Why Biological Control Fails: Encapsulation May be the Culprit

Brown soft scale
Brown soft scale attacks a wide range of ornamental and greenhouse crops, as well as citrus.

Mealybugs and scales are insect pests of many horticultural plants grown in greenhouses, nurseries, and interior plantscapes. One option associated with plant protection is the use of biological control by releasing parasitoids. A number of parasitoids are commercially available currently to address problems with specific mealybug and scale species. For example, Leptomastix dactylopii and Anagyrus pseudococci are available for use against specific mealybug species. Aphytis melinus and Metaphycus helvolus are sold for use against specific scales. However, there may be instances when parasitoids are not providing sufficient regulation of mealybugs or scales because some mealybug and scale species are able to encapsulate the eggs of certain parasitoids.

Encapsulation is the main defensive response affiliated with the cellular immune system of insects such as mealybugs and scales to prevent attack by parasitoids or prevent parasitism, and may be responsible for determining the suitability of a host (prey) in supporting the development of parasitoids.

What is Encapsulation?

Encapsulation is a process by which the immune system of an insect responds to a foreign body (parasitoid egg) by surrounding the egg, resulting in death by means of asphyxiation or suffocation. Encapsulation may occur within 24 hours. Consequently, parasitoid development can be hindered by encapsulation, thus reducing the ability of parasitoids to provide sufficient regulation of mealybug or scale populations. The host blood cells are responsible for encapsulation of eggs, a process mediated by hemocytes. Hemocytes are free-floating blood cells that circulate within the hemolymph (blood) of an insect and are involved in insect immune system responses.

Once an egg enters the body cavity of the host by means of the parasitoid female laying an egg, hemocytes form a capsule composed of melanin (a dark-brown to black pigment) that surrounds and adheres to the surface of a parasitoid egg (this process is called melanization); consequently killing the egg and preventing development of the parasitoid. Encapsulation may substantially reduce the efficacy of parasitoids released against mealybugs or scales. This diminishes the numbers of parasitoids (offspring) in the next generation, thus decreasing regulation/suppression of mealybug populations. Encapsulation is effective primarily against endo-parasitoids, which are parasitoids that lay eggs within the host body and have to contend with the host immune system, whereas ecto-parasitoids lay eggs on the outside of the host body.

Leptomastix dactylopii is a parasitoid of the citrus mealybug (Planococcus citri) and the eggs inserted into citrus mealybug crawlers by females are susceptible to encapsulation.

How Encapsulation Affects Parasitoid Efficacy

Encapsulation by an insect host is an important parameter affiliated with host suitability and influences the efficacy of mealybug or scale parasitoids. High encapsulation rates may occur with mealybugs. For instance, the parasitoid Leptomastix dactylopii can suffer encapsulation rates of 100% when attempting to parasitize the citrus mealybug (Planococcus citri). High encapsulation rates may result in outbreaks of mealybugs or scales due to insufficient regulation by parasitoids.

Factors that can affect encapsulation include: parasitoid species, host plant, and host age. Host and parasitoid-type may differ in regard to the incidence of egg encapsulation. Host plant or plant type may affect encapsulation by mealybugs. In addition, the host plant may influence the ability of insects such as mealybugs or scales to encapsulate eggs due to nutritional quality (nitrogen content). This can either enhance or lessen immune system responses based on the nutrient value of the plant to mealybugs or scales.

Older, or larger hosts (later instar larvae), contain higher numbers of hemocytes responsible for encapsulation of parasitoid eggs, whereas younger, or smaller hosts (early instar larvae) have fewer hemocytes available for encapsulating eggs. Therefore, parasitoids that parasitize young or smaller hosts (early instars) may be more successful biological control agents or natural enemies because they parasitize hosts before the host has developed the ability to encapsulate eggs.

Parasitoids have evolved mechanisms that allow them to avoid or overcome encapsulation by a host. For example, some parasitoids inject a virus into a host along with an egg, which interferes with the immune system and protects the egg from encapsulation. Furthermore, certain parasitoids are not affected by encapsulation in any host.

Be sure to consider the possibility of encapsulation when biological control does not seem to be working for you.

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