Pests

How to recognize the danger?

The onion fly is one of the most serious pests of onions. In appearance, it resembles the common housefly but is smaller: the adult insect is only 6–8 mm long, with an ash-grey colouration (Akhatov et al., 2013).

The main danger is not the flies themselves, but their larvae, which develop inside the bulbs. The larvae are dirty-white, legless, up to 10 mm long, with characteristic hook-shaped mouthparts (Brewster, 2008).

How can you tell if onions are affected?

Signs of damage are easy to spot with careful inspection:

1. Wilting and yellowing of leaves. Seedlings at the cotyledon or first true-leaf stage wilt in groups, as if lacking moisture. In older plants, leaves turn yellow from the tip, take on a greyish-yellow tint, and gradually dry out (Akhatov et al., 2013).

2. Bulb rotting. Bulbs damaged by larvae become soft, rot, and give off an unpleasant odour. When cut open, larval tunnels can be seen inside (Brewster, 2008).

3. Characteristic holes. Small holes may be noticed at the base of the leaves or near the root plate—entry points for the larvae (Akhatov et al., 2013).

Why does this happen? Life cycle

In the temperate zone, the onion fly produces two generations per season (in southern regions, a third is possible). The first flight of flies coincides with the flowering of dandelions or lilacs—this is a signal for the gardener: it is time to be on alert (Akhatov et al., 2013; Brewster, 2008).

Females lay eggs in small groups (5–20 at a time) in the soil near the plants or between leaves. After 3–6 days, larvae emerge, penetrate the bulb through the leaf base or root plate, feed on bulb tissues, tunnel inside, and can move to neighbouring plants. After 2–3 weeks, the larvae pupate in the soil, and after some time, the second generation of flies appears (Brewster, 2008).

The pests overwinter as puparia (false cocoons) with pupae in the soil at a depth of 10–20 cm, in areas where onions were grown (Akhatov et al., 2013).

What to do? Practical protective measures

1. Prevention is the key to success

• Crop rotation. Do not return onions to the same spot for at least 3–4 years. The best preceding crops are perennial grasses, grain legumes, cabbage, and cucumber (Autko et al., 2012). Why does this work? Onion fly pupae overwinter in the soil, and if onions are planted in the same place year after year, the pest population builds up.

• Deep autumn ploughing (digging) helps bring pupae deep into the soil so they cannot emerge in spring (Brewster, 2008).

• Early sowing. Sow onions as early as possible so that by the time of the mass fly flight, the plants have grown and become stronger. Young seedlings are easy prey for larvae, while mature plants are less damaged (Akhatov et al., 2013).

• Mulching. Cover the soil around the plants with peat—this makes it harder for flies to lay eggs (Autko et al., 2012).

• Removal of post-harvest residues. Thoroughly remove all plant debris from the beds, as pupae may survive in them (Brewster, 2008).

2. Choosing resistant varieties

Experience shows that pungent varieties with high dry matter and essential oil content are damaged by onion fly 3–4 times less than sweet varieties. For example, varieties such as Strigunovsky, Skvirsky, and Zolotisty are more resistant, while sweet varieties (Yaltinsky mestny, Oranzhevy) are more severely affected (Akhatov et al., 2013).

3. Biological and chemical agents

• To control larvae, products based on diazinon (e.g., Mukhoed, Zemlin) are used—applied to the soil at planting. Important! Strictly follow the dosage indicated on the package (Akhatov et al., 2013).

• When growing onions for greens (feathers), the use of insecticides is not permitted! Use only preventive and agronomic methods (Akhatov et al., 2013).

How to recognize the danger?

The small onion fly (onion bulb fly) is another dangerous pest that often acts in conjunction with the onion fly but appears later, in the second half of summer. In appearance, it is a stocky fly 5–7 mm long, with a bronze-greenish or metallic-blue abdomen. Two light-grey stripes are clearly visible on the back (Akhatov et al., 2013). Do not confuse it with the onion fly—the bulb fly has a more robust body and a characteristic metallic sheen.

The larva is yellowish-grey, wrinkled, up to 10–11 mm long, with a convex back and flattened underside. At the rear end, there is a brown, hard protrusion with breathing openings, flanked by two fleshy processes (Akhatov et al., 2013).

How can you tell if onions are affected?

The symptoms of damage largely resemble those caused by the onion fly, but there are differences:

1. Wilting and yellowing of leaves. Leaves turn yellow from the tips, the plant is stunted and looks depressed. However, unlike the onion fly, the bulb fly attacks already formed, older plants—in the second half of the growing season (Brewster, 2008).

