The louse of San José (Quadraspidiotus perniciosus), also known as San Jose scale, is one of the most damaging agricultural pests These pests affect numerous fruit crops, especially pome and stone fruit trees. Their economic impact is significant because they can significantly reduce the quality and commercial value of fruits such as apples, pears, peaches, plums, and numerous other plant species. Below, we discuss in detail the biology, characteristics, damage, life cycle, detection and diagnostic methods, as well as integrated treatments and strategies for the control and management of this highly significant agricultural pest.
What is the San Jose louse?
The San Jose louse is a hemiptera belonging to the Diaspididae family, its scientific name being Quadraspidiotus perniciosus (also know as Diaspidiotus perniciosus).
This pest has been reported in more than 150 species of plants, which include most stone fruit trees (peach, nectarine, plum, cherry, etc.), pome fruit trees (apple, pear, quince), and some ornamental shrubs. Their presence not only affects the health of the trees but also restricts fruit exports, given their status as quarantine pests.
Morphological characteristics of the San Jose louse
The San Jose louse shows a marked sexual dimorphism, which means that there are clear differences between males and females.
- Females: Their body is oval, yellow, and they are always wingless. During the first stages of development they are mobile, but later they attach to the plant and are covered by a grayish protective shell, known as a scale (or follicle), which can reach between 1,3 and 2 mm in diameter, beneath which the adult female remains. The morphology of this shield is circular, generally dark gray, with centralized exuvia (molt remains).
- malesAfter passing through four nymphal stages, they emerge as winged adults. Adult males are about 1 mm in size, with a yellowish-yellow body and whitish wings. Under the nymphal shield, they develop a more elliptical shape than the females.
El only mobile stadium In the life of the female it is that of a newborn nymph, while in males the adult stage is mobile thanks to its wings.
Hosts and distribution
The San Jose scale is widely distributed in temperate and subtropical zones. It can infest a wide variety of plants, but has a particular predilection for:
- Apple tree (Malus domestica)
- Peara (pyrus communis)
- Peach (Prunus)
- Plum (Prunus domestica)
- Cherry, apricot, quince, among many others
It can also be found on ornamental species and shrubs, such as Acer, Ligustrum, Rosa, Salix y ulmus.
Biology and life cycle
The life cycle of the San Jose louse presents differences between males and females, as well as a generation dynamic that can vary depending on the weather. Normally, they develop three generations a year, although this number may fluctuate depending on the local climate.
Much of the population winter passes In the first or second nymphal stage (black shield stage) attached to the branches of the host, although adult females can also be found.
- La biological activity It restarts with the rise in temperatures in spring, when the first adults of both sexes appear (males usually emerge before females).
- El mating and fertilization of females usually occurs shortly after the emergence of adults.
- The females are viviparous, that is, they give birth to live larvae (they do not lay eggs) that, once born, move towards the upper parts of the tree to look for a suitable place to attach themselves, feed and begin to form the protective shield.
- Each season, several waves of neonates are produced: the first generation of mobile larvae usually begins in late spring, the second generation in mid- to late summer, and the third in early autumn. It is the last generation that overwinters attached to wood or fruit.
A female can generate between 90 to 400 larvae Under favorable conditions, each of them is capable of initiating the infestation cycle.
Stages of development
- Crawler nymph: This is the first stage after birth, yellow in color, measuring 0,2–0,3 mm. This is the only time the female moves.
- Fixed nymph: Soon they attach and insert their mouthparts into the plant tissue. Here they begin to secrete whitish filaments and lose their legs and antennae, beginning the formation of the shield.
- Successive nymphal stagesIn both males and females, the scutes grow. In the second instar, the main difference is the larger scutes and sexual differentiation. The female retains the circular scale, while the male develops an elliptical scale and organs to form wings.
- AdultsThe male is winged and mobile, the female remains under the shield all her life.
Damage caused by the San Jose scale
The San Jose scale causes direct and indirect damage to crops:
- Food: The insect sucks sap from branches, shoots, leaves, fruits and bark, weakening the plant, slowing its growth and facilitating the entry of secondary diseases.
- Fruit spots: Where the pest feeds, circular reddish or purplish spots remain, which reduces the commercial quality and value of the fruit.
