Insect Pests of Wheat and Their Management Strategies

Common wheat (Triticum aestivum L.) is the third most important staple food crop worldwide, and it is widely cultivated in more than 150 countries throughout the world, occupying approximately 220 million hectares worldwide and feeding approximately 4.5 billion of the world population.

(Source: FAO 2019)

However, the productivity is limited by a large number of abiotic and biotic constraints.

Insect pests are one of the major biotic factors that limit wheat production worldwide. These pests can cause significant damage to the crop, resulting in reduced yield and quality.

Insects such as aphids, cereal leaf beetle, armyworm, and brown wheat mite can feed on different parts of the wheat plant, including leaves, stems, roots, and grain, leading to a range of symptoms such as stunted growth, reduced photosynthesis, decreased nutrient uptake, and yield loss.

Controlling insect pests in wheat production is essential to minimize yield losses and maintain sustainable agriculture. Various methods are used to control insect pests, including chemical insecticides, biological control agents, cultural practices, and the use of resistant crop varieties.

Aphids

Aphids are a major pest of wheat, causing significant damage to the crop. There are several modes of aphid damage to wheat, including direct feeding damage, vectoring of plant viruses, and secretion of honeydew, which can lead to fungal infections.

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Fig 1. Aphids on wheat spikes (A) and on leaves (B).

▶Life Cycle:

Wheat aphids have a complex life cycle that involves both sexual and asexual reproduction. The overwintering eggs hatch in the spring, and the nymphs emerge and begin feeding on the wheat plant. The nymphs molt several times before reaching maturity and can produce up to ten generations per year. During the summer, the aphids reproduce asexually, giving birth to live young that are genetically identical to the parent. In the fall, the aphids produce sexual females, which mate with males to produce overwintering eggs.

▶Feeding and Reproduction:

Wheat aphids are piercing-sucking insects that feed on the phloem sap of wheat plants. The aphids insert their stylets into the plant tissue and remove the sap, which contains nutrients that the plant needs for growth and development. Aphids also secrete a sugary substance called honeydew, which can lead to the growth of sooty mold on wheat plants. Aphids reproduce rapidly and can produce large numbers of offspring in a short period. The asexual reproduction of aphids allows for rapid population growth,

as each individual aphid can produce up to ten offspring in a single day. The sexual reproduction of aphids produces genetically diverse offspring, which can increase their ability to adapt to changing environmental conditions.

▶Population Dynamics:

The population dynamics of wheat aphids are influenced by several factors, including temperature, humidity, and host plant availability. High temperatures and low humidity can lead to a decrease in aphid populations, while moderate temperatures and high humidity can promote population growth.

▶Common Modes of Damage

1. Direct Feeding Damage

Aphids are piercing-sucking insects that feed on the phloem sap of plants. When they feed on wheat, they remove nutrients from the plant, which can result in reduced plant growth and yield. Aphid feeding can also cause leaf curling and yellowing, stunted growth, and reduced root development.

2. Secretion of Honeydew

Aphids secrete a sugary substance called honeydew, which can lead to the growth of sooty mold on wheat plants. Sooty mold can reduce photosynthesis and cause a decrease in plant growth and yield. Studies have shown that the severity of sooty mold damage to wheat is dependent on the density of aphid infestations. High aphid densities can result in a significant increase in sooty mold growth on wheat plants, leading to a reduction in plant growth and yield.

3. Vectoring of Plant Viruses

Aphids are known vectors of several plant viruses, including barley yellow dwarf virus (BYDV), cereal yellow dwarf virus (CYDV), and wheat streak mosaic virus (WSMV). When aphids feed on wheat plants infected with these viruses, they acquire the virus and can transmit it to healthy plants. This can result in reduced plant growth, yield, and quality.

Cereal Leaf Beetle

The cereal leaf beetle, an invasive insect native to Europe that feeds on cereal crops such as wheat, barley, and oats, is now present in most cereal-growing regions worldwide.

