The many insects that are selected for research are easy to rear; artificial diets have been developed for hornworms, drosophila, wax moths, and cabbage white butterflies. Other insects are herbivores so only one plant needs to be produced to rear them. Even predators some parasitoids can be easy to rear as they may be reared on easy to rear alternative hosts or artificial eggs. Unfortunately such is not the case with GWSS egg parasitoids. While the wasps themselves are relatively easy to rear, obtaining a reliable year-round supply of eggs is very hard. In the field, GWSS feed on large perennial plants and switch hosts as the season changes. They can oviposit on plants on which they rarely feed and once the nymphs emerge, they frequently leave the ovipositional plant to feed on other plants. And even in the field GWSS cease to oviposit in California winters. In an attempt to rear GWSS biological control agents, we are attempting not only to duplicate field conditions but improve on them. There are three major components to the production process:
The production process starts with plant selection and soil preparation. The preferred host plants in the field are often inappropriate for production because they are too big for cages or are not sufficiently vigorous to survive GWSS feeding (e.g. citrus). Alternatively, plants may be susceptible to disease in greenhouse conditions (e.g. mildew on grapevines). The plants ultimately selected for GWSS production are select varieties of cowpea, sorghum, and sunflower. Ovipositional plants include sorghum, chrysanthemum and Euonymus.
Once plants have been selected, conditions for cultivation are critical. Optimal growing conditions involve the careful control of fertilizer applications, water quality and irrigation, lighting, and temperature. Any deviation can result in poor plant growth, early maturation of plants, or poor nutrient content of xylem fluid, the liquid on which GWSS feeds. This can ultimately lead to GWSS mortality or GWSS that develop to maturity but never produce eggs; something that can take two months to discover.
Then the next step is rearing GWSS. Care in cage design, insect stocking and selection of plants appropriate for each age is needed. Plants take a minimum of three weeks to grow to a size that can be used in GWSS production.
As the GWSS develop, attention needs to be taken regarding infestation by pests such as whitefly, spider mite, and thrips. Plants need to be replaced on a regular basis as they outgrow the cages, mature, or die. When GWSS reach maturity, the host plants need to be stocked with oviposition plants and the plants must be checked twice a week to collect egg masses. The GWSS egg masses can be stored if they are laid on certain plant species. A proportion of eggs are retained for maintenance of GWSS colonies, otherwise they need to be taken and placed in wasp sting cages immediately.
Colonies can be initiated from populations of GWSS originating from differing locations but care must be given not to inadvertently transport the diseases they carry to new locations; insects originating from some areas are easier to maintain than others. GWSS escaping from cages into the greenhouse should be minimized and no plant material should be outside the cages for the insects to survive on. Greenhouses should be screened to prevent escape of GWSS.
Once colonies have been initiated they can be maintained by introducing offspring from the founding population (i.e. a closed system) or fresh material can be introduced from the field. There are benefits and drawbacks for each method; closed colonies can select for laboratory strains or inbreeding depression, introduction of field insects can inadvertently introduce disease and other insect species such as pests and parasitoids. Adding field collected insects can augment production of GWSS eggs when field collected sharpshooters are physiologically prepared to oviposit (i.e. in spring and summer months) while closed colonies are most valuable for the production of GWSS eggs when field populations are undergoing ovipositional quiescence during the winter months.
This is the easiest part of the production process and a number of production techniques have been developed depending on the production scale, host plant, and parasitoid involved. At the smallest, scale an egg mass on an individual Euonymus leaf is placed in a floating raft in a 20 dram vial half filled with water to which a single wasp is added. At the largest scale, egg masses on 10-16 plants are placed in a 2’ cubed cage to which are added wasps. After incubating for 10-14 days wasp offspring emerge and are collected for maintenance of colonies or release. During this part of the production process, environmental conditions do not need to be so strictly controlled but care needs to be taken to maintain the vigor of plants. Drops of honey are added to cages to keep adult wasps alive for more than one day.
Storage and Other Factors
Occurrences in the real world often lead to disruption in the smooth running of the production process. Plants may die due to disease, colonies may become infested with undesirable pests, greenhouses may break down, water can be cut off and power cuts occur. All such events can lead to collapse of one or more stages of the production process so it is important to maintain back-up material to get back up to speed with minimum delay.
Most plants used in GWSS production are annual fast-growing species. However, a back-up of perennial plants are maintained should there be production problems. These include citrus and Euonymus. Many of these plants can be purchased from nurseries but are often pre-treated with insectides with long residuality. For instance, potted Hibiscus treated with Imidacloprid can remain fatal to GWSS for over two years post-treatment.
A considerable amount of work has been carried out into the preservation of GWSS and parasitoids. Nymphal and adult GWSS have poor survival at low temperatures but eggs, if laid in certain plants, can be stored for over three months at low temperatures without significant mortality. Wasps can also be stored in their larval stage (in GWSS eggs) but adults loose quality if stored for over two weeks at low temperatures.
Because of the need to maintain populations of parasitoids that are adapted to the field rather than the laboratory, it is important to add fresh stock to colonies on a regular basis. This can be done by importing new stock from the wasps’ original range or by re-introducing individuals that have been recovered from release sites.