9.5 Nematode and insect pests

Nematodes and insects are major plant pests that can wreak havoc on crops. These tiny roundworms and diverse arthropods feed on various plant parts, causing damage and yield losses. Understanding their life cycles and feeding habits is crucial for effective management.

Nematodes pierce plant cells with stylets, while insects have specialized mouthparts for chewing or sucking. Both can cause significant economic impacts through yield reduction, quality loss, and increased control costs. Integrated pest management combines cultural, biological, and chemical methods to minimize damage sustainably.

Types of nematode pests

• Nematode pests are microscopic roundworms that feed on plant roots, stems, and leaves causing significant damage to crops and ornamental plants
• Nematodes have specialized mouthparts called stylets that pierce plant cells and extract nutrients leading to various symptoms and reduced plant health

Root-knot nematodes

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• Root-knot nematodes (Meloidogyne spp.) are the most economically important group of plant-parasitic nematodes
• Infect a wide range of crops including vegetables, fruits, and ornamentals (tomatoes, potatoes, cotton)
• Cause distinctive swellings or galls on the roots disrupting water and nutrient uptake
• Galls can also serve as entry points for other soil-borne pathogens

Cyst nematodes

• Cyst nematodes (Heterodera and Globodera spp.) are another major group of plant-parasitic nematodes
• Form protective cysts containing eggs that can survive in the soil for many years (soybean cyst nematode, potato cyst nematode)
• Cysts hatch in response to root exudates and larvae invade the roots causing stunting and yellowing

Lesion nematodes

• Lesion nematodes (Pratylenchus spp.) are migratory endoparasites that feed and move within the root cortex
• Cause necrotic lesions on the roots reducing root function and predisposing plants to other pathogens (banana root nematode)
• Can also feed on above-ground plant parts such as stems and leaves

Foliar nematodes

• Foliar nematodes (Aphelenchoides spp.) are unique in that they feed on leaves and buds rather than roots
• Cause angular leaf spots, distortion, and necrosis on the foliage (chrysanthemum foliar nematode)
• Spread by water splash and infested plant material
• Require high humidity for survival and reproduction

Nematode pest life cycles

• Understanding the life cycle of nematode pests is crucial for developing effective management strategies and timing control measures appropriately
• Most plant-parasitic nematodes have a similar basic life cycle consisting of an egg stage, four larval stages, and an adult stage

Egg stage

• Nematode eggs are typically laid in the soil or within plant tissues (root galls, cysts)
• Eggs are protected by a tough, resistant shell that helps them survive adverse conditions
• Embryonic development occurs within the egg and the first-stage larva (J1) molts to the second-stage larva (J2) before hatching

Larval stages

• After hatching, nematodes go through four larval stages (J2, J3, J4) before reaching adulthood
• Each larval stage is separated by a molt where the old cuticle is shed and a new one is formed
• J2 larvae are the infective stage that locates and penetrates plant roots using chemotaxis and thigmotaxis
• J3 and J4 larvae feed and develop within the root, causing damage to plant tissues

Adult stage

• Adult nematodes are sexually dimorphic with distinct male and female forms
• Females are typically sedentary and continue feeding to produce eggs (root-knot and cyst nematodes)
• Males are usually vermiform and migrate out of the root to mate with females
• Some species like lesion nematodes have both migratory males and females

Reproduction and survival

• Nematodes can reproduce sexually or parthenogenetically (without males)
• Females can lay hundreds of eggs either in the soil, within root galls, or retained within their bodies (cysts)
• Eggs and larvae can survive in the soil for extended periods (several years for cyst nematodes) until a suitable host is present
• Survival structures like cysts, egg masses, and anhydrobiotic larvae help nematodes persist in the absence of hosts

Nematode damage to plants

• Nematode feeding on plant roots and other tissues leads to various symptoms that can significantly impact plant growth, yield, and quality
• The type and severity of damage depends on the nematode species, population density, host plant, and environmental conditions