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Fig 2. Adult (A), larvae (B) and eggs (C)of cereal leaf beetle on wheat crop.

▶Life Cycle

Cereal leaf beetle has a single generation per year in temperate regions, although it can have multiple generations per year in warmer regions. Cereal leaf beetle overwinters as adults in the soil or in plant debris, and emerge in the spring to feed and mate. The females lay eggs on the undersides of wheat leaves, and the larvae hatch from these eggs and feed on the leaves and stems of the wheat plant. The larvae molt several times before pupating, and the adults emerge from the pupae in the summer. The adults mate and feed before overwintering in the soil or plant debris.

▶Feeding and Reproduction

Cereal leaf beetle larvae and adults are chewing insects that feed on the leaves and stems of wheat plants. The larvae are more damaging than the adults, and heavy infestations can cause significant yield losses. Cereal leaf beetle reproduction is dependent on several factors, including temperature, host plant quality, and population density. High temperatures and high-quality host plants can lead to increased reproduction and population growth.

▶Population Dynamics

The population dynamics of cereal leaf beetle are influenced by several factors, including weather conditions, host plant availability, and natural enemies. Temperature and rainfall can affect the development and survival of cereal leaf beetle larvae and adults, while host plant availability can impact population growth.

▶Common Modes of Damage

1. Direct Feeding Damage

Cereal leaf beetle larvae and adults are chewing insects that feed on the leaves and stems of wheat plants. The feeding activity of cereal leaf beetle larvae and adults can cause the defoliation of wheat plants, leading to reduced photosynthesis and plant growth. In severe cases, defoliation can cause significant yield losses. Several studies have shown that the severity of cereal leaf beetle damage to wheat is dependent on the larval and adult densities and the timing of infestation. For example, cereal leaf beetle larvae cause more damage to wheat than adults, and earlyseason infestations cause more significant yield losses than late-season infestations.

2. Oviposition

Cereal leaf beetle females lay eggs on the undersides of wheat leaves. The larvae hatch from these eggs and feed on the leaves and stems of the wheat plant. The feeding activity of cereal leaf beetle larvae can cause the defoliation of wheat plants, as described above. In addition to direct feeding damage, oviposition by cereal leaf beetle females can also cause indirect damage to wheat plants. The egg-laying activity of cereal leaf beetle females can lead to the transmission of fungal pathogens, such as Fusarium graminearum, which can cause fusarium head blight in wheat. Fusarium head blight can cause significant yield losses and reduce grain quality.

Brown Wheat Mite (Petrobia latens)

Brown wheat mite occur in wheat in the summer rainfall regions and expected to be severe where drought conditions are frequently encountered. It has gained importance in recent years.

Fig 3. Brown wheat mite on wheat leaves.

▶Identification and Life Cycle

Mites are tiny, spider-like creatures with four pairs of legs and they are about as big as the period at the end of this sentence. Mites are oval with dark red-brown bodies and lighter yellow-orange legs; the front legs are about twice as long as the other three pairs of legs. Unlike mite species that produce webbing, the brown wheat mite is free-living without webbing. In spring and early summer, mites lay red eggs that hatch after a short period of time, producing multiple generations. However, as the season progresses, female mites begin to produce white eggs that remain dormant until fall. The white eggs are a resting stage indicating that mite activity is declining.

▶Plant Damage

Brown wheat mites move from the soil to the host plant to feed on foliage. Feeding produces a fine white speckling called stippling, which is caused by the removal of chlorophyll from each feeding site. Stippling typically coalesces causing the leaves to turn light green to white colored and droughty in appearance; mite damage is often confused with symptoms of drought stress. Heavily infested fields present a scorched withered appearance. It is also responsible for the transmission of barley yellow streak mosaic virus disease.

It has been found that Showers of 12mm or more may lead to the eradication of the mite population, but eggs present in the soil may start a new generation.

Management of volunteer wheat is an important preventive measure for brown wheat mites and other small grain pests. However, once an outbreak occurs, chemical control is the only effective (albeit temporary) management option.

