Pests

1. Coarse defoliation (eating of leaves)

If you see leaves partially or completely eaten, sometimes down to the main vein, with only petioles and stems left on the bushes, this is the work of pests with powerful chewing mouthparts. This is the most visible and rapid type of damage.

• Who does it: The main "culprit" is the Colorado potato beetle. Both adult beetles and their voracious larvae cause damage. Besides the Colorado potato beetle, severe defoliation can also be caused by caterpillars of some noctuid moths (Noctuidae) and larvae of leaf beetles (e.g., the 28‑spotted potato ladybird Henosepilachna vigintioctomaculata in Asia) (Kroschel et al., 2020; De Jong et al., 2011).
• Why it is dangerous: Severe leaf loss before flowering and at the start of tuber formation can reduce yields by 30‑50%, because the plant loses its ability to accumulate nutrients for the tubers (Alyokhin, 2009).

2. Numerous small holes ("shot‑hole" damage)

Leaves are covered with many small, nearly round holes, resembling shotgun pellet marks.

• Who does it: Flea beetles (Flea beetles) — small beetles (about 1.5 mm) that jump when disturbed. Particularly dangerous are species of the genus Epitrix (De Jong et al., 2011).
• Why it is dangerous: The "shot‑hole" damage itself rarely kills the plant, but it weakens young bushes and makes leaves more vulnerable to fungal diseases, such as early blight (Alternaria solani). In greenhouses, leaves can also be damaged by whiteflies and thrips, leaving characteristic pale spots and dots.

3. Leaf mines — winding tunnels inside the leaf

Inside the leaf blade, between the upper and lower epidermis, thin winding pale tunnels (mines) are visible. Inside, you may notice black dots — larval droppings.

• Who does it: Larvae of leafminer flies (Leafminer flies), particularly Liriomyza huidobrensis. This is a serious pest in tropical and subtropical regions (Kroschel et al., 2020; Radcliffe & Lagnaoui, 2007).
• Why it is dangerous: The damaged leaf tissue loses its ability to photosynthesise. In heavy infestations, leaves turn brown and die, which can reduce yields by 30‑40% (Mujica & Kroschel, 2013).

4. Leaf curling, yellowing, and discolouration

If the leaves are not eaten but look diseased — curled, yellowed, reddened, or covered with sticky honeydew — then sucking pests have likely settled on them. They feed on plant sap by piercing the tissues with their stylets.

• Who does it: Primarily aphids (Aphids), especially the green peach aphid and the potato aphid. In heavy infestations, leaves become sticky from their excretions ("honeydew"), on which black sooty mould rapidly develops (Kroschel et al., 2020; Radcliffe & Lagnaoui, 2007).
• Why it is dangerous: Aphids and leafhoppers (Leafhoppers) are the main vectors of dangerous viral diseases, such as Potato leafroll virus (PLRV) and Potato virus Y (PVY). Yield losses from these diseases can reach 50‑80% (Radcliffe & Lagnaoui, 2007). Therefore, the appearance of aphids is a "red alert" signal, especially if you are growing seed potatoes.
• "Hopperburn" and yellowing: If leaves turn yellow at the edges, curl upward and dry out ("hopperburn"), this may be the work of the potato leafhopper (Potato leafhopper). These pests suck sap and inject toxic saliva (Radcliffe & Lagnaoui, 2007).

1.1. Green peach aphid (Myzus persicae)

This is the most dangerous and widespread species. Its origin is East Asia, but today it is found in virtually all potato‑growing regions of the world (Kroschel et al., 2020).

• How to recognise: Colour is highly variable — from light green and yellow to pink or red. Body length 1.2‑2.3 mm. A distinctive feature is that the antennal tubercles on the head converge inward, forming a characteristic groove. Adults can be wingless (apterous) or winged (alate), allowing rapid dispersal.
• Life cycle: In regions with cold winters, aphids overwinter as eggs on peach trees (the primary host). In spring, winged females appear, fly to potatoes and other herbaceous plants, where they reproduce parthenogenetically (without fertilisation) at enormous speed — one female can produce up to 100 offspring in 2‑3 weeks. Under favourable conditions, the population doubles every 1.7 days (Radcliffe & Lagnaoui, 2007).

1.2. Potato aphid (Macrosiphum euphorbiae)

The second most important species, found worldwide.