2. Softening and rotting of bulbs. With mass feeding, the inside of the bulb turns into a black, rotting mass with a characteristic sharp odour. The bulb becomes soft and hollow inside (Akhatov et al., 2013).

3. Damage near the root plate. Larvae more often enter the bulb from the root plate, gnawing tunnels, or through mechanical damage. The holes may be unnoticeable externally, but when squeezed, the affected bulb deforms easily (Brewster, 2008).

Why does this happen? Life cycle

The small onion fly overwinters as mature larvae—inside bulbs left in the field after harvest, or in the soil at a depth of 5–8 cm. In spring, when the soil warms above +8 °C, the larvae pupate (Akhatov et al., 2013).

The first generation of flies emerges in early June, during rose hip flowering. The flight lasts about a month. Females lay up to 100 eggs in small groups directly on bulbs or in the soil nearby, choosing weakened plants—often those already damaged by onion fly or stem nematode (Brewster, 2008).

Larvae appear in 5–10 days and penetrate the bulb, feeding for 18–25 days. They then go into the soil to pupate. At the end of July, the second generation of flies appears, whose larvae cause damage in August–September (Akhatov et al., 2013).

What to do? Practical protective measures

Control measures for the small onion fly largely coincide with those for the onion fly, but there are some nuances.

1. Prevention

• Thorough harvesting. Remove all bulbs, even small and damaged ones, after harvest. Do not leave them in the field—they become ideal overwintering sites for larvae (Brewster, 2008).

• Deep autumn digging (ploughing) to a depth of at least 20 cm. This helps destroy pupation sites and brings larvae to the surface, where they will die from frost or be eaten by birds (Akhatov et al., 2013).

• Crop rotation. As with the onion fly, maintain a 3–4-year break between onion plantings in the same place (Autko et al., 2012).

• Destruction of plant residues. Burn or deeply bury the tops and culled bulbs, as larvae may persist in them (Brewster, 2008).

2. Strengthening plants

The bulb fly prefers weakened plants. Therefore, anything that promotes good growth and development of onions reduces the risk of infestation:

• Balanced fertilisation (especially phosphorus-potassium during bulb formation) increases resistance (Autko et al., 2012).
• Timely watering and loosening ensure healthy growth.
• Control of other pests (especially the onion fly) reduces the number of "weakened" plants on which the bulb fly lays eggs (Brewster, 2008).

3. Chemical agents

There are currently no specific insecticides recommended for home gardens in Russia against the small onion fly. However, products effective against the onion fly (e.g., those based on diazinon) may also be effective against the bulb fly if applied to the soil in a timely manner. In commercial vegetable production, foliar sprays with organophosphorus compounds are used during the growing season (Akhatov et al., 2013). Important: when growing onions for greens, chemical treatments are not permitted!

How to recognize the danger?

Onion thrips (also called tobacco thrips) is one of the most insidious pests. The adult insect is so small (only 1.0–1.3 mm) that it is difficult to see with the naked eye. The body is narrow, elongated, yellowish, with fringed wings folded along the back (Akhatov et al., 2013). The larvae are even smaller, light yellow or whitish.

The main danger is that thrips not only suck sap from plants but also transmit dangerous viruses, in particular the iris yellow spot virus (IYSV), which can lead to total loss of seed crops and a severe reduction in bulb quality (Brewster, 2008).

How can you tell if onions are affected?

The symptoms of thrips damage are very characteristic and visible even at low numbers:

1. Silvery or whitish streaks and spots on leaves. Thrips puncture epidermal cells and suck out their contents, creating air spaces in the tissues that give the leaves a silvery sheen (Brewster, 2008). In severe infestations, leaves become completely bleached (Akhatov et al., 2013).

2. Small dark dots—these are dried excreta of the pest (Akhatov et al., 2013).

3. Deformation and drying of leaves. Leaves become twisted, turn yellow from the tip, and dry out. Onions grown for greens lose their marketable appearance and become unfit for sale.

4. Damage to bulbs during storage. Some thrips hide under the dry scales of the bulbs, where they continue to feed, causing desiccation, roughness, and spoilage. The quality of sets and marketable bulbs is reduced (Akhatov et al., 2013).

5. Damage to inflorescences on seed plants: flowers dry out, seeds become shrivelled with low germination rates (Brewster, 2008).

