Visual Key for Identifying Hazelnut Pests¶
Content generated by artificial intelligence — verify before application
This article was automatically compiled from a collection of hazelnut literature using artificial intelligence and may contain errors or outdated information. It serves as a starting point for further research, not as professional advice. Dosages, products, and timing are informative and depend on the cultivar, location, and applicable regulations — always verify with advisory services and official sources before application.
When you notice a swollen bud, a chewed leaf, or a wormy nut in your hazelnut orchard, the first question is always the same: who did this? Accurate pest identification is crucial, as it determines the control decision, the choice of product, and — perhaps most importantly — the timing of the intervention. An incorrect diagnosis means delayed or ineffective treatment, unnecessary costs, and further yield loss. This article provides a practical, tabular visual key that, based on visible damage (by plant organ: bud, leaf, catkin, shoot/branch, nut, root/trunk), leads to the most likely pest and refers to the corresponding monograph in this volume (15. Pest Compendium). We have specifically highlighted cases where similar damage is caused by a disease or abiotic stress — in such cases, it is not a pest issue, and you should refer to volume 17 (diseases and abiotic disorders).
How to Use the Key¶
The key is organized by the plant organ where you first notice the damage. Follow these steps:
- Determine where the damage is located (bud, leaf, catkin, shoot/branch, nut, root/trunk).
- Describe how the damage looks (deformation, holes, chewed edges, slime, sticky coating, dropping).
- Consider the phenological stage and season — the same symptom at different times points to different causes.
- If necessary, confirm the finding with a magnifying glass or by cutting open (e.g., mites in the bud, larvae in the nut).
- If the symptom matches a described disease/abiotic cause, do not treat against pests; instead, check volume 17.
Golden Rule of Diagnostics
One symptom is rarely sufficient. Combine location, appearance, and timing of occurrence, and whenever possible, find the actual culprit (insect, larva, mite) before making a control decision.
Quick Overview of Major Hazelnut Pests in Croatia¶
Before the detailed tables, here are the main actors you will most commonly encounter:
| Pest (Latin name) | Main attacked organ | Typical sign |
|---|---|---|
| Hazelnut bud mite (Phytoptus / Phytophus avellanae) | bud | swollen, round "large" buds |
| Hazelnut weevil (Curculio nucum) | nut (and leaf) | wormy nuts, exit holes, small holes on leaves |
| Hazelnut aphid (Myzocallis coryli) | leaf, summer shoot, nut | honeydew, sooty mold, "wet leaf" |
| Phytophagous bugs (Gonocerus, Palomena, Nezara etc.) | nut | bug damage, empty/poor nuts, kernel rot |
| Green fruit weevil (Phyllobius spp.) | leaf | chewed edges and small holes in leaves |
| Catkin bug (Pantilius tunicatus) | catkins | necrosis and dropping of catkins |
| Hazelnut longhorn beetle (Oberea linearis) | shoot/branch | wilting and drying of summer shoot tips |
| Walnut moth (Cydia spp.) | nut | wormy nuts (less common, secondary) |
| Scale insects (Eulecanium spp.) | branch, shoot | brown "shells" on bark, honeydew |
| Clover mite / red spider mite (Tetranychus urticae) | leaf | leaf marbling, small mites on the underside |
Diagnostic Key by Organ¶
Bud (Buds)¶
Buds are the first place where the most important hazelnut pest—the hazelnut bud mite—can be recognized in winter and early spring.