• How to recognise: Larger than the green peach aphid (2‑4 mm), usually green, but pink forms also occur. Body elongated, with long legs and antennae. At the end of the abdomen, long siphunculi (tubes) with a reticulate structure are visible.
• Characteristics: This species is also an efficient virus vector, though less so than the green peach aphid. It overwinters on roses and other ornamental shrubs.

1.3. Why are aphids dangerous?

Aphids cause double damage:

1. Direct damage: By sucking sap, they weaken plants, slow growth, and cause leaf curling and deformation. At high numbers, leaves become covered with sticky honeydew, on which sooty moulds develop, impairing photosynthesis (Kroschel et al., 2020).

2. Indirect damage — viruses: This is the main problem. Aphids transmit more than 30 potato viruses. The two most dangerous are:

• Potato leafroll virus (PLRV) — a persistent virus transmitted only by aphids that feed on the plant for several hours. It causes leaf rolling, chlorosis, and tuber necrosis ("net necrosis"). Yield losses — up to 50‑80% (Radcliffe & Lagnaoui, 2007).
• Potato virus Y (PVY) — a non‑persistent virus transmitted very quickly (within seconds) during brief probes. It is carried not only by colonising aphids but also by many other species simply flying over the field. PVY causes mosaic, leaf and tuber necroses, and some strains (PVY^NTN) lead to tuber necrotic ringspot disease. Yield losses can reach 80% (Radcliffe & Lagnaoui, 2007; Akhatov et al., 2013).

Key difference: PLRV is transmitted only by "its own" aphids that live on potatoes, whereas PVY is transmitted by dozens of species that may simply land on a leaf for a few seconds. This is why PVY is harder to control: insecticide treatments do not always kill the aphid before it transmits the virus (Ragsdale et al., 2001).

How to tell if the field has a virus, not just aphid damage?

From symptoms on leaves and tubers:

• PLRV: Upper leaves curl upward, become brittle and "leathery", often developing a reddish‑purple (anthocyanin) colour. On cut tubers — a brown net in the vascular ring.
• PVY: Mosaic pattern appears on leaves (alternating light‑ and dark‑green areas), with necrotic spots and streaks on veins. Some strains cause deep ring necroses on tubers.

2.1. Potato leafhopper (Empoasca fabae)

Widespread in North America, but also found in other regions.

• How to recognise: Small, wedge‑shaped, pale green. Adults jump and fly away quickly when disturbed. Nymphs (immatures) resemble adults but are wingless and paler.
• Damage: When feeding, leafhoppers inject toxic saliva, causing yellowing of leaf margins, curling, and drying — so‑called "hopperburn". Severe damage can reduce yields by 30‑75% (Radcliffe & Lagnaoui, 2007).
• Disease transmission: In some regions, leafhoppers transmit phytoplasmas (e.g., agents of purple top and stolbur diseases). These cause stunting, aerial tuber formation, and plant death (Akhatov et al., 2013).

2.2. Beet leafhopper and other species

In Europe and Asia, species of Macrosteles and Hyalesthes are dangerous as they transmit phytoplasmas (e.g., solanaceous stolbur). Symptoms are similar to viral ones: leaves turn yellow and curl, plants are stunted.

4.1. Monitoring — the foundation of success

• Yellow sticky traps. Place them at canopy level to catch winged aphids and whiteflies. Regular counting helps determine the start of flight and take action.
• Visual inspection. Once a week, check the underside of leaves for aphid colonies and leafhopper nymphs. This is especially important before flowering, when plants are most vulnerable to viruses.

4.2. Virus prevention: healthy planting material

The only reliable way to avoid viral epidemics is to use certified (virus‑free) seed potatoes. Viruses are transmitted through tubers, so even a small percentage of infected seed can lead to widespread infection in the field (Radcliffe & Lagnaoui, 2007).

What to do: Purchase seed only from reputable nurseries. If you multiply potatoes yourself, regularly renew the seed stock (every 3‑4 years).

4.3. Cultural methods

• Spatial isolation. Place seed potato plots at least 500 m away from commercial potato fields and other crops that attract aphids (e.g., rape, sunflower).
• Barrier crops. Planting tall cereals (oats, barley) or sunflower around the field perimeter intercepts flying aphids, causing them to land first on the barrier, where they lose the ability to transmit non‑persistent viruses (DiFonzo et al., 1996). A barrier width of just 1‑2 metres already provides an effect.
• Weed control. Many weeds (nightshade, jimsonweed, goosefoot) serve as virus reservoirs and alternative hosts for aphids. Regular weeding reduces the risk of infection (Hanafi et al., 1995).