Root galling and malformation

• Root-knot nematodes induce the formation of characteristic galls or knots on the roots
• Galls result from hyperplasia and hypertrophy of root cells in response to nematode feeding and secretions
• Galled roots have a reduced ability to absorb water and nutrients and are more susceptible to other pathogens

Nutrient deficiencies

• Nematode feeding disrupts the root system's ability to take up and translocate nutrients to the shoots
• Plants infested with nematodes often exhibit symptoms of nutrient deficiencies such as chlorosis (yellowing), stunting, and reduced vigor
• Deficiencies are particularly pronounced for nutrients with limited mobility in the plant (iron, manganese)

Stunted growth

• Nematode damage to the root system leads to reduced plant growth and stunting
• Infested plants may be smaller, have fewer leaves and branches, and show an overall decline in health
• Stunting is often patchy within a field with heavily infested areas showing more severe symptoms

Yield reduction

• Nematode infestations can cause significant yield losses in susceptible crops
• Yield reductions result from the combined effects of root damage, nutrient deficiencies, and stunted growth
• Losses can range from 10-30% in moderately infested fields to over 50% in heavily infested areas
• Quality of the harvested product (fruits, vegetables, grains) may also be compromised due to nematode damage

Types of insect pests

• Insect pests are a diverse group of arthropods that feed on various plant parts causing damage and yield losses
• Insects have different feeding habits and mouthpart structures that determine the type of damage they inflict on plants

Chewing insects

• Chewing insects have biting-chewing mouthparts adapted for consuming plant tissue
• Include caterpillars, beetles, grasshoppers, and leaf-feeding insects (Colorado potato beetle, Japanese beetle)
• Cause visible holes, skeletonization, or complete defoliation of leaves reducing photosynthetic capacity
• Some chewing insects also feed on roots (white grubs), stems (corn borers), and fruits (codling moth)

Sucking insects

• Sucking insects have piercing-sucking mouthparts that penetrate plant tissues and extract sap
• Include aphids, leafhoppers, mealybugs, whiteflies, and scale insects
• Cause stippling, yellowing, and distortion of leaves due to removal of plant fluids
• Excrete honeydew which promotes the growth of sooty mold fungi on leaf surfaces
• Can transmit plant viruses through their feeding activities (aphids, leafhoppers)

Boring insects

• Boring insects have chewing mouthparts adapted for tunneling into plant stems, trunks, or roots
• Include wood-boring beetles, weevils, and moth larvae (emerald ash borer, corn rootworm)
• Cause structural damage to plants weakening them and increasing susceptibility to other stresses
• Larval feeding inside plant tissues disrupts vascular transport and can lead to plant death

Leaf miners

• Leaf miners are the larval stages of certain flies, beetles, and moths that feed between the upper and lower leaf surfaces
• Create characteristic serpentine or blotch-shaped mines in the leaves (citrus leaf miner, spinach leaf miner)
• Mining reduces photosynthetic area and can cause premature leaf drop
• Heavy infestations can result in significant defoliation and yield losses

Insect pest life cycles

• Insect pests undergo different types of development and metamorphosis from egg to adult
• Understanding the life cycle of a particular pest is important for timing control measures and predicting population dynamics

Complete metamorphosis

• Insects with complete metamorphosis have four distinct life stages: egg, larva, pupa, and adult
• Larvae are the primary feeding stage and often cause the most damage to plants (caterpillars, beetle grubs)
• Pupae are a non-feeding, transitional stage where the larva transforms into an adult
• Adults have different morphology and behavior than larvae and may or may not feed on plants (moths, beetles)

Incomplete metamorphosis

• Insects with incomplete metamorphosis have three life stages: egg, nymph, and adult
• Nymphs resemble small, wingless adults and feed on the same plant parts as adults (aphids, leafhoppers)
• Nymphs molt several times before reaching the adult stage, gradually developing wing buds and reproductive organs
• Adults have fully developed wings and are capable of reproducing and dispersing to new host plants

