The codling moth (Cydia pomonella), or apple worm, is one of the most troublesome pests affecting apple and pear trees. It’s a phytophagous parasite capable of harming other crops like walnuts. When left unchecked, it causes severe damage to fruits, often leading to them falling to the ground. Biological defense against it is possible and is implemented through a diversified strategy. This strategy accounts for the prevailing season and the insect’s developmental stage.
Let’s understand the codling moth better by studying its biological cycle and the damage it inflicts on tree crops. Additionally, we’ll explore various preventive and biological defense measures that can be undertaken.
Identifying the Codling Moth
The codling moth belongs to the Lepidoptera order, Tortricidae family. Other lepidopterans we’ve discussed include the tomato leafminer, tomato cutworm, pine processionary, cabbage moth, and red tree borer. As an adult, the codling moth appears as a small butterfly, with a wingspan of 15-22 mm. Its wings are gray-streaked near the head and have more intense bronze transverse veins towards the rear. The codling moth larvae are whitish with a dark head. Over time, the rest of their body also darkens, turning yellowish and eventually pinkish. They reach a final length of about 15-20 mm. It’s the larval stages that cause damage to the fruits.
Biological Cycle of the Codling Moth
The biological cycle of the codling moth is regulated by neurohormonal mechanisms influenced by external stimuli such as ambient temperature and photoperiod. In our climatic region, the codling moth overwinters as a mature larva in diapause inside a cocoon placed in the tree trunk’s crevices or in the soil. At the start of spring, the larvae pupate, and between April and May, adult moths emerge. The peak of moth emergence occurs in the latter half of May. The moths are active at dusk, and when temperatures exceed 15 °C, mating occurs. A few days later, egg-laying takes place. The females lay eggs individually. Typically, those from the first generation are laid on leaves or branches near the fruits. The newly hatched larvae spend some days outdoors, moving among leaves and branches before penetrating the fruits. Inside the fruits, they complete their development in 20-30 days. Once mature, they emerge to pupate beneath the tree bark.
Adult moths from the second generation appear from late June onwards, continuing through August, with most sightings in the first decade of July. These moths directly lay eggs on already developed fruits. Second-generation larvae have a very short incubation period and are active from July to late August. At this point, the larvae have two developmental options:
- Enter diapause, ending the annual cycle
- Pupate to give rise to the third generation, with adults emerging from August to September
In the latter case, we observe two overlapping moth emergences in August: the delayed second flight and the adults from the third generation.
The peak emergence of the third generation occurs in the second or third decade of August. The last-generation larvae remain active until October. As cold approaches, with changes in photoperiod and temperature drops, the larvae seek shelter in cocoons to overwinter. In summary, the codling moth completes three generations per year (or two if the third remains incomplete).
Damage Caused by the Codling Moth
The codling moth is a fruit-eating insect, feeding exclusively on fruits. The most affected crops are pome fruits, especially apple and pear trees, but serious damage can also occur on walnuts. Damage is caused by the larvae, which tunnel galleries into the flesh of the fruit at any stage of ripening. These galleries are deep, reaching the fruit’s core where the seeds are. They can enter the fruit at any point on its skin, with a preference for the calyx area. A small cluster of rasping and excrement around the entry hole aids in easily identifying the attack. As the larvae feed, the gallery fills with irritating excrements. Once their development is complete, the larvae exit the fruit by creating another exit tunnel. The most severe damage occurs in the later larval generations when their feeding activity is faster. This is because many fruits are attacked in a short span. Apart from the aesthetic damage caused by galleries, fruit drop is also common. Young larvae are more prone to natural mortality or post-biological treatments. In such cases, they cause only superficial erosion on the fruit, which, upon healing, results in the so-called “dry borer”.
Biological Defense against Cydia pomonella
Biological defense against the apple worm Cydia pomonella is quite complex and involves various intervention strategies. Firstly, setting up traps to capture adult moths using a specific pheromone is crucial. These traps can be found here, while the associated pheromones can be found here. These pheromone traps serve two purposes: monitoring the presence of codling moths and establishing thresholds for subsequent intervention. These traps should be placed in the orchard (on tree branches) towards late April, with 2-3 traps sufficient for an acre of land. In a small family orchard, one trap is enough.
When more than 2 male specimens are captured in a week, it’s time to intervene with other products. Let’s see what they are.
Summer white mineral oils can be used against codling moth eggs. It’s an effective ovicide we’ve previously discussed in the article on scale insects. It’s a product allowed in organic agriculture. It’s easy to apply since the apple worm eggs are relatively large and visible, predominantly found in the tree canopy’s shaded areas.
Microbiological Control: Beauveria bassiana and Granulosis Virus
When the codling moth is still in its young larval stage, effective microbiological control is necessary. This is achieved by using Beauveria bassiana. This is a biological product available in specific formulations for agriculture. Another example of microbiological control is the use of the so-called Granulosis Virus Cydia pomonella G.V.. This is a specific virus isolated in a laboratory, targeting only codling moth larvae.
This virus must be applied promptly at the time of egg hatching. It’s applied in an aqueous suspension, with water that must maintain a neutral pH. The Granulosis Virus, when ingested by the larvae, reaches the intestine and dissolves the protective protein matrix of the viral particle. Subsequently, the virus is capable of crossing the intestinal wall, spreading throughout the larva’s body. This disrupts the activities of vital organs, causing larvae to perish within 5 days of ingestion. The treatment must be repeated at an 8-day interval. The use of this product is permitted in organic farming, and you can purchase it without needing a license. It’s recommended to alternate the use of these microbiological control products. Granulosis Virus for the first generation of larvae, Beauveria bassiana for the second. Another suggestion is to use white summer oil first for the eggs. Both of these products have proven highly effective as substitutes for chemical insecticides.
To combat overwintering larvae, namely those of the third generation, entomopathogenic nematodes can be employed (as previously explained concerning the vine weevil). This biological treatment is carried out in autumn, using the Steinernema feltiae strain. The product should be applied at the onset of rain or when the tree trunks and branches are thoroughly wet. This ensures a film of water on the vegetation, essential for the efficacy of the bio-treatment. External temperatures must be 10°C or higher. Commercial formulations come as dehydrated masses in an inert substrate, reactivating as an aqueous suspension. Following the product label instructions poses no particular application difficulties, even for the inexperienced.
Mechanical remedies can also be used against the codling moth. The first of these involves the use of paraffin-coated corrugated cardboard band traps. These bands help eliminate larvae present on the trunk that descend to pupate or overwinter.
The mechanism is quite simple: when the larva finishes its development, having attacked the fruits, it descends along the bark and becomes trapped in the corrugated cardboard band placed on the trunk. The corrugated cardboard bands are easily installed on the trees. These give the larva the impression of finding an ideal place for metamorphosis. At this point, collecting and burning the bands is sufficient to eliminate the insect. The corrugated cardboard band should be placed on the trunk about 20 cm above the ground. Once placed, it should be checked every 15 days to observe if any larvae are trapped. If numerous larvae are found at this point, remove and burn the band (which is easy due to the paraffin). If larvae are few, for example, during the first generation, they can be manually removed by shaking the band. This allows reuse of the trap. This solution is very convenient in a family orchard and can be easily homemade.
Insect-proof netting is a rather complex codling moth capture system. It’s discouraged in a large specialized orchard, primarily due to setup costs. However, in a small family orchard with small-sized trees, it’s an option to consider. To implement it, simply use fine-mesh hail netting that prevents adult moths from coming into contact with the tree’s vegetation. Suitable netting can be found here.