Target pest: silverleaf whitefly (Bemisia tabaci biotype b)
This damaging pest has moved rapidly around the world. It feeds on a very wide range of crops including cotton, vegetables and legumes. Vegetable hosts include melon, zucchini, tomato, lettuce and eggplant. Silverleaf whitefly can downgrade fruit and vegetables by the production of a sticky secretion called honeydew which, in turn, leads to sooty mould. In large numbers it will also cause a reduction in yield and quality. Importantly this pest is known to transmit several virus diseases that can be debilitating to crops.
Beneficial insects, especially tiny wasps, prefer a protected environment. A crop that has good foliage development, is well irrigated and free from pesticide residue and dust is best.
Recommended release rates
- Field crops: Make 5 releases of 3,000 wasps (10 vials) per hectare. The first releases should commence early in the crop or at the fist sign of silverleaf whitefly. Release at intervals of 2-3 weeks depending on the crop cycle, insect pressure and prior history. Further releases may be required following adverse conditions or during periods of intense whitefly activity.
- Greenhouses: Make at least 4 releases of 2-3 wasps per square metre at intervals of 7 days. Aim to start releases at the very first sign of whitefly activity. If whitefly is already active in the crop higher release rates may be required. Continue making releases until control is achieved or parasitism rates reach 80%.
A note on release rates: Unlike chemicals which generally exhibit a clearly defined dose response curve, with beneficial insects, more is always better. However, they are costly to produce and the goal should be to achieve the best results at minimal cost. We are constantly trying to strike a balance between cost and efficacy. There are many factors that should be considered including the value of the crop, the magnitude of the pest population and the activity (or otherwise) of naturally occurring beneficial species. Also unlike chemicals, where it is common to respond to pest populations that have already exceeded some ‘economic threshold’, we recommend establishing beneficials early in the life of the crop before pest populations reach threatening levels. In most cases our releases are inoculative and we anticipate that our beneficials will establish and breed up within the crop to give long term control. As a general principle, 2-3 releases of modest numbers is better than a single large release – this reduces risk, improves establishment and accelerates the development of multiple overlapping generations of the beneficial species.
Releasing the parasites
Eretmocerus hayati are supplied in small plastic vials with a cotton plug insert. Wait until several wasps are visible inside the vials before releasing into the crop. They can be placed in the foliage by locating the vial cap in a leaf petiole, a tie wire or similar. Remove the cotton plug to allow the wasps to emerge.
Place the vials uniformly throughout the crop at a rate of 10 vials per hectare.
Watch for ant activity as aggressive ant populations can interfere with wasp establishment. Ants may need to be controlled if this is the case.
It is not easy to find the adult wasp parasites after release. However with experience it is possible to assess the levels of parasitism by examining the whitefly nymphs (immature stages) with a handlens or microscope. The video below demonstrates how to do this.
Area wide management
Silverleaf whitefly is a highly mobile pest that moves quickly through entire production areas. It is an excellent target for an area wide management approach. If growers across an entire production area can co-operate to release E. hayatistrategically, best results will be achieved. Current research is focussed on improving the control of this difficult pest with an area wide management approach.
Silverleaf whitefly has a history of rapidly developing resistance to pesticides. Pesticides may be useful tools but they should only be used as a last resort and with a clear understanding of their efficacy, impact on beneficial insects and the need to conserve them for the future. Overuse of pesticides is generally counter-productive especially with difficult pests such as silverleaf whitefly. Hayati are very effective parasites but they are delicate organisms and are easily harmed by pesticides. Nutritional sprays, copper fungicides and some miticides are relatively safe. No synthetic pyrethroids should ever be used. Organophosphate and some carbamate insecticides are toxic and must also be avoided if at all possible. New generation pesticides are increasingly available however they are also prone to resistance development. Therefore should only be applied with caution to conserve them for the future. An IPM consultant should be able to give you advice on compatible materials.