4.4. Biological control

In nature, aphids are effectively suppressed by natural enemies:

• Predators: ladybird beetles (larvae and adults), green lacewings, hoverflies (syrphids), predatory bugs (Orius, Nabis).
• Parasitoids: small parasitic wasps (Aphidius spp.) that lay eggs inside aphids, turning them into "mummies".
• Entomopathogenic fungi: Beauveria bassiana and Lecanicillium spp. They are effective at high humidity.

Advice for growers: Avoid broad‑spectrum insecticides, as they kill beneficial fauna too, and aphids, released from their natural enemies, reproduce even faster (Lagnaoui & Radcliffe, 1998). Use selective products that spare entomophages.

4.5. Chemical protection

Applying insecticides against sucking pests requires special care.

• Against aphid virus vectors:
• Systemic products (e.g., neonicotinoids such as thiamethoxam) penetrate plant tissues and protect from within. They are applied to the soil at planting or as tuber treatments. This provides 6‑8 weeks of protection, critical in the early period when plants are most vulnerable to viruses (De Jong et al., 2011).
• Mineral oil emulsions. Spraying plants with a thin oil film reduces the efficiency of non‑persistent virus (PVY) transmission, because the oil interferes with virus attachment to the aphid's stylets. This method is safe and can be used in organic farming.
• Important: Contact insecticides do not kill the aphid quickly enough to prevent PVY transmission, so they are practically useless against non‑persistent viruses (Perring et al., 1999).

• Against leafhoppers and whiteflies:
• Use pyrethroid (e.g., lambda‑cyhalothrin) or neonicotinoid products, spraying when the first signs of hopperburn appear or when the threshold (5‑10 nymphs per leaf) is exceeded.

Important: Insecticide resistance is a serious problem. Aphids, especially Myzus persicae, have developed resistance to most chemical classes (more than 60 cases of resistance have been described). Rotate products with different modes of action (follow IRAC recommendations) and do not exceed the recommended dose.

How to recognise:

• Adult beetle (click beetle): dark brown or black, 7‑25 mm long. If turned on its back, it snaps and flips into the air — hence the name.
• Larva (wireworm): slender, yellow or brown, with a hard chitinous covering, up to 25‑30 mm long. The body is segmented, resembling a piece of wire (hence the name). It has three pairs of short legs on the thoracic segments.
• Life cycle: Wireworm development takes 3‑5 years. Larvae and adults overwinter in the soil at depths of 15‑60 cm. Adult beetles lay eggs in the topsoil, especially in areas with plant debris.

What damage they cause:

Wireworms bore round, neat holes 1‑3 mm in diameter in tubers. The tunnels often go deep inside. Larvae can also damage seed tubers, gnaw roots, and young stems, leading to poor emergence.

One larva can damage several tubers per season. In outbreak years, losses can reach 30‑45% of the crop (Steele, 2011).

Who is at risk? Wireworms are most abundant in fields previously under perennial grasses (meadows, pastures) or cereal crops. Fallow land and fields with many weeds, especially couch grass, are particularly dangerous (Parker & Howard, 2001).

How to detect before planting:

• Bait traps. 2‑3 weeks before planting, bury pieces of raw potato or carrot on sticks (for easy retrieval) at a depth of 10‑15 cm. After 3‑4 days, dig them up and count the wireworms. If more than 3‑5 larvae are collected in one trap, the risk is high (De Jong et al., 2011).
• Soil samples. Dig over several plots (0.5 m² each) to a depth of 20‑25 cm, pick out all wireworms by hand. If there are more than 5 larvae per 1 m², it is time to take action.

Control methods:

1. Crop rotation (the most effective method!). Do not plant potatoes after cereals (wheat, rye, oats, maize) and especially after perennial grasses. Best preceding crops — legumes, brassicas (cabbage, rapeseed), onions, carrots.

2. Deep autumn ploughing. Ploughing to 25‑30 cm in autumn brings larvae and pupae to the surface, where they die from frost or are eaten by birds.

