Overview of Aphids

Introduction

Decisions on managing and controlling aphids in potato crops depend on whether the crop is grown for seed or consumption. For seed potato crops, aphid control is critical to prevent the spread of viruses such as Potato Leafroll Virus (PLRV) and viruses causing mild or severe mosaic (e.g., Potato Virus Y, PVY). Aphids act as vectors for these viruses, and effective control is essential to maintain the health of seed potatoes for further production. In consumption crops, heavy aphid infestations can reduce yields through direct feeding on plant sap, leading to "false top roll" (leaf curling due to sap loss), exacerbated by drought stress, and through primary infection by certain viruses. Chemical control of aphids in consumption crops may be necessary in these cases to protect yield and quality.

Origin and Distribution

Aphids affecting potatoes have diverse origins but are now mostly cosmopolitan due to global trade and agriculture. The green peach aphid (Myzus persicae) originated in Asia, likely alongside its primary host, the peach tree (Prunus persica), and has spread worldwide, first reported in North America in the 1870s and now present in temperate, subtropical and tropical regions except the coldest areas. The potato aphid (Macrosiphum euphorbiae) is native to North America, specifically the northeastern USA, but has become nearly cosmopolitan, spreading to Europe, Asia and beyond through human activities. The glasshouse potato aphid (Aulacorthum solani) is of European origin and is now distributed almost worldwide, particularly in temperate zones and greenhouses. Historically, aphids became significant pests on potatoes after the crops introduction from the Andes to Europe in the 16th century, with widespread recognition as pests in the 18th-19th centuries as potato cultivation expanded globally, facilitated by trade and migration.

Aphid Biology

Reproduction Methods: Aphids reproduce in two ways, laying eggs (oviparity) or giving birth to live young (viviparity). The reproductive method depends on environmental conditions, such as temperature and food availability.

Parthenogenesis and Live Birth: When food is abundant typically in warmer months, aphids use parthenogenesis, where females produce live young without fertilization. This leads to rapid population growth as aphids produce many offspring (up to 60 per female), have a short lifespan (about 23 days), and even pre-adult females can reproduce. In potato crops, this rapid reproduction increases the risk of virus transmission, such as Potato Virus Y (PVY).

Life Cycle: Eggs when laid, hatch in three to four days. Young aphids called nymphs, take five to eight days to mature into adults, depending on temperature and food supply. Warm conditions accelerate this cycle, amplifying aphid damage in potato fields. Up to 30 generations can occur per year, with a complete cycle as short as 12 days.

Seasonal Adaptations: In colder seasons aphids often switch to laying eggs to survive winter, which are laid on host plants or crop residues. These eggs remain dormant until favorable conditions return, ensuring population persistence. In northern climates, adults return to Prunus plants in autumn for mating.

Species Diversity: Common aphid species affecting potatoes include the green peach aphid (Myzus persicae), potato aphid (Macrosiphum euphorbiae), glasshouse potato aphid (Aulacorthum solani), melon aphid (Aphis gossypii) and turnip aphid (Lipaphis erysimi). Each species varies in virus transmission efficiency and host preference, impacting potato crop management. M. persicae is the most significant vector for over 100 viruses, including PVY and Potato Leaf Roll Virus.

Feeding and Damage: Aphids feed on plant sap using piercing mouthparts, weakening potato plants and causing leaves to curl or growth to stunt. Their feeding also produces honeydew, promoting sooty mold, which further reduces photosynthesis and crop quality.

Favorable Conditions

Optimal Climate: Aphids thrive in potato crops at temperatures between 20–25°C, ideal for development and reproduction of species like the green peach aphid (Myzus persicae). Population growth is optimal at around 26.7°C with lower and upper thresholds of 6.5°C and 37.3°C.

Population Decline: Aphid populations sharply decline when relative humidity exceeds 73%, as high moisture levels hinder their reproduction and survival. They prefer moderate to low humidity (around 60-70%).

Seasonal Influence: Warm, dry conditions in spring and early summer favor rapid aphid population growth, increasing the risk of infestations in potato fields. Aphids are sensitive to temperature changes, with survival and fecundity peaking at 15-20°C for some strains.

Host Plant Availability: Abundant food sources, such as young potato plants or nearby weeds (e.g., nightshade), enhance aphid colonization and reproduction rates. Fields near overwintering hosts like Prunus spp. are at higher risk.

Natural Enemies of Aphids

Parasitoids: Several parasitic wasps, including Lysiphlebus sp., Diaeretiella sp., Aphelinus sp., and Aphidius colemani, lay eggs inside aphids, killing them as the larvae develop. These are effective in reducing aphid populations in potato crops.