Overwintering strategies

• Insect pests have various strategies for surviving unfavorable winter conditions
• Some insects overwinter as eggs (gypsy moth), larvae (corn earworm), pupae (tobacco hornworm), or adults (bean leaf beetle)
• Overwintering stages are often hidden in protected sites such as soil, leaf litter, or under bark
• Diapause is a hormonally mediated state of arrested development that allows insects to survive cold or dry periods

Migration and dispersal

• Many insect pests are capable of long-distance migration to find suitable host plants and escape unfavorable conditions
• Migratory species often have multiple generations per year and can rapidly colonize new areas (armyworm, potato leafhopper)
• Dispersal can occur by active flight (moths, beetles) or passive transport by wind or human activities (aphids, whiteflies)
• Understanding migration and dispersal patterns is important for predicting pest outbreaks and implementing regional control strategies

Insect damage to plants

• Insect pests cause various types of damage to plants depending on their feeding habits and the plant parts they attack
• Damage can range from minor cosmetic injury to complete crop loss and can have significant economic consequences

Leaf damage and defoliation

• Many insect pests feed on leaves causing holes, skeletonization, or complete defoliation
• Defoliation reduces the plant's photosynthetic capacity leading to reduced growth and yield
• Examples include caterpillars (armyworms, loopers), beetles (Japanese beetle, bean leaf beetle), and sawflies

Stem and trunk damage

• Boring insects tunnel into plant stems and trunks disrupting vascular transport and weakening the plant
• Stem borers (European corn borer) and trunk borers (emerald ash borer) can cause lodging, dieback, and plant death
• Girdling of stems or trunks by beetles (Asian longhorned beetle) can also lead to plant mortality

Fruit and seed damage

• Some insect pests specifically target fruits and seeds reducing yield and quality
• Fruit feeders (codling moth, plum curculio) cause internal damage and premature fruit drop
• Seed feeders (pea weevil, bean weevil) reduce germination and vigor of infested seeds
• Damage to fruits and seeds can also provide entry points for secondary pathogens

Transmission of plant diseases

• Many insect pests are vectors of plant viruses, bacteria, and other pathogens
• Sucking insects like aphids and leafhoppers can acquire and transmit viruses from infected to healthy plants during feeding
• Beetles and thrips can spread bacterial and fungal pathogens on their bodies or through feeding wounds
• Insect-vectored diseases (citrus greening, tomato spotted wilt virus) can cause significant yield losses and are often more difficult to control than the insect itself

Integrated pest management

• Integrated pest management (IPM) is a holistic approach to managing pests that combines multiple tactics to reduce pest populations and minimize economic, health, and environmental risks
• IPM programs rely on regular monitoring, accurate pest identification, and use of economic thresholds to guide management decisions

Cultural control methods

• Cultural control involves modifying crop production practices to create unfavorable conditions for pests or promote plant health
• Examples include crop rotation, sanitation (removal of crop residues), adjusting planting dates, and using pest-resistant varieties
• Proper irrigation, fertilization, and pruning can also help plants tolerate or compensate for pest damage

Biological control agents

• Biological control uses natural enemies (predators, parasitoids, pathogens) to suppress pest populations
• Conservation biocontrol involves protecting and enhancing existing natural enemy populations through habitat management and selective pesticide use
• Augmentative biocontrol involves the mass rearing and periodic release of natural enemies (ladybugs, lacewings, parasitic wasps)
• Classical biocontrol involves the introduction of exotic natural enemies from the pest's native range to provide long-term control

Chemical control options

• Chemical control uses pesticides (insecticides, nematicides) to kill or suppress pest populations
• Pesticides should be used judiciously and only when other control methods are insufficient to prevent economic damage
• Selective pesticides (Bt, insect growth regulators) are preferred over broad-spectrum products to minimize non-target effects
• Proper pesticide selection, timing, and application technique are critical for effective control and resistance management

