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Dealing with Olive Moth (Prays oleae) Infestations

Prays oleae, also known as the olive moth, poses a significant threat to olive orchards. Discover effective methods for managing and preventing infestations.

by BioGrow

One of the most destructive pests for olive groves is the olive moth. It can be found everywhere in olive orchards, especially in areas near the sea. This pest, also known as Prays oleae, has a complex life cycle with distinct generations. Understanding the differences between these generations is crucial for knowing when to intervene using methods approved for organic farming. Undoubtedly, the first step is thorough monitoring to check for the presence of these insects on our olives.

In this article, we will explore various aspects of olive moth infestations.

Description of the Olive Moth

Adult olive moth butterfly

The Prays oleae is an insect belonging to the Lepidoptera order and the Yponomeutidae family. Due to its small size as an adult butterfly, it falls under the category of micro-lepidoptera, similar to the more well-known tomato leafminer.
It attacks olive cultivations and, when fully mature, appears as a butterfly with a wingspan of about 14 mm. When its wings are closed, it measures approximately 7 mm. Its distinctive appearance includes silvery-gray coloration with evident black spots on its forewings.

Olive Moth Eggs

The eggs of the olive moth are tiny, measuring around 0.5 mm in diameter, and oval in shape.

Olive Moth Larvae

The fully developed olive moth larvae, responsible for damaging the olives, are about 7-8 mm long. They are brownish-green, leaning towards a lighter shade. They have greenish bands on their backs and two yellowish bands on their sides. The color of their heads varies from brown to black. Before reaching full development, the larvae go through five growth stages during which their heads become larger.

Olive Moth Pupae

Finally, the pupae, up to 6 mm long, are brown in color and have a somewhat conical shape.

Life Stages of the Olive Moth

Larva of Prays oleae (olive moth)
Monophagous insects are those that grow and feed on a single plant species. The olive moth falls under this category. As expected, the host plant for this pest is the olive tree. The moth completes three generations in a year, each of which goes through its cycle on a different vegetative organ. These generations are:

  • First generation: Anthophagous, targeting flowers;
  • Second generation: Carpophagous, damaging fruits;
  • Third generation: Fillophagous, overwintering on olive leaves.

Adults overwinter as pupae and begin emerging in spring, coinciding with the tree’s floral bud differentiation.

First Generation

During this phase, olive moths lay eggs on the calyx of the flower, and after 10-15 days, the first larvae emerge. They enter the flowers, extract pollen, and in the final stage of development, penetrate the inner organs. These larvae are highly active and can attack up to 15 flowers, enveloping them with a thin thread of silk, making the affected flower quite visible.
The larvae of the first generation complete their development in about a month. They then pupate within the tree’s tissues and emerge as adults after 10-15 days, giving rise to the second generation.

Second Generation

This generation of olive moths (carpophagous) is the most damaging to olive oil production, as it occurs during fruit development and poses a threat to olive harvesting. Adult moths lay eggs on the calyx of small fruits near the peduncle. Within a week, new larvae hatch and penetrate deep into the olives. Once their development is complete, they pupate, often inside the olives.

Third Generation

In September, adults of the third and overwintering generation of olive moths emerge. These butterflies lay their eggs on the upper surface of leaves, and the larvae go through five stages of development inside the leaves. This causes significant erosion on the foliage.

Damage Caused by Olive Moths on Olive Trees

Damage of the olive moth on a leaf

Damage of the olive moth on a leaf

The extent of damage caused by the olive moth varies depending on the severity of the infestation and the generation responsible for the damage.

Damage from the First Generation

The attack of the anthophagous olive moth generation (which targets flowers) varies from a few flowers affected in the field to values as high as 90%. Clearly, the extent of damage varies widely.

Damage from the Second Generation

The attack of larvae from the carpophagous olive moth generation (which damages fruits) is the most concerning. This is particularly true because it leads to early fruit drop. This occurs in two distinct phases:

  • In June and July, during larval penetration into the fruit;
  • In September and October, when the olive moth larva pupates.