Predators: Natural predators such as ladybird beetles (Coccinellidae), lacewings (Chrysopidae), spiders, and hoverfly larvae (Syrphidae) feed on aphids, helping to control infestations naturally. Lady beetles and lacewings are particularly abundant in untreated fields.

Fungal Pathogens: Entomopathogenic fungi, like Beauveria bassiana, can infect and kill aphids under humid conditions, serving as a natural control mechanism in potato fields. These are more effective in high humidity (>70%).

Role in IPM: Encouraging natural enemies through habitat diversification (e.g., planting wildflower strips near potato fields) enhances biological control, reducing reliance on chemical pesticides. Flower strips have been shown to decrease aphid density by attracting enemies.

Methods of prevention and control

Monitoring

Scouting Practices: Regularly monitor potato fields for aphids by checking 25-50 plants per field section, focusing on lower leaves and stems. Use yellow sticky traps to detect winged aphids early in the season.

Economic Thresholds: Initiate control when aphid populations reach 2-5 aphids per leaf for seed potatoes or 10-20 per leaf for consumption crops, especially if virus transmission risk is high. Monitor 6-8 weeks before harvest for potato aphids.

Virus Monitoring: Use aphid traps and test for virus presence in plants to predict outbreaks, integrating with regional monitoring programs.

Cultural Control

Healthy Seed Selection: Use certified, virus free seed potatoes to reduce the risk of aphid transmitted viral infections, such as PVY and PLRV, ensuring healthier crop establishment.

Early Planting: Plant early bulking or early-maturing potato cultivars to align with aphid free periods, allowing seed crops to escape peak aphid vector populations and reduce virus transmission.

Crop Rotation: Rotate potatoes with non host crops (e.g., cereals) to disrupt aphid life cycles and reduce overwintering egg populations in potato fields.

Weed Management: Remove weeds like nightshade, which serve as alternate hosts for aphids, to limit their population buildup and virus reservoirs.

Irrigation and Fertilization: Maintain balanced irrigation and avoid excessive nitrogen fertilizer, as lush growth attracts aphids. Proper water management also mitigates drought stress, which exacerbates aphid damage.

Green Manuring: Incorporate green manure crops (e.g., clover, mustard) to improve soil health and deter aphids by altering field microclimates or attracting natural enemies.

Trap Crops: Plant attractive crops like mustard or radish near potatoes to draw aphids away, then treat or remove the trap crops to reduce main crop infestation.

Resistant Varieties: Select potato varieties with partial resistance to aphids or viruses (e.g., 'Russet Burbank', 'Shepody' for some tolerance to PVY), though full resistance is rare.

Biological Control

Neem-Based Sprays: Apply neem seed kernel extract (NSKE) at 5% concentration to repel aphids and disrupt their feeding and reproduction, offering an organic control option for potato crops.

Encouraging Natural Enemies: Promote populations of predators (e.g., ladybird beetles, lacewings) and parasitoids (e.g., Aphidius colemani) by maintaining diverse habitats, such as wildflower strips, near potato fields.

Microbial Agents: Use entomopathogenic fungi like Beauveria bassiana or Metarhizium anisopliae as bio-pesticides, particularly in humid conditions to target aphid populations without harming beneficial insects.

Mineral Oils: Apply paraffinic mineral oils weekly to interfere with aphid feeding and virus transmission, reducing populations organically.

Chemical Control for Aphid Management

• Carbofuran 3% CG – Apply 6.64 kg/acre (16.5 kg/ha) as a soil treatment to control aphids effectively.
• Oxydemeton-methyl 25% EC – Use 0.4 L in 200–400 L of water/acre (equivalent to 1 L/ha in 500–1000 L water/ha) as a foliar spray for systemic control of aphids.
• Thiamethoxam 25% WG – Apply 40 g in 200 L of water/acre (or 100 g/ha in 500 L water/ha) as a foliar spray. Alternatively, use as a soil drench at 80 g/acre (200 g/ha) in 200–500 L of water for longer-lasting protection.
• Phorate 10% CG – Apply 4 kg/acre (10 kg/ha) as a granular soil application to target aphids early in the crop cycle.

Additional options

• Imidacloprid – At planting, apply 0.35 fl oz/1000 ft of row for 60–90 days of protection.
• Spirotetramat – Foliar spray at 4–5 fl oz/acre.
• Flupyradifurone (Sivanto Prime) – Foliar spray at 7–10.5 fl oz/acre.

"Effective aphid management in potato crops is not just about pest control it is about safeguarding yield and quality against the silent threat of virus transmission."