Why does this happen? Life cycle

Thrips are heat-loving pests. In southern regions, they develop faster and produce more generations per season. Adults overwinter under dry bulb scales in storage facilities, as well as on plant debris and in the upper soil layer (Akhatov et al., 2013).

In spring, after onion planting, females lay eggs singly under the leaf skin (in the leaf tissue). In 3–6 days, larvae hatch and immediately begin to feed. Development of one generation takes 14–30 days in southern regions, longer further north. Several generations may occur per season (Brewster, 2008).

By harvest time, most thrips migrate to other cultivated plants (cabbage, cucurbits, sunflower) and weeds, where they feed before going into overwintering. Some remain on bulbs and enter storage with them (Akhatov et al., 2013).

What to do? Practical protective measures

1. Preventive and agronomic measures

• Destruction of plant residues. Thoroughly collect and burn tops, especially on seed plots, to deprive thrips of overwintering sites (Akhatov et al., 2013).

• Weed control. Thrips feed on many weed species and can move from them to onions. Keep beds clean (Brewster, 2008).

• Sprinkler irrigation. In hot, dry weather, overhead watering (sprinkling) washes thrips off the leaves and increases their mortality. This method is especially effective in combination with other measures (Brewster, 2008).

• Straw mulching. In experiments, straw mulching reduced thrips numbers, increased populations of their natural enemies (predatory mites), and increased yields (Brewster, 2008).

• Maintaining optimal humidity. Thrips multiply greatly in dry, hot weather. Timely watering, especially during active growth, helps plants better tolerate damage (Brewster, 2008).

2. Variety selection

It has been observed that varieties with bright green, glossy leaves are less affected by thrips than varieties with greyish-green leaves. This is related to the characteristics of the waxy coating and rosette structure: in resistant varieties, the leaves spread more widely from the neck, providing fewer hiding places for the pest (Brewster, 2008).

3. Biological and chemical agents

• Use of "soft" insecticides. In commercial vegetable production, products based on spinosad are used to control thrips; they are effective against the pest but less dangerous to beneficial insects (Brewster, 2008).

• For home gardens in Russia, insecticides based on thiamethoxam (Aktara) and lambda-cyhalothrin (Karate Zeon) are registered. Treatments are carried out during the growing season, strictly following the instructions. It is important to carry out two treatments with an interval of 5–7 days to destroy not only adults but also hatching larvae (Akhatov et al., 2013).

• Important! On onions grown for greens (feathers), the use of insecticides is not permitted. Use only agronomic and biological methods.

• Heating bulbs before storage. Heating at 40–42 °C for 8 hours significantly reduces bulb infestation by thrips and other pests (Autko et al., 2012).

How to recognize the danger?

Stem nematode is one of the most dangerous and difficult-to-eradicate pests of onions. It is a microscopic worm: females are only 1.1–1.8 mm long, 0.02–0.04 mm thick. It is impossible to see with the naked eye (Akhatov et al., 2013). But the consequences of its activity are very clearly visible.

The main danger is that the nematode penetrates inside plants and lives there, feeding on cell sap. It secretes enzymes that break down pectin, causing tissues to loosen, cells to disintegrate, and the plant to become deformed (Brewster, 2008). In addition, the nematode is a carrier of bacterial and fungal infections that finish off the weakened plant.

How can you tell if onions are affected?

The signs of nematode damage cannot be confused with anything else:

1. Curling and thickening of leaves. In young plants, the first leaf becomes shortened and deformed. In older plants, leaves become twisted, thickened at the base, wrinkled or curled (Akhatov et al., 2013; Brewster, 2008).

2. "Blotch" – swelling and cracking of bulbs. The tissues of the fleshy scales grow unevenly, the bulb becomes loose and soft, whitish spots appear, followed by cracks. Voids form inside (Brewster, 2008). On cross-sections, loose, greyish areas are visible (Akhatov et al., 2013).

3. Thickening of the neck and root plate. The bulb may appear "swollen," the root plate cracks, often right in the middle (Akhatov et al., 2013).

4. Stunted growth. Plants look depressed, develop poorly, leaves turn yellow and die prematurely.

5. External reddish-brown stripes on the scales closer to the base of the bulb—a characteristic sign (Akhatov et al., 2013).

Important! During storage, affected bulbs become soft, often rot, and rot quickly spreads to neighbouring healthy bulbs (Brewster, 2008).