Armyworm

Mythimna separata Walker (Lepidoptera: Noctuidae) is a pest of wheat and is prevalent in U.P, Bihar, Rajasthan, and Punjab. The adult moths are stoutly build and pale brown in colour. Larvae have three, orange, white and brown stripes running the length of each side. The larvae will also have a narrow broken stripe down the center of its back. The presence of black spots located at the top of the four pairs of prolegs. The major damage to the seedlings is caused by caterpillars which move in swarm. After destroying crop of one field, they move to the other crop. The caterpillars feed upon the leaves of the seedlings and devour the ear heads as a result further growth of the plant ceases. They infests spring wheat fields primarily, although winter wheat and spring barley may also be affected.

Fig 4. Armyworm larva with parasitoid eggs on wheat crop.

Whiteflies

Whiteflies are “true bugs” (Hemiptera) that feed on plant sap, much like aphids. Adults are very small (1/16 - 1/10 inch) with powdery white wings. Females lay eggs directly on the undersides of plant leaves. The eggs are elliptical and often deposited in circles. Whiteflies cause direct damage to plants by sucking sap, which alters the leaf color and creates spots. Additionally, as these pests feed on plant sap, they inject toxic substances into the phloem, spreading throughout the entire plant and leading to metabolic imbalance, weakening, chlorosis, and diseases in grains. Indirect damage occurs when the honeydew excreted by nymphs enables fungi (such as sooty mold) to colonize the leaves, covering the surface and forming a barrier that reduces the plant's photosynthetic efficiency. But the most severe damage whiteflies inflict on crops lies in transmitting viruses.

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Fig 5. Adult (A) and eggs (B) of whiteflies.

Management Strategies for Insect Pests of Wheat

Insect pests can have a significant impact on crop production, causing economic losses and reducing yields. Effective management of insect pests requires an integrated approach that considers ecological and social impacts. Here are some of the different approaches for the management of insect pests:

▶Cultural Control Methods:

Cultural control methods involve modifying the crop environment to reduce pest populations. Crop rotation, which involves alternating crops in a field, can reduce the buildup of pest populations by interrupting their life cycle. Sanitation practices, such as removing crop debris after harvest, can also reduce pest populations.

▶Biological Control Methods:

Biological control methods involve the use of natural enemies such as predators, parasites, and pathogens to control insect pests. Predators such as ladybugs and lacewings can be introduced into a field to control aphid populations. Parasites, such as wasps, can be used to control the populations of caterpillars other insects. Pathogens, such as Bacillus thuringiensis (Bt), can be used to control the populations of insects that feed on crops.

▶Physical Control Methods:

Physical control methods involve using physical barriers to prevent insect pests from accessing crops. For example, netting can be used to prevent insects from accessing crops or sticky traps can be used to capture insects.

▶Biotechnology:

Biotechnology involves the use of genetic engineering to develop insect-resistant crop varieties. For example, Bt genes can be inserted into crops to make them resistant to certain insect pests. Biotechnology can be effective in controlling insect pests, but there are concerns about the potential ecological and social impacts of genetically modified organisms.

▶Chemical Control Methods:

Chemical control involves using chemical substances to kill insects. The main advantages of chemical control technology are its high effectiveness, ease of use, and minimal impact by time and space constraints.

However, the use of chemicals has certain impacts on the soil and environment. With long-term and extensive use, resistance in pests can develop, affecting subsequent effectiveness.

Therefore, when using chemicals for control, it is essential to select appropriate pesticides, rotate them frequently, and apply appropriate amounts.

▶Nutrition-Mediated Pest Resistance

Scientific fertilization is key to boosting wheat's nutrition and resilience against stresses and pests.

With the continuous advancement of agricultural science and technology in recent years, people have gained a deeper understanding of the nutrients required for wheat growth and soil characteristics.

Scientific fertilization requires us to precisely determine the types of fertilizers needed based on factors such as the growth stage of wheat, soil conditions, and climatic environment.

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