Parasitic wasps

Trichogramma species

Trichogramma are minute parasitic wasps (less than1 mm in length) that attack the egg stage of their hosts (mainly different moth species). Adults of most Trichogramma species typically have red eyes and a yellowish to brown head and thorax, often with a somewhat darker abdomen. The female wasps lay one or more eggs inside the host moth egg, 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 stage, the host egg turns black, and a few days later, the adult wasps emerge from within,

Most Trichogramma species will attack the eggs of a relatively wide range of moth species, with host range being determined by factors such the size and thickness of the host eggs, and to some extent by the plant host and habitat. Movement of the adult wasps can be impaired an 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 temperatures below 20°C and at high temperatures combined with low humidity. Adults without access to food will live only a few days, but they 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 target moth; wherever possible, monitoring the latter with an appropriate pheromone trap is  therefore recommended. Trichagramma are not strong fliers: although some adults may disperse 14 m (46 ft) or more from their release site, dispersal is generally better if adjacent plants are touching (particularly at temperatures below 20°C, when flight is inhibited and dispersal is mainly through walking).

The species of Trichogramma available include T. minutum and T. platneri. These species are closely related and cannot be distinguished by their external appearance.
However, their native geographical ranges differ, and it is better to use the species native to the target region T. minutum is native to the eastern USA (i.e , east of the Rocky Mountains) while T. platneri is native 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 also attack the eggs of various other moths. T. platneri has been recorded from more than 33 host species in 8 different math 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). However, it will also attack the egg stage of various other moths.