Why does this happen? Life cycle

The nematode is an obligate parasite, meaning it can live and reproduce only inside a living plant. However, it has a unique ability: when unfavourable conditions occur (drought, cold, lack of food), it enters a cryptobiotic stage—fourth-stage larvae that can survive without food and water in dry plant debris for up to 8 years, and at low temperatures up to 25 years (Akhatov et al., 2013; Brewster, 2008).

In dry onion scales, the nematode survives for more than 2 years, and in dried garlic for 4–5 years (Autko et al., 2012). In soil without plants, it lives for 12–18 months and can move to a depth of up to 1.5 m (Akhatov et al., 2013).

Reproduction occurs inside the plant. A female lays up to 400 eggs. At favourable temperatures (12–18 °C), one generation develops in 19–23 days, and 3–5 generations can occur per season (Akhatov et al., 2013).

The nematode enters the plant through the roots or through microcracks at the base of the leaves. It spreads with seeds, planting material, soil particles on tools and footwear, as well as with rainwater and irrigation (Brewster, 2008). There are about 20 races of this pest, specialised on different plants—onion, strawberry, clover, oat, and others (Akhatov et al., 2013).

What to do? Practical protective measures

Control of stem nematode is difficult, so the main emphasis is on prevention and preventing infection.

1. Crop rotation and "clean fallow"

• The most effective method is a 3–4-year break between onion plantings in the same area, with the inclusion of crops not affected by nematodes (cereals, legumes) in the rotation (Akhatov et al., 2013; Brewster, 2008).
• In severe cases (with heavy infestation), it is recommended to leave the plot under clean fallow (without plants) for 2–3 years—this can significantly reduce the pest population, as the nematode cannot feed and gradually dies (Brewster, 2008).

2. Healthy planting material—the main condition

• Use only seeds and sets from reliable producers.
• Carefully inspect bulbs before planting—discard any with signs of deformation, cracks, or softness.
• Heat treatment. For small batches, heating sets in hot water is effective: at 45–46 °C for 12–15 minutes or 50–52 °C for 7–10 minutes (time depends on bulb size—the larger, the longer). After heat treatment, it is useful to dip the onions for 1 minute in a 1% formalin solution at room temperature (Akhatov et al., 2013; Brewster, 2008).
• If infection is suspected, store sets at about 14 °C—under these conditions, affected bulbs rot more quickly and can be discarded in time before planting (Akhatov et al., 2013).

3. Drying and storage

• Thoroughly dry bulbs before storing. Nematodes actively multiply in moist environments. Dry scales are a poor environment for their survival (Autko et al., 2012).
• Maintain low humidity in storage (no more than 70%) (Brewster, 2008).

4. Removal of diseased plants

• During the growing season, regularly inspect plantings and remove (with roots) all suspicious plants, as well as neighbouring ones, to prevent spread (Akhatov et al., 2013).

5. Chemical agents

There are practically no specific nematicides for use in home gardens. In commercial production, soil fumigation (e.g., with methyl bromide) is used, but this is not applicable in amateur gardening. The main reliance is on prevention and agronomic practices (Brewster, 2008).

5.1. Bulb (Root) Mite (Rhizoglyphus echinopus)

5.2. Four-legged (Garlic, Tulip) Mite (Aceria tulipae)

6.1. Onion Leaf Beetle (Lily Leaf Beetle relative) (Lilioceris merdigera)

6.2. Onion Weevil (Ceuthorrhynchus jakovlevi)

6.3. Onion Moth (Acrolepia assectella)

6.4. Cutworms (Polyphagous Caterpillars)

General preventive measures against leaf-eating pests

1. Keep beds free of weeds—they serve as additional food and shelter for many pests.

2. Regularly inspect plantings—early detection allows you to limit control to manual collection or local measures.

3. Carry out deep autumn digging—it destroys overwintering stages (pupae, eggs, beetles) in the soil.

4. Observe crop rotation—alternating crops disrupts the food chains of specialised pests.

5. Remove and burn plant residues immediately after harvest to deprive pests of overwintering sites.

6. Use biological products (based on Bacillus thuringiensis)—they are effective against caterpillars and safe for humans and beneficial insects. Apply them in dry, calm weather, strictly according to instructions.

7. Use chemical insecticides only as a last resort, when the economic threshold of harmfulness is exceeded, and only on onions grown for bulbs (not for greens). Strictly observe dosages and waiting periods before harvest (Akhatov et al., 2013).

In the final chapter, we will combine all the knowledge gained and discuss integrated protection of onions from pests—how to build a system of preventive measures to minimise chemical use and preserve a healthy harvest.