| Visible damage / symptom | Most likely cause | How to distinguish / confirm | See monograph |
|---|---|---|---|
| Bud swollen, round, significantly larger than healthy; dark reddish color (autumn/winter); does not open in spring | Hazelnut bud mite (Phytoptus avellanae) | Cut the bud and use a magnifying glass to look for numerous small mites (~0.2 mm). The "big bud" can be up to twice as large (~10 mm) | Monograph: Hazelnut bud mite |
| Deformed, stunted, pale yellowish leaves develop from the bud and dry out | Hazelnut bud mite (consequence of infestation) | Most pronounced in wet and cold springs | Monograph: Hazelnut bud mite |
| Bud not swollen, but spotted on the bud scales; bud dries out | Disease (e.g., Labrella coryli / leaf spot and bud drying) | No mites in cross-section; ring spots on nearby leaves → Section 17 | — (disease) |
| Buds chewed/damaged by supplementary feeding of adult insects | Hazelnut weevil or leafrollers (supplementary feeding) | Signs of chewing, no "big bud" type deformation | Monographs: Hazelnut weevil / Leafrollers |
Hazelnut bud mite vs. bud disease — key distinction
A swollen, round "big bud" is almost certainly a sign of hazelnut bud mite, especially if you find colonies of small mites when cutting it open. If the bud is dry and spotted, but not swollen, it is more likely a fungal disease (leaf spot and bud drying)—in this case, acaricides/sulfur against the mite will not help; instead, refer to Section 17.
Threshold and monitoring (informative): literature cites a decision threshold for hazelnut bud mite when around 10 % of buds are observed to be infested in February (sample of 100 buds). The mite can damage up to 20 % of buds, and in severe infestations, yield losses can be very high. The optimal time for treatment is the phase of mass migration of mites into new buds at air temperatures above 15 °C (approximately when the hazelnut has 5–10 leaves). Thresholds and timing are informative and depend on cultivar, location, and regulations.
Leaf¶
On leaves, traces of several groups "cross": defoliators (chew), aphids (suck and produce honeydew), and mites (marble).
| Visible damage / symptom | Most likely cause | How to distinguish / confirm | See monograph |
|---|---|---|---|
| Chewed leaf edges + small holes in the blade, during leaf unfolding | Green fruit weevil (Phyllobius spp.) | Leafrollers have a grayish-silver or greenish body; damage is mainly spring-related | Monograph: Leafrollers / Phyllobius |
| Small holes (punctures) in leaves from supplementary feeding | Hazelnut weevil (adult females) | Often accompanied by dimpling of young nuts; late spring–summer | Monograph: Hazelnut weevil |
| Sticky, shiny coating on the leaf ("wet leaf"), later black sooty mold | Hazelnut leaf aphid (Myzocallis coryli) | Aphid colonies on the underside of the leaf; honeydew attracts sooty molds | Monograph: Leaf aphids |
| Leaves deformed, stunted, pale, drying out (from bud) | Hazelnut bud mite (indirect) | Associated with swollen buds (see above) | Monograph: Hazelnut bud mite |
| Marbled (mosaic-lightened) leaves; small mites (~0.7 mm) on the underside | Nettle mite / red spider mite (Tetranychus urticae) | Summer heat; fine webs and dotted discoloration | Monograph: Mites |
| Leaves with ring spots (light zone surrounded by dark), necrosis, shedding | Disease (leaf spot, e.g., Labrella coryli) | After prolonged cold and rainy periods → Section 17 | — (disease) |
| Entire leaf turns yellow/brown without insects or honeydew | Abiotic stress (drought, nutrient deficiency, phytotoxicity) | No visible organisms → Section 17 | — (abiotic) |
Chewed edges vs. honeydew — different groups
Mechanical chewing of edges and hole-punching of leaves is done by insects with chewing mouthparts (leafrollers, hazelnut weevil, caterpillars). Sticky coating, shine, and sooty mold are the result of sucking aphids and their honeydew—there is no chewed tissue here. Do not confuse them: measures and products differ.
Catkins (Male flowers)¶
Catkins are an easily overlooked but specific organ with its own pest.
| Visible damage / symptom | Most likely cause | How to distinguish / confirm | See monograph |
|---|---|---|---|
| Catkins wither, partially necrotize, and fall prematurely | Catkin bug (Pantilius tunicatus) | Feeds by sucking sap from catkins | Monograph: Bugs |
| Catkins do not develop from swollen, deformed buds | Hazelnut bud mite (attack on generative organs) | Associated with "big buds" | Monograph: Hazelnut bud mite |
| Catkins turn brown and rot after frost/cold | Abiotic cause (winter/spring freezing) | No insects; related to temperature event → Section 17 | — (abiotic) |
Shoot / Branch¶
Traces on shoots and branches are rarer but important because they can indicate long-term decline.