- Loss of vigor and death of branchesIn severe infestations, it can dry out entire branches and even cause the death of the host tree.
- Impact on exports: It is a quarantine pest, so its presence limits or prohibits the export of fruit from infested plots.
Early detection is essential to avoid significant damage, since the protective follicle hinders the penetration of phytosanitary products and its control in advanced stages is complicated.
Signs, symptoms and diagnosis
The most characteristic signs of a San Jose louse attack include:
- Circular greyish shields on branches and fruits, which correspond to the protective shell of the adult female or the elliptical scales of the male.
- reddish spots on the bark and surface of the fruits where the nymphs feed.
- Deformed fruits and dry branches in severe attacks.
The diagnosis must be made by means of direct observation with a magnifying glass, paying attention to the follicles and collecting samples for entomological analysis if in doubt.
Annual cycle and generation of the plague
Depending on the climatic conditions, the San Jose scale can develop different numbers of generations. In temperate climates, they usually occur three generations a year:
- First generation: The first mobile larvae emerge in late spring and actively move around the tree before settling.
- Second generation: It occurs in summer, with new waves of mobile nymphs that colonize branches and fruits.
- Third generation: The larvae of this generation will seek to overwinter, protecting themselves under the shield and waiting for the plant to restart its vegetative growth.
A few individuals may develop additional generations or advance to a new phase depending on microclimatic conditions.
Factors that favor the proliferation of the San Jose louse
Among the most determining factors for the spread of the plague are:
- Mild temperatures and absence of severe frosts during the winter.
- Intensive use of broad-spectrum insecticides that eliminate natural predators of the pest.
- Fruit monocultures and lack of crop rotation.
- Deficiencies in tree pruning and management, which hinder aeration and access to infested areas.
Methods of control and treatment of San Jose scale
The management of this pest must be design, combining chemical, biological and cultural methods to minimize damage and reduce resistance.
culture control
- Perform sanitation pruning to remove infested branches and improve aeration.
- Remove and destroy fallen fruit and plant debris on the ground that could serve as shelter.
- Promote biodiversity within the garden to maintain populations of natural enemies.
Biologic control
- Natural predators: Among the most effective are plants that attract beneficial insects , the Coccidophilus spp., Rhizobius pulvhellus y Lindorus lophanthae.
- Parasitoids: They stand out Encarsia perniciosi and several species of the genus Aphytis, which parasitize the nymphs of the San Jose louse.
The conservation and promotion of these natural enemies is essential for effective biological control.
Chemical control
- The use of mineral paraffinic oil Applied during vegetative dormancy (winter), it is one of the safest and most effective methods to reduce wintering.
- Specific insecticides such as diazinon, malathion, or methyl azinphos can be effective, but their use should be limited to critical moments such as the emergence of mobile nymphs, where the follicle is not yet impermeable.
- The choice of product and timing of application should be based on pest monitoring and always following local legal recommendations and restrictions.
The indiscriminate use of chemicals should be avoided, as it can encourage the development of resistance and cause imbalances in the agroecosystem.
Technologies and monitoring for efficient treatments
Today, there are decision support software , the FuturCrop, which calculate the life cycle of pests in real time based on agroclimatic conditions and send alerts for the most timely and efficient treatment. These tools allow for:
- Receive advance warnings about the start of nymph emergence.
- Reduce the number of phytosanitary treatments and adjust the timing of application to achieve maximum effectiveness.
- Optimize costs and minimize environmental impact.
Prevention and complementary measures
- Purchase pest-free seedlings from certified nurseries.
- Carry out regular inspections throughout the annual crop cycle.
- Use visual sampling methods and pheromone traps to monitor the presence and levels of infestation.
- Strengthen plants through proper fertilization and irrigation, making them less susceptible to attack.
Economic and phytosanitary importance
The San Jose louse is considered a quarantine plague in numerous countries. Its presence in fruits and plants destined for export can lead to rejection at the border and serious economic losses for producers.
Correct identification, monitoring, and timely treatment are key to maintaining crop health and ensuring the commercial viability of affected agricultural products. Know integrated strategies for the control of the San Jose scale helps farmers minimize damage, protect their farms, and increase the production of quality fruit.