Good spray coverage and timing prove central to bio-insecticide performance

Aphid control is less than that achieved with a synthetic insecticide, but often good enough to warrant a place in the programme, writes David Townley, Agrii vegetable agronomist.

In our trials, biological insecticides have given at best 60% control of aphids in field vegetables. This is less than what growers have come to expect from synthetic insecticides, but with few alternatives and the need to retain the best of these products for periods when they can contribute the most to protection, bio-insecticides have a place in a carefully devised programme.

Bio-insecticides are different in every imaginable way. The term covers a diverse range of active substances and modes of action. From naturally occurring plant extracts to entomophagus fungi, bacteria or virus, to materials which irritate the pest or render it immobile.

The control is typically 60% of that achieved with a conventional insecticide and is often far more variable. The contact mode of action of many products means spraying technique is important to success as is the time of day and conditions at application.

Nymphs (wingless aphids) often reside on the underside of the leaf or at the base of the petiole – parts of the plant that are difficult to reach with the vertical deposition delivered by a conventional boom sprayer – and are least active at the start and end of the day.

The advantage of these products is that many have a zero or otherwise unrestrictive harvest interval and are often exempt from residue limits: an important concern for growers of fresh produce.

Given the limited forms of conventional insecticide and the gaps in protection this creates, Agrii has investigated the potential of bio-insecticides to better understand their performance and where they fit in a programme. During 2021, we focussed on aphid control in cabbage, particularly that of the peach-potato aphid (Myzus persicae) given its significance.

Discussion

Many of the bio-insecticides that we have tested require thorough contact between the active substance and the pest for control. Achieving this may require the adoption of air-sleeve or drop-leg systems to improve spray distribution within the canopy and support efficacy.

The photographs below illustrate the dispersion of aphids on the cabbage plant. While the aphids on the upper surface of the leaves are relatively easy to target with spray droplets from a horizonal boom sprayer, the aphids present on the underside of the leaf present more of a challenge.

The practical challenges of application make getting the best from bio-insecticides difficult, but there are situations where their use may be preferable to that of a conventional product.

Young plants, such as the relatively small cabbages in these trials or the seedling stages of crops such as carrots and parsnips, or drilled salad crops, or other cropping situations where crop architecture is relatively open will allow the spray to penetrate the canopy. This should improve the control that can be expected.

• The results of this and other bio-insecticide trials will be shared with growers later or can be requested from your Agrii agronomist.