Capnodis tenebrionis, the flat-headed root borer of stone fruit

The flat-headed root borer is one of the few insects capable of killing an apparently healthy tree, and it does so unnoticed until it is too late. The damage happens below ground, done by a larva feeding on the root and the crown. What follows sets out how it lives, why it has spread so widely across central Macedonia, which signs to look for in the field, and — most importantly — why irrigation turns out to be the strongest weapon against it.

1. Identity of the insect

The insect goes by several common names — flat-headed root borer, peach flat-headed root borer — and its scientific name is Capnodis tenebrionis. It belongs to the order Coleoptera and the family Buprestidae, the jewel beetles. Its main hosts are the stone fruits — sweet cherry, apricot, plum, peach, almond — and secondarily the pome fruits.

The adult is about 12 to 30 mm long and 7 to 12 mm wide. It is broad, solid and black, with characteristic pale markings on the pronotum, that is on the dorsal plate of the thorax.

The larva is the dangerous stage. It is white and legless, with the first body segment broad and flattened — hence the name flat-headed. It starts at a few millimetres on hatching and, on completing its development, can reach a considerable length, of the order of nine to twelve centimetres. Its head darkens over time and becomes brown.

2. Its life cycle, step by step

The cycle lasts one year or two, depending on conditions, and the insect can overwinter in any stage — even as an adult, in a sheltered position in the soil or in neighbouring vegetation.

The adult

The first adults appear in the orchard from early May. The highest density, and in particular the young adults, is seen in July. From December to the end of March they are usually not found in the blocks at all. The adult is long-lived — three to four months — and feeds and lays eggs throughout that period.

Oviposition — where everything is decided

The female lays its eggs through the summer, in small batches, near the base of the trunk. On light soils it leaves them in the soil itself; on heavy soils it hides them in cracks or under stones. A single female can lay more than a thousand eggs in its lifetime.

Two conditions must hold simultaneously for oviposition to take place, and both are of direct practical interest:

• The temperature must be above 26 °C.
• The soil must be dry. Females systematically avoid soil with moisture above 10%. Indeed, where the soil around the trunk is moist, they will move away and look for a dry spot even at more than 1.2 metres from the base of the tree.

The larva

The egg hatches within a few days, depending on temperature. The young larva does not enter the trunk but heads for the roots. It has the remarkable ability to locate a root at up to 60 centimetres from where it hatched. Once it finds one it bores in and begins feeding on the bark, opening galleries that start small and widen as the larva grows. Behind it, it leaves compacted residues, the so-called frass. Larval development lasts from six to eighteen months.

The pupa

Pupation takes place preferentially at the base of the trunk and lasts three to four weeks. From there the new adult emerges and the cycle begins again.

3. The two different kinds of damage

It is important to grasp that the same insect causes two entirely different kinds of damage, at two different points of the tree, in two different stages of its life.

That is precisely why the problem is underestimated. The damage done by the adult is visible and annoying, but it does not kill. The damage done by the larva is not visible at all, and it kills.

4. Symptoms in the field

Because the main damage is underground, recognition rests on indirect signs. The signs follow, in order of priority:

• Dieback of the tip of young shoots, while the remaining leaves on the branch stay green. This is among the first visible signs and results from adult feeding at the base of the shoot.
• Leaves severed at the petiole, falling to the ground while still green.
• Isolated trees wilting within an otherwise healthy block, particularly in July and August.
• Gum and frass at the base of the trunk. Digging around the crown and finding galleries packed with compacted residues makes the diagnosis certain.
• The adults themselves, motionless on the branches during the hot hours. They drop to the ground and feign death on approach, so collection needs care.

5. The key: dry soil and a thirsty tree

An earlier version of this text noted, from field experience, that regularly irrigated orchards are less affected, because the tree stays vigorous and produces abundant gum that drowns the larva. The international literature confirms that observation in full, and adds a second mechanism that reinforces it.

First mechanism: moist soil prevents oviposition

As set out above, females lay exclusively in dry soil and avoid soil with moisture above 10%. Moisture also sharply reduces egg hatch, and in fully saturated soil no egg hatches at all. An irrigated block, in other words, is not merely less attractive — it is unsuitable as a breeding site.

Second mechanism: a vigorous tree kills the larva

In healthy, well-watered trees with strong sap flow, young larvae drown in the gum exuded by the plant in response to attack. A tree suffering water stress, by contrast, cannot mount that defence. Adults, moreover, actively prefer weakened trees both for feeding and for oviposition.

6. Management

6.1 Cultural measures — the foundation

• Adequate and regular irrigation, especially through summer and especially around the base of the trunk. This is the measure with the highest return, because it strikes simultaneously at oviposition, at hatching and at larval survival.
• Sound nutrition. Deficiencies weaken the tree and make it a preferred target.
• Prompt removal and destruction of dead or dying trees. Inside them, larvae complete their development and will produce next year's adults.
• Particular care with new plantings. Young trees are dramatically more vulnerable, since one larva is enough.

6.2 Mechanical measures

• Hand collection and destruction of the adults. This remains the first and basic measure, especially on small holdings. It is done during the hot hours, when the beetles are motionless on the shoots. A good technique is to spread a sheet under the tree and shake the branches.
• Covering the soil with plastic. Polyethylene sheeting in a strip of about one metre either side of the tree row has proved effective, because it prevents newly hatched larvae from reaching the root.

6.3 Biological control

Natural predators and parasites of this insect are very few, so biological control rests on formulated products:

• Entomopathogenic nematodes, mainly Steinernema carpocapsae. They are applied to the soil together with irrigation and have given good results against the larvae. It is essential that the soil be kept moist during and after application, or the nematodes do not survive.
• Entomopathogenic fungi, such as Metarhizium brunneum and Beauveria bassiana, applied preventively to the soil. They are also being tested against the eggs.

6.4 Chemical control

This is difficult, because the damaging stage sits protected inside the root, but it is feasible when correctly targeted:

• The target is the adult, not the larva. Applications are made to the trunk and the canopy, on the basis of adult monitoring through the warm period.
• In young plantings and nurseries, soil applications have given protection.
• Once a serious population is detected, control is carried out with a purpose-chosen insecticide programme.

6.5 Rootstocks

Rootstocks with relative resistance exist and are used as part of the strategy. There is, however, one critical proviso that has to be stated plainly: under water stress every rootstock becomes vulnerable to some degree. Rootstock resistance does not substitute for irrigation.

7. A monitoring calendar

8. Sources

• Capnodis tenebrionis (Coleoptera: Buprestidae), an important pest of stone fruits in the Mediterranean basin: current management strategies and prospects for integrated pest management. Journal of Integrated Pest Management 15(1):20 (2024) — a current review.
• Capnodis tenebrionis — Plant Pests of the Middle East, Hebrew University of Jerusalem, Department of Entomology — biology, oviposition in dry soil, polyethylene mulching.
• Control of the flat-headed root borer Capnodis tenebrionis with the entomopathogenic nematode Steinernema carpocapsae in a chitosan formulation in apricot orchards. BioControl (2007).
• Preventative approach to microbial control of Capnodis tenebrionis by soil application of Metarhizium brunneum and Beauveria bassiana (2020).
• Effectiveness of local entomopathogenic fungal isolates against eggs of Capnodis tenebrionis. Egyptian Journal of Biological Pest Control (2025).
• Mendel, Z., Assael, F. & Ben-Yehuda, S. (2003). Host selection and root colonization of cyanogenic stonefruit species by Capnodis spp. Annals of the Entomological Society of America 96:127–134.

Symptom photographs: Pastopoulos Agronomics archive, from cherry orchards in our area.