3. Liming of acid soils. Wireworms prefer acid soils (pH 4.5‑5.5). Applying lime, dolomite flour, or ash reduces acidity and makes the soil less attractive to the pest.

4. Biological methods. Entomopathogenic fungi Metarhizium anisopliae and Beauveria bassiana infect wireworms in the soil. Products based on them (e.g., "Metarizin") are applied at planting. Efficacy reaches 60‑70% with proper technique (Kabaluk et al., 2005).

5. Chemical products. If other methods fail, apply neonicotinoid insecticides (thiamethoxam, imidacloprid) to the soil before planting. They protect the seed tuber and young roots for 4‑6 weeks. Always follow application rates and safety intervals strictly.

How to recognise:

• Adult insect: large (up to 5‑6 cm), brownish, with powerful digging legs resembling rakes. The front limbs are adapted for tunnelling in the soil.
• Lifestyle: The mole cricket lives underground, digging branched tunnels at depths of 10‑30 cm. It is active at night. It overwinters in the soil at depths of up to 1 metre.

What damage it causes:

The mole cricket gnaws roots, stems at the base, and eats large irregular cavities in tubers. One mole cricket can destroy up to 10‑15 plants per season. Damage is often confused with that of other pests — the cavities are larger and have ragged edges, unlike the neat holes made by wireworms.

Control methods:

1. Traps. In early spring, place fresh manure around the plot (mole crickets like to settle in it) or bury jars with beer (the smell attracts them). Collect pests regularly.

2. Deep ploughing. Autumn and spring digging destroys tunnels and kills eggs and larvae.

3. Physical barriers. When planting, you can wrap the root collar of seedlings with netting or nylon stockings to prevent the mole cricket from cutting the stem (more relevant for tomatoes and peppers, but more difficult for potatoes).

4. Chemical baits. Apply granular insecticides (based on diazinon) to the soil or place poisoned baits (a mixture of grain and insecticide) into mole cricket tunnels.

How to recognise:

• Larva: thick, white, fleshy body, up to 4‑5 cm long, with three pairs of legs. It is shaped like the letter "C". The head is dark brown, with powerful jaws.
• Life cycle: Development takes 3‑4 years. Larvae overwinter at depths of 30‑60 cm. Adult beetles emerge in spring and lay eggs in soil with vegetation.

What damage they cause:

Larvae gnaw roots and stems at the base, and eat large deep cavities in tubers. In young plants, root damage causes wilting and death. Yield can decrease by 40‑80% (Pathania & Chandel, 2016).

Control methods:

1. Crop rotation. Avoid planting potatoes after perennial grasses and pastures.

2. Deep ploughing. Autumn ploughing to 25‑30 cm helps destroy some of the larvae and pupae that overwinter in the upper soil layers.

3. Biological products. Fungi Metarhizium anisopliae and Beauveria bassiana effectively infect white grub larvae. Applying them to the soil before planting gives good results.

4. Entomopathogenic nematodes (Heterorhabditis, Steinernema). These microscopic worms penetrate the larvae and kill them from the inside. Efficacy reaches 70‑80% in moist soil.

5. Chemical products. At high densities (more than 5 larvae per 1 m²), apply soil insecticides (neonicotinoids, organophosphates).

How to recognise:

• Symptoms: Dark, dry spots appear on the tuber surface, sometimes with a greyish tinge. The skin shrivels, and cavities with dry, crumbly tissue form beneath it. On cut sections, brown necroses are visible, and the tissue becomes dry and powdery. The spots gradually enlarge, and the tuber dries out completely.
• The pest: Microscopic worms 0.7‑1.4 mm long. They cannot be seen with the naked eye, but characteristic damage reveals their presence.

How it spreads:

Transmitted through infected tubers, through soil (survives up to 3 years), with plant debris, and on agricultural tools.

Prevention and control:

1. Healthy planting material. Use only certified seed potatoes. Visual inspection does not always detect hidden infection.

2. Crop rotation. Return potatoes to the same site no earlier than after 3‑4 years.

3. Heat treatment. Warming tubers before planting at 45‑50 °C for 30 minutes (in water) can reduce nematode infestation, but requires care to avoid seed damage.

4. Chemical methods. Soil nematicides (e.g., phosphides) are expensive and environmentally hazardous, used only in commercial production.

