (f)   =          common (C)

Occasional (o) =      uncommon (U)

Rare (r)           =          rare (R)

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Pest Damage Assessment

The term injury is used to connote slight (i.e. non-damaging) effects of insects feeding or other activities on the growth or appearance of crop plants, and damage is an injury resulting in a measurable loss of yield or reduction in quality.

This distinction would be most useful, but to date in the literature, these two terms are used more or less synonymously, and of course, the term ‘economic injury level’ is well established.

The extent of crop damage is usually proportional to the number of insects present, and would accordingly be rated as follows:

In some systems of recording a numerical categorization is used, and a six-point scale seems to be quite popular; presumably in this case number 6 (and ‘very mild’) come under the ‘injury’ category of Bardner & Fletcher (1974) and would be detectable but not of any economic importance.

With simple damage, such as leaf lamina being eaten or apples infested with Codling Moth larvae, damage can be expressed easily as a proportion of lamina destroyed or the percentage of fruits infested per tree.

In some systems number of pests present is correlated empirically against the expected loss of yield (percentage), on a scale of no loss (0%) to total loss (100%).

Because each crop has its own growth characteristics (see below), and the vast diversity of types of pests and pest damage and all the other factors involved with crop production, it is not possible to generalize extensively.

Damage assessment will remain different for each crop and sometimes also for each major locality as the pest complex usually varies regionally.

It is however generally agreed that for most purposes a damage assessment scale of not more than six levels is preferable, for easy recognition in the field by Non-experts, and it is recommended that a large number of small samples be taken rather than a small number of large samples; this also caters better for the uneven pest distribution within the crop, which is usual.

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