| Visible damage / symptom | Most likely cause | How to distinguish / confirm | See monograph |
|---|---|---|---|
| Tips of summer shoots wither and then dry out | Hazelnut clearwing (Oberea linearis) | Larva bores/chews inside the shoot; check for presence of galleries | Monograph: Clearwings / Borers |
| Brown, hemispherical "shells" (scales) on bark; aphids underneath; honeydew | Scale insects (Eulecanium spp.) | Immobile scales on branches; stickiness and sooty mold | Monograph: Scale insects |
| Cankers on twigs and branches, yellow bacterial exudate in wet weather; branch drying | Disease (bacterial canker, Pseudomonas spp.) | Exudate and cankers, no insects → Section 17 | — (disease) |
| Branches lag in growth, general weakening with aphid colonies | Leaf / scale insects (heavy infestation) | Colonies + honeydew; remove infested branches during pruning | Monograph: Aphids |
Tip drying: pest or disease?
Withering and drying of individual summer shoot tips is typical for borers (clearwing)—look for a gallery inside the shoot. If, on the other hand, entire branches dry out with cankers and exudate, it is likely bacterial canker (Section 17), not a pest. Infested branches are removed 60–90 cm below the diseased site with disinfected tools.
Nut (Hazelnut)¶
The nut is the organ of greatest economic importance, and market value is decided here.
| Visible damage / symptom | Most likely cause | How to distinguish / confirm | See monograph |
|---|---|---|---|
| Nut "wormy": kernel chewed and filled with frass; creamy-white larva with brown head inside the nut | Hazelnut weevil (Curculio nucum) | Most common and well-known cause of worminess; one generation per year | Monograph: Hazelnut weevil |
| Round exit hole on the shell of a ripe/fallen nut | Hazelnut weevil (larva left the nut) | Hole means the larva has already exited; shell chewed from inside | Monograph: Hazelnut weevil |
| Young, just-set nuts pierced and fall off | Hazelnut weevil (supplementary feeding and egg laying) | Small holes; nuts deform and fall | Monograph: Hazelnut weevil |
| Nuts "buggy": empty or poorly filled, kernel deformed | Phytophagous bugs (Gonocerus, Palomena, Nezara etc.) | Bug punctures; suck nut contents | Monograph: Bugs |
| Kernel rotting, unpleasant smell; nut remains on tree of poor quality | Bugs as vectors (Nematospora coryli — kernel rot) | Bugs transmit the fungus; along with the pest, a disease occurs → see also Section 17 | Monograph: Bugs |
| Nut wormy, but larva/damage different from hazelnut weevil | Walnut moth (Cydia spp.) | Secondary pest; caterpillar of the moth, not the weevil larva | Monograph: Moths |
| Nut rot at puncture sites (Monilia, Botrytis, Fusarium, Nematospora) | Disease on pest wounds | Pest punctures are entry points for fungi → primarily Section 17, but control the pest as well | — (disease, secondary) |
Wormy nut: hazelnut weevil vs. moth
By far the most common cause of hazelnut worminess in Croatia is the hazelnut weevil (Curculio nucum): you find a creamy-white larva with a brown head surrounded by frass in the nut, and later a round exit hole. The walnut moth (Cydia spp.) is secondary and produces a caterpillar (butterfly larva) of a different appearance. Distinction is important because monitoring (pheromone traps for the moth) and control timing differ.
Figures (informative): hazelnut weevil causes 10–40 % damaged nuts in bearing orchards, and foreign literature reports yield losses of over 80 % in unfavorable years without protection. The critical number for treatment is often cited as the appearance of more than one imago (adult insect) per bush, or 3–4 adult insects per bush according to other sources. Adults appear from early May to August. Figures are indicative and depend on cultivar, location, monitoring method, and regulations.