During the first phase, it is essential to determine whether the fruit drop is due to olive moth attacks or natural thinning. Factors like high temperatures and extended periods of drought could also cause premature fruit drop.
In the second period, olives close to harvesting fall. During this phase, olive growers are deeply concerned, as effective treatments become challenging to implement.

Damage from the Third Generation

The larva of the third generation, the overwintering fillophagous olive moth (which erodes leaves), damages vegetation by limiting photosynthetic activity. From an operational perspective, these damages are not alarming enough to warrant defense interventions. Nevertheless, it lays the groundwork for future issues.

Natural Factors Hindering Olive Moth Development

The olive moth develops in mild climatic conditions during its various stages. In hot summer seasons with daytime temperatures consistently above 30°C and high humidity, eggs and larvae naturally perish. However, pupae can survive temperatures exceeding 40°C. Another natural limiting factor can be observed in the carpophagous generation of the olive moth. As mentioned earlier, small attacked olives fall prematurely, leading to the death of the fruit and the larva inside.

Predators of Olive Moths Impacting Olive Trees

The olive moth has numerous natural enemies, often referred to as beneficial insects in olive groves. Several studies have demonstrated their effectiveness. Unfortunately, the use of chemical pesticides, prevalent in conventional agriculture, reduces the presence of these natural predators, undermining their impact.

List of Antagonistic Insects to the Olive Moth

Here is a list of antagonistic insects to the olive moth:

  • Hymenopterans Clelonus elaphilus and Ageniaspis fuscicollis var. praysincola. These insects can parasitize eggs from all three generations of the olive moth.
  • Hymenopterans Apantheles xanthostigma, Elasmus stefani, and Itoplectis alternans. These insects also parasitize a significant number of eggs.
  • Chrysopidae Chrysoperla carnea. This is a predator of olive moth larvae.

Monitoring Olive Moths

Cocoon of Prays oleae

Cocoon of Prays oleae

Similar to the approach used for the olive fly, monitoring olive moths can be carried out using pheromone traps. These traps capture male specimens and provide insights into the flight patterns of various generations. However, this method does not provide an accurate indication of the total population present due to its lack of precision. Therefore, it is not suitable for mass capture. Where it has been tried, it yielded poor results.
Nevertheless, Pheromone traps like this can provide insight into the presence of the olive moth and are therefore worth considering. Visual monitoring of olive moths can also be done, especially for the carpophagous generation. Sampling of fruit can be done in two ways:

  • Evaluating early fruit drop;
  • Cutting a small sample of olives on a suspected tree and examining them cross-sectionally. This helps determine whether there are eggs or larvae inside the olives.

Preventing Olive Moths

Unfortunately, there are not many paths to prevent olive moths. When establishing a new olive grove in an area known for the presence of these insects, choose varieties with small drupes. Smaller olives make life more challenging for the olive moth, and damage has less impact on the overall crop. For soil management, favor the technique of cover cropping over cultivation. This promotes the presence of beneficial insects. Pruning olive trees should not be overly intensive in high-risk attack zones. Excessive pruning can expose the few remaining olives to a higher risk of infestation the following season. Pruning should always be balanced to meet both production and defense needs.

Combatting Olive Moths with Bacillus Thuringiensis

Controlling olive moths can be achieved using the biological insecticide Bacillus thuringiensis, which we’ve discussed before. The most effective strains against this insect are:

Typically, it is not recommended to intervene during the antophagous generation unless the percentage of infested flowers exceeds 45%. The carpophagous generation, however, is the most damaging and requires action. The intervention threshold is set at 15% of infested fruits. The best time to act is during egg hatching, before the small larvae penetrate the fruit.
Once inside, the effectiveness of Bacillus thuringiensis diminishes, as it only acts through ingestion.

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