Monitoring and decision-making

• Regular monitoring of pest populations and crop damage is the foundation of an IPM program
• Monitoring methods include visual inspection, trapping (pheromone traps, sticky cards), and sampling (sweep nets, beat sheets)
• Economic thresholds are pre-determined pest densities at which control actions are justified to prevent economic losses
• Decision-making in IPM considers multiple factors such as pest biology, crop stage, weather conditions, and available control options

Plant resistance to pests

• Plant resistance is the inherent ability of a plant to tolerate, suppress, or overcome the effects of pest infestations
• Resistant plants can reduce pest populations and damage without the need for external control measures

Genetic resistance

• Genetic resistance is conferred by specific genes or gene combinations that provide defense against pests
• Resistance genes can encode physical barriers (thicker cuticle, trichomes), chemical defenses (alkaloids, terpenes), or physiological responses (hypersensitive reaction)
• Examples of genetically resistant crops include Bt cotton (resistant to bollworms) and Mi tomato (resistant to root-knot nematodes)

Induced resistance

• Induced resistance is a plant's ability to enhance its defense mechanisms in response to pest attack or other stresses
• Induced resistance can be triggered by chemical elicitors (salicylic acid, jasmonic acid) or by exposure to non-damaging levels of pests
• Induced defenses include production of toxins, digestive inhibitors, or volatile compounds that attract natural enemies

Tolerance vs resistance

• Tolerance is a plant's ability to withstand pest damage without significant loss of growth or yield
• Tolerant plants may have compensatory growth mechanisms or the ability to recover from pest injury
• Resistance, on the other hand, is the plant's ability to reduce pest preference, performance, or reproduction
• Resistant plants actively defend themselves against pests through antixenosis (non-preference), antibiosis, or tolerance mechanisms

Breeding for pest resistance

• Breeding for pest resistance involves the selection and development of plant varieties with enhanced resistance traits
• Resistance genes can be introduced through traditional breeding methods (crossing, backcrossing) or genetic engineering (transgenic crops)
• Breeding programs often aim to pyramid multiple resistance genes to provide more durable and broad-spectrum resistance
• Challenges in resistance breeding include the potential for pests to evolve counter-resistance and the need to balance resistance with other desirable agronomic traits

Economic impact of pests

• Pests can have significant economic consequences for crop production, food security, and international trade
• The economic impact of pests includes direct losses from reduced yield and quality as well as indirect costs associated with pest management and trade restrictions

Crop yield losses

• Pests can cause substantial yield losses in agricultural crops ranging from 10-30% or more depending on the pest species and infestation level
• Yield losses result from direct damage to harvestable plant parts (fruits, grains) or from reduced plant growth and photosynthetic capacity
• Examples of major yield-reducing pests include soybean cyst nematode, cotton bollworm, and potato late blight

Quality reduction

• Pest damage can also reduce the quality and marketability of harvested crops
• Quality losses include cosmetic damage (scarring, discoloration), reduced shelf life, and contamination with pest fragments or frass
• Pests that directly attack fruits and vegetables (codling moth, tomato fruitworm) can render them unmarketable or reduce their grade and value

Control costs

• Pest management requires significant investments in monitoring, cultural practices, biological control agents, and pesticides
• Control costs can include material inputs (traps, lures, pesticides), labor (scouting, application), and equipment (sprayers, tractors)
• Indirect costs of pest management may include environmental and human health impacts, pesticide resistance, and disruption of natural enemy populations

Trade and export implications

• Pests can also have economic impacts beyond the farm level by affecting trade and export markets
• Quarantine pests are those that are absent or have limited distribution in an area and are subject to official control measures
• Presence of quarantine pests can result in trade restrictions, export bans, or costly phytosanitary treatments (fumigation, irradiation)
• Examples of quarantine pests include Mediterranean fruit fly, khapra beetle, and golden nematode