How to recognise:

You will not see the worms themselves (they are only 0.5‑1 mm long), but you can notice indirect signs:

• On roots: At the end of the growing season, small white, yellowish, or golden‑brown balls the size of a pinhead (0.5‑1 mm) appear on the roots. These are cysts — the bodies of dead females filled with eggs and larvae. In G. rostochiensis they are golden‑yellow, in G. pallida they are pale cream.
• Above‑ground symptoms: Plants in infested patches are stunted, leaves turn yellow and wilt, flowering is absent or very weak. Bushes look stressed, as if lacking water or nutrients. Affected areas appear as patches — "bald spots" in the field (Akhatov et al., 2013; Abrantes et al., 2023).

Life cycle: Second‑stage juveniles emerge from cysts under the influence of potato root exudates, penetrate the roots, create feeding cells (syncytia), and develop into adult females. Males leave the roots and fertilise the females. After fertilisation, the female dies, and her body becomes a cyst containing hundreds of eggs. The cycle lasts 45‑60 days.

What damage it causes:

The nematode disrupts water and nutrient uptake, causing stunting. At densities of 5‑10 cysts per 100 g of soil, yield losses can reach 30‑50%, and in severe infestations up to 100% (Turner & Rowe, 2006).

Control methods:

1. Quarantine and diagnosis. If infestation is suspected, contact the quarantine service. Only laboratory analyses (soil washing, molecular diagnostics) can confirm the presence of nematodes.

2. Long crop rotation. Return potatoes to an infested field no earlier than after 6‑8 years. During this time, most juveniles in cysts die (up to 30% per year) (Trudgill et al., 2003).

3. Growing resistant varieties. Some potato varieties are resistant to G. rostochiensis (e.g., 'Impala', 'Cardinal', 'Agria'). However, resistance to G. pallida is almost non‑existent, so it is important to know exactly which species is present (Fuller et al., 2008).

4. Trap crops. Some plants (e.g., Solanum sisymbriifolium) stimulate juvenile emergence from cysts but are not hosts themselves. The juveniles die without finding food. This is effective but labour‑intensive (Dias et al., 2012).

5. Biological control. Applying antagonistic fungi (Pochonia chlamydosporia) and bacteria (Pasteuria spp.) to the soil can reduce nematode numbers by 50‑70% (Tobin et al., 2008).

6. Chemical methods. Nematicides (e.g., phosphides, metam‑sodium) are expensive and environmentally hazardous. Used only on a commercial scale at high infestation levels.

How to recognise:

• On roots: Numerous swellings (galls) form on the roots. They can range from pea‑sized to walnut‑sized.
• On tubers: Small warty outgrowths or bumps appear on the tuber surface. On cut sections, white dots are visible — these are the bodies of females and their egg masses (De Jong et al., 2011). In some species (e.g., M. chitwoodi), galls can be very small and almost invisible, but dark spots and cracks appear on the tubers.
• Above‑ground symptoms: Plants are stunted, leaves turn yellow and wilt, especially in hot weather. The root system is poorly developed.

Life cycle: Second‑stage juveniles penetrate the roots, create giant cells (syncytia) in the vascular system, and develop into sedentary females. Females lay eggs in a gelatinous matrix that remains on the root surface or inside the gall. The cycle lasts 4‑6 weeks.

Control methods:

1. Crop rotation. The wide host range of root‑knot nematodes (over 3,000 plant species) makes rotation less effective. However, some non‑host crops exist: certain cereals (rye, oats) and legumes (lupine).

2. Resistant varieties. Work is ongoing to develop varieties resistant to M. chitwoodi and M. hapla. For example, varieties carrying the Rmc‑1 gene from the wild species Solanum bulbocastanum (Bali et al., 2019).

3. Trap crops. Some Solanum sisymbriifolium cultivars effectively reduce M. chitwoodi populations (Perpétuo et al., 2021).

4. Biological control. Fungi Pochonia chlamydosporia and Purpureocillium lilacinum, as well as the bacterium Pasteuria penetrans, effectively attack eggs and juveniles of root‑knot nematodes (Stirling, 2014).

5. Chemical methods. Nematicides (fluazindolizine, fosthiazate) are used, but their use is limited due to toxicity and cost.

How to recognise:

• On tubers: Dark, slightly sunken spots appear on the surface. The skin becomes flabby, and cavities with dry, crumbly tissue form underneath. On cut sections, brown necroses are visible, which later merge. The tuber turns into a "mummy", completely drying out (Akhatov et al., 2013).
• During storage: Losses can reach 30‑50% of stored tubers, especially at high humidity and temperature.