Root / Trunk¶
Traces at ground level and on the trunk are rarer in hazelnut but must not be ignored.
| Visible damage / symptom | Most likely cause | How to distinguish / confirm | See monograph |
|---|---|---|---|
| Galleries and boreholes in the lower part of the trunk/shoot; larvae inside the wood | Borers / clearwings (e.g., Oberea linearis) | Sawdust (frass), galleries; check by cutting | Monograph: Borers |
| Damaged bark, chewed underground shoot/root | Soil pests (wireworms, grubs — general fruit species) | Less specific to hazelnut; check soil | Monograph: Soil pests |
| Drying of the entire plant from the root, no insects; cankers on the trunk | Disease (root rots, bacterial canker) | No boreholes/insects → Section 17 | — (disease) |
| Root suckers (shoots) grow vigorously around the trunk | Not a pest — agrotechnics | Removed by pruning/herbicide for suckers | — (agrotechnics) |
When the cause is NOT a pest: diseases and abiotic stress¶
A large portion of "suspicious" symptoms on hazelnut is not the work of an insect or mite. Typical examples that are easily mistaken for pest attacks:
- Leaf spot and bud drying (Labrella coryli / Gloeosporium coryli): ring spots on leaves (light zone surrounded by dark), necrosis and leaf shedding, bud drying—occurs after cold and rainy periods. Buds dry out but are not swollen as with mites.
- Bacterial canker (Pseudomonas syringae pv. avellanae etc.): cankers on branches, yellow exudate, drying of branches and entire plants. Spreads via undisinfected tools.
- Nut rot (Monilia, Botrytis, Fusarium, Nematospora): occurs at puncture/wound sites—often in combination with a pest, but insecticide will not stop the fungus.
- Abiotic stress: freezing of catkins and buds, drought, nutrient deficiency, phytotoxicity—discoloration and drying without any visible organism.
In all these cases, refer to Section 17 (diseases and abiotic disorders). The key test is: if a thorough inspection (magnifying glass, cutting bud/nut, checking leaf underside) yields no pest, it is very likely a disease or abiotic cause.
Pests open the door to disease
Many fungal and bacterial infections enter precisely through wounds made by pests (punctures from hazelnut weevil, bugs, pruning wounds). Therefore, consistent pest control is also a preventive measure against secondary nut rot and drying.
Seasonal orchard inspection reminder¶
| Period | What to inspect | Target pest |
|---|---|---|
| February (dormancy) | Buds (sample 100 buds) | Hazelnut bud mite — "big buds" |
| Bud swell/bud break | Buds, first leaf tissue | Hazelnut bud mite (migration at >15 °C) |
| Leaf emergence | Leaf edges, holes | Leafrollers (Phyllobius), Hazelnut weevil (supplementary feeding) |
| Flowering / post-flowering | Shoots, young nuts | Hazelnut weevil, Hazelnut weevil |
| May–August | Nuts, leaves, honeydew | Hazelnut weevil, bugs, aphids |
| Summer heat | Leaf undersides, mottling | Eriophyid mite / red spider mite |
| Pruning (dormancy) | Branches with colonies, scales | Leaf and scale insects (remove infested branches) |
Practical recommendations and common mistakes¶
- Do not treat "blindly." First identify the causal agent. A sulfur preparation against mites will not help if the bud is actually diseased by a fungus.
- Cut and use a magnifying glass. Hazelnut bud mite (~0.2 mm) and hazelnut weevil larvae are confirmed only up close. Without confirmation, diagnosis is guesswork.
- Pay attention to timing. For mites, the migration phase is critical (temperature >15 °C); for hazelnut weevil, the adult emergence period (May–August). Late treatment is wasted money.
- Distinguish between chewing and sucking. Chewed leaf edges = chewing insects; sticky "wet leaf" and sooty mold = sucking insects (aphids).
- Watch for resistance. Practice shows that aphids are becoming increasingly resistant to certain active substances (e.g., deltamethrin) — do not rely on the same product every year.
- Mechanical measures pay off. In smaller orchards, removing and destroying deformed ("big") buds and removing branches with aphid colonies during pruning reduce infestation without chemicals.
- Check registration. Always verify the product authorization in the Ministry's database (FIS — Phytosanitary Information System) and adhere to IPM rules and pre-harvest intervals.