7.1. The principle of integrated protection

Integrated protection is not just a set of methods, but a well-thought-out strategy that includes:

• Prevention—creating conditions unfavourable to pests.
• Monitoring—regular observation of plantings to detect problems early.
• Use of biological and mechanical methods as a priority.
• Use of chemical agents only as a last resort, when the economic threshold of harmfulness is exceeded (Brewster, 2008; Akhatov et al., 2013).

Such a strategy reduces the environmental burden, preserves beneficial insects, and ensures the production of environmentally safe products.

7.2. Prevention: the foundation of protection

Most problems can be prevented even before planting.

7.3. Monitoring: spotting early is half the success

Regularly inspect plantings, especially during critical periods:

• Seedling stage — 2–3 true leaves: this is when the onion fly is active. Check plants for wilting, yellowing, and damage at the leaf base.
• Second half of the growing season (July–August): bulb fly, thrips, moth caterpillars, and cutworms are active. Pay attention to silvery streaks (thrips), mines on leaves (moth, weevil), and bulb damage (bulb fly, cutworms).
• Before harvest: assess the condition of bulbs, especially the root plate and neck, to identify hidden damage from mites and nematodes (Brewster, 2008).

Simple monitoring techniques:

• Use sticky traps (yellow or white) to catch adult thrips and flies. They can be placed on beds from mid-May.
• When the first signs of damage appear, dig up a few suspicious plants and inspect roots and bulbs—this will help you identify the "culprit."
• Keeping an observation diary helps track pest population dynamics over the years and adjust treatment schedules.

7.4. Mechanical and biological methods

These are safe for humans and beneficial insects and should be used first.

• Manual collection of beetles (leaf beetle, weevil) and caterpillars (moth, cutworms) when numbers are low gives good results.
• Mulching with peat, straw, or compost around plants makes it difficult for flies and bulb flies to lay eggs, and also reduces thrips damage (Brewster, 2008).
• Sprinkler irrigation in hot weather washes thrips off the leaves and increases their mortality (Brewster, 2008).
• Use of biological products based on Bacillus thuringiensis (e.g., Lepidocide, Bitoxibacillin) is effective against moth and cutworm caterpillars. They are safe for bees, birds, and humans (Akhatov et al., 2013).
• Use of pheromone traps (for onion moth, cutworms) is currently limited in amateur gardening, but in commercial farms they help determine treatment timing accurately (Brewster, 2008).

7.5. Chemical agents: when and how to apply

Chemicals should be used only when the pest population exceeds the economic threshold of harmfulness (i.e., when the damage caused by pests becomes greater than the cost of treatment). For home gardeners, this means: if you see clear signs of mass infestation and mechanical and biological methods are no longer sufficient.

7.6. Post-harvest protection: preserving the harvest

Even after harvest, pests can continue to destroy your labour. Mites and nematodes actively multiply in storage if storage conditions are violated.

What to do:

• Dry onions thoroughly. The moisture content of the outer scales should be no higher than 14–15%. To achieve this, onions are dried in the field (in good weather) and then finished in a ventilated room at 25–27 °C for 7–10 days (Autko et al., 2012).
• Store onions at 0–1 °C and relative humidity of 75–80%. Under such conditions, mite and nematode reproduction practically ceases (Autko et al., 2012; Brewster, 2008).
• Periodically sort through onions in storage—remove all soft, sprouted, or mouldy bulbs. They are sources of infection.
• Treatment of the storage before stocking. Sulphur smoke generators (Klimat, FAS) are a reliable means of disinfection. Carry out the treatment at least 2 weeks before storing the harvest (Akhatov et al., 2013).

7.7. Conclusion: remember the main points

Integrated protection of onions from pests is not a single "magic pill," but a combination of several complementary techniques. Its goal is not to destroy every single pest, but to keep their numbers at a safe level, while preserving the harvest and soil health.

Your main allies:

1. Healthy planting material—inspect, heat-treat, discard.

2. Proper crop rotation—3–4 years without onions in the same place.

3. Regular inspections—the earlier you spot a problem, the easier it is to deal with.

4. Clean beds—without weeds, pests have nowhere to hide.

5. Proper storage—a dry and cool cellar is the best protection against mites.

And remember: the use of chemical insecticides is always a last resort. Start with prevention, attract beneficial insects (e.g., predatory mites effectively suppress thrips), use biological products—and your onions will reward you with strong, healthy bulbs and abundant greens.