Control methods:

1. Healthy planting material. The main route of spread is infected tubers. Use only certified seed potatoes.

2. Tuber warming. Heat treatment (45‑50 °C for 30 minutes) reduces infestation but can damage seed if done improperly.

3. Crop rotation (3‑4 years). The nematode survives in soil for up to 3‑4 years, so return potatoes to an infested plot should be delayed.

4. Biopreparations. Antagonistic fungi (Trichoderma spp.) can reduce nematode numbers in soil.

3.1. Colorado potato beetle (Leptinotarsa decemlineata)

• Young larvae (1st‑2nd instar): threshold — 20‑30 larvae per plant (or 4‑6 larvae per stem) (Ferro, 1985; Martel et al., 1986). First‑ and second‑instar larvae are still relatively small but highly susceptible to biological products (Bacillus thuringiensis) — treatment at this stage is most effective.
• Older larvae (3rd‑4th instar): threshold — 10‑15 large larvae per plant (or 1.5‑2 larvae per stem). They are the most voracious, consuming up to 75% of leaf mass. Treatment should be done as early as possible, before they cause serious damage.
• Adult beetles: threshold — 0.5‑1 beetle per plant (or 5‑10 beetles per 100 plants). If beetles are few, hand collection is sufficient. Mass emergence of overwintered beetles in early spring is a signal to start monitoring.

When you can skip treatment: If only old lower leaves are damaged (less than 10% defoliation) and larvae are few, wait 3‑5 days and re‑inspect. Plants before flowering can recover from minor leaf loss.

3.2. Aphid virus vectors (especially Myzus persicae)

For table potatoes (not seed), thresholds may be quite high — 50‑100 aphids per 100 leaves (Flanders et al., 1991). Direct damage from aphids is rarely critical.

But for seed potatoes, the threshold drops sharply to 1‑10 winged aphids (alates) per 100 leaves or even 1‑2 aphids per trap per day during the active flight period (Radcliffe & Ragsdale, 2002). This is because even one viruliferous aphid can infect an entire plant, and the virus will spread across the field.

What to do: In seed production, aphid monitoring is continuous using yellow sticky traps. At the first winged individuals (especially during early flowering), systemic insecticides or oil sprays are recommended to block virus transmission.

3.3. Wireworms

It is difficult to set an exact threshold for wireworms because they are unevenly distributed in the soil and not visible on the surface. However, common guidelines are:

• More than 5‑7 larvae per 1 m² of soil — high risk of tuber damage (Parker & Howard, 2001).
• More than 3‑5 larvae in a bait trap (a piece of potato buried 10 cm deep for 2‑3 days) — treatment at planting is required (De Jong et al., 2011).

When you can skip treatment: If there have never been wireworms on the plot, the soil is light, and no cereals were grown previously, you can rely on preventive measures (deep ploughing, liming) without chemicals.

3.4. Root‑knot nematodes (Meloidogyne spp.)

For root‑knot nematodes, the threshold depends on the species:

• M. chitwoodi — threshold 1 egg/juvenile per 250 cm³ of soil (about 1 juvenile per 1 kg of soil) (Santo et al., 1981).
• M. javanica — threshold 0.5 eggs per 1 cm³ of soil (Russo et al., 2007).
• M. incognita — threshold 1.2 eggs per 1 cm³ of soil (Vovlas et al., 2005).

Important: If nematodes are detected, even low densities are already dangerous because they will multiply. In such cases, long crop rotation or the use of resistant varieties is recommended, rather than emergency treatments that are of limited effectiveness.

3.5. Potato tuber moth

The threshold for the tuber moth is usually determined by male catches in pheromone traps:

• More than 15‑20 moths per trap per night — treatment required (Kroschel & Schaub, 2013).
• Detection of egg masses on leaves or larvae in stems — a signal for immediate action, especially in southern regions with year‑round cultivation.

3.6. Potato leafhopper (Empoasca fabae)

• 10‑30 nymphs per 100 leaves — threshold at which noticeable yield reduction begins (Johnson & Radcliffe, 1991; Cook et al., 2004).
• "Hopperburn" symptoms (yellowing and drying of margins) on more than 10% of leaves — time to apply insecticides.