Dosages and products mentioned in literature (informational)¶
The following data are taken from a literature collection for orientation purposes. They are not recommendations for application — check current registration, dosage, and pre-harvest interval before each use.
| Pest | Approach / product (from literature) | Dosage/concentration (informational) |
|---|---|---|
| Hazelnut bud mite | Sulfur preparations (Chromosul 80, Kumulus DF, Microthiol Special Disperss, Cosavet DF) | e.g., Chromosul 80 ~0.5–1 % (500–1000 g / 100 l water); effect better at >15 °C |
| Hazelnut bud mite + bacteriosis (combined) | Cuprablau Z + Chromosul 80 | Cuprablau Z ~0.5–0.6 % + Chromosul 80 ~1 % |
| Aphids | Spirotetramat (Movento, systemic); deltamethrin (Decis 100 EC); + silicone polymers (Silmax EC) | according to label; deltamethrin products limited to BBCH 50–59 |
| Honeydew (adjuvant) | Honeydew solvent (e.g., FitoDimel) | 250–300 ml / 100 l water (50–100 ml as adjuvant) |
| Hazelnut weevil / bugs | Registered insecticides (May–August) | according to label and FIS |
In Croatia, there are no registered acaricides specifically for hazelnut bud mite, so an indirect effect of sulfur fungicides is used; for hazelnut weevil and bugs, insecticides with Ministry authorization are used during the period from late May to mid-August. All dosages, products, and timings are informational and depend on cultivar, location, and current regulations.
Sources¶
This article is an AI summary generated from an internal literature collection on hazelnut (RAG corpus) supplemented with publicly available sources. Main sources used:
- Hazelnut bud mite — very important / important pest of hazelnut (Gospodarski list / AgroPortal.hr) — descriptions of "big bud" symptoms, two forms of the mite, sulfur preparations.
- Control of harmful organisms in hazelnut orchards (2009.) — hazelnut weevil, nut worminess, mite migration, mottling from red spider mite.
- Diseases and pests of hazelnut — Repository (2020.) — green fruit leafroller (Phyllobius piri), bugs, and Nematospora coryli.
- Most significant hazelnut pests (Plant Protection Bulletin, 2011.) — Myzocallis coryli, Oberea linearis, Eulecanium, Cydia amplana, biological control (Trioxys/Trioxys pallidus).
- How to get rid of hazelnut weevil naturally (Agroklub, 2016.) — description and biology of hazelnut weevil.
- Time for early spring hazelnut protection (Agroklub, 2017.) / Do not delay with the first early spring protection (2019.) — thresholds, sulfur dosages, bacteriosis vs. mite.
- Notice to hazelnut commercial producers — Ministry of Agriculture (2025.) — protection recommendations and deadlines, FIS.
- Recommendation for permanent hazelnut orchard owners (2025.) — aphids, "wet leaf" / honeydew, spirotetramat and deltamethrin, resistance.
- Protect your hazelnut orchards after heavy rains (2025.) — leaf spot and bud drying (Labrella coryli).
- Hazelnut bug (Pantilius tunicatus) and other pests — Environmental Protection Elaborate (PPO Hazelnut).
- Fitoeko catalog 2024 — honeydew solvent (FitoDimel).
- Curculio nucum (hazelnut weevil) — Wikipedia, CABI Compendium, MDPI Agronomy: https://en.wikipedia.org/wiki/Curculio_nucum ; https://www.mdpi.com/2073-4395/12/12/3059
- Phytoptus avellanae (hazelnut big bud mite) — NDP-39 (Plant Biosecurity Diagnostics), PNW Pest Management Handbooks, Wikipedia: https://www.plantbiosecuritydiagnostics.net.au/app/uploads/2018/11/NDP-39-Hazelnut-big-bud-mite-Phytoptus-avellanae-Nalepa-V1.pdf ; https://pnwhandbooks.org/insect/nut/hazelnut/hazelnut-filbert-bud-mite
For detailed biology, monitoring, and control of individual pests, see the corresponding monograph in this volume (15. Pest Compendium). When a symptom suggests a disease or abiotic cause, see volume 17.
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