BIOLOGICAL CONTROL AGENTS FOR LEPIDOPTERAN PESTS

Podisus maculiventris (spined soldier bug)

Podisus maculiventris is a large predatory stink bug, the adults of which are approximately 11–13 mm long by 6.5–8 mm wide, with males usually being slightly smaller than females. The adults are brown in color, with two prominent thorn-like spines projecting to each side just behind the head (as pictured). 

First instar nymphs are red and black and are usually found clustered close to the eggs from which they hatched. The nymphs gradually become more colorful as they mature, developing paler areas that include partial abdominal bands. There are a total of 5 nymphal instars, and all stages are predatory except the first instar nymphs, which are not thought to feed.

The recorded prey range of P. maculiventris includes more than 80 insect species, of which approximately 50 are economic pests. Usually it is the immature stages that are attacked, with various beetle and moth larvae being common prey. However, P. maculiventris will also attack the immature stages of other stink bugs, squash bugs, the larval stages of some fly species, and various others, including the larval and/or adult stages of some coccinellid beetles. Moisture is needed for optimal survival and reproduction, and if prey or water is scarce they may take plant sap; such feeding does not cause noticeable damage. 

The optimum temperature range for P. maculiventris lies between 22 to 27 °C (72–81 °F); at 24 °C, the life-cycle (egg to adult) is completed in approximately 28 days, and adults may live approximately 30–60 days at this temperature. With adequate prey, females are capable of laying more than 500 eggs during that time, usually in small groups on the leaves. 

Under field conditions, towards the end of the summer, older nymphs that develop under short-day conditions (approx. 10 days with a day-length of 8 hours and a temperature of 23 °C) will develop into adults that are in reproductive diapause, when reproduction is suspended and predation rate is reduced. Diapause is a natural adaptation for overwintering and is reduced or prevented at higher temperatures and longer day-lengths.

Podisus maculiventris is available in packs of 50 nymphs.

Trichogramma (various species)

Trichogramma are minute parasitic wasps (less than 1 mm in length) that attack the egg stage of their hosts (mainly different moth species). 

Adults of most species typically have red eyes and a yellowish to brown head and thorax and a somewhat darker abdomen. The female wasps lay one or more eggs inside host moth eggs, with the number depending on the size of the host egg. The wasp larvae feed on the host embryo and eventually kill it before it can hatch. At that point, the host egg turns back, and a few days later, the new adult wasp emerges.

Most Trichogramma species will attack the eggs of a relatively wide range of moth species, with host range being determined by factors such as the size and thickness of host eggs, and to some extent by the plant host and habitat. Wasp movement can be impaired on plants with very hairy leaves.

The life-cycle (egg to adult) can be completed in approximately 8–14 days, depending on temperature and humidity. Optimal conditions for most species of Trichogramma are in the region of 25–30 °C (77–86 °F), with relative humidities of 60–80%. For most Trichogramma species, efficacy is reduced at high temperatures and low humidity, and at temperatures below 20 °C. Adults without access to food will live only a few days, but will survive considerably longer when sugars are available (e.g. in plant nectar or aphid honeydew). 

Control of moth pests with Trichogramma can be improved by timing releases to coincide with the flight period of the adult moth; monitoring the latter with an appropriate pheromone trap is therefore recommended. Trichogramma are not strong fliers; in outside environments, some individuals may disperse 14 m or more from their release site, but adult dispersal is generally better if adjacent plants are touching (particularly at temperatures below 20 °C, which reduce flight activity).

The species of Trichogramma available include Trichogramma minutum and T. platneri, which are closely related species that cannot be distinguished from their external appearance; however, their native geographical ranges differ and cross-breeding between the two can result in only male offspring being produced. Hence it is better to use the species native to the target region: T. minutum is native to the eastern USA and T. platerni to the western USA. 

Both species are generally considered well-adapted to tree-dwelling hosts and have been used in orchards to suppress codling moth and oriental fruit moth. However, they will attack the eggs of various other moths as well: Trichogramma platneri has been recorded from more than 33 host species in 8 different moth families, and it is likely that T. minutum has a similarly broad host range. 

Trichogramma brassicae has been widely used to control moth pests of vegetable crops, including the European corn borer (Ostrinia nubilalis), cabbage looper (Trichoplusia ni) and leek moth (Acrolepiopsis assectella). 

Trichogramma species are supplied in units of 100,000 parasitized host eggs containing wasp pupae at or near the point of adult emergence.

Diglyphus isaea 

Diglyphus isaea is a small parasitic wasp that attacks the larval stages of at least 9 different species of leaf-mining flies (primarily Liriomyza spp.) found on various crops (including tomatoes, cucumbers, eggplant, peppers and various ornamentals (such as chrysanthemum and gerbera). This wasp is most effective at warmer temperatures (approx. 25–33 °C (77–91 °F), depending on host species and crop).

Adults of D. isaea range in size from approximately 1 to 2.5 mm, with males being slightly smaller than females. The body is generally dark with a metallic greenish sheen, while the legs show variable patterns of dark and light bands. 

The adult female wasps attack second and third instar host larvae within mined leaves. Each host larva may be penetrated by the wasp's ovipositor (egg-laying apparatus) from one to several times, during which a toxin is injected that paralyzes the host larva and prevents it from feeding. The adult wasp may either feed directly on the fluids seeping from the wounded host (a process termed 'host feeding'), or lay 1–3 eggs beside it. Both processes eventually result in the death of the host larva. 

On hatching from the egg, the wasp larvae (which lack both legs and a distinct head) feed externally on the paralyzed host within the mine, undergoing 3 larval instars and becoming blue-green in color as they mature. Shortly before pupating, the wasp larvae move a short distance from the dead host and construct six dark-colored 'pillars' of frass (fecal matter) between the upper & lower surface of the mine; these dark spots can be seen through the leaf tissue surrounding the pupa, which is initially bluish green, becoming darker just prior to adult emergence.

Diglyphus isaea seems to tolerate a relatively wide range of humidities, but performs best at warmer temperatures; fastest development occurs between 30–35 °C (86–95 °F). At the latter temperature, the life-cycle (egg to adult) can be completed in 6–7 days (13–15 days at 20 °C/68 °F). 

The adult wasps live approximately 10 days at 25 °C (77 °F) and 30 days at 20 °C (68 °F), with each female producing a total of approximately 200 eggs. Temperatures at or below 10 °C (50 °F) or above 40 °C (104 °F) do not support development. 

When scouting for evidence of this insect, look for short mines where paralysis of the host has prevented further damage; within the mine, wasp eggs or larvae may be observed next to parasitized hosts, whereas those killed by host-feeding may show multiple dark puncture wounds. Pupating wasp larvae can be detected by looking for the surrounding dark illars/spots of fecal matter. 

Note that adult wasps can be caught on yellow sticky traps, so it may be necessary to temporarily remove such traps around leaf-miner 'hot-spots' until control is achieved.