Ant receiving aphid honeydew
Photo: Dawidl/ Wikimedia Commons
By Gary Hartley

How insects’ sweet excretions could be used to benefit growers

Managing honeydew, the sugary excretions of some plant-feeding insects, should be considered as part of integrated pest management strategies, according to the authors of a study examining its role in growing environments.

Honeydew is mainly produced by species from the order Hemiptera or ‘true bugs’, but also some caterpillars. It has traditionally been viewed only as a ‘problem’ by growers, but with more understanding of its multifaceted role, there needs to be a re-evaluation of how it can fit into successful crop protection, wrote Maite Fernández de Bobadilla from Instituto Valenciano de Investigaciones Agrarias in the journal Current Opinion in Insect Science.

Attracting friends, stopping ants

Her team suggested five ways this could be achieved. Firstly, non-pest insects which produce honeydew should be actively encouraged around growing environments, as the sweet substance provides a carbohydrate source for natural enemies of economically important pests. This could be done using flower strips and appropriate cover crops.

The second task is to break the association between ants and honeydew-producing pests, she said. Ants often protect such species, posing a problem for natural methods of pest control.  Sugar baits have been successfully used to attract ants away from their pest ‘farms’ in a number of crops, she noted, but the presence of flowering plants and non-pest honeydew producers near crops has yet to be fully tested.

Tapping honeydew’s chemical cocktail

The use of airborne chemicals from honeydew could also help with pest control, by attracting potential biological controls and putting off pests which could be ‘fooled’ into perceiving a food source is already over-populated.

“Identifying which part of the volatile blend attracts biological control agents and repels pests is the first step to develop artificial infochemical blends to promote pest suppression in our crops,” she explained.

Tapping components in honeydew which prompt plant defences is another possible avenue to explore, albeit one where knowledge is limited to date. A number of studies have demonstrated plant responses to the presence of honeydew, explained Fernández de Bobadilla, though the exact chemical signals have yet to be unravelled. Doing so could potentially enable crops to be ‘vaccinated’ and made more resistant to attack.

Risks to manage — and pesticide warning

Despite all these potential advances, there are still risks to be managed. Primary among them is limiting sooty mould growth on honeydew, as it can affect plant growth and crop quality. Then, there are some pests such as fruit and root flies and feed on honeydew as adults, meaning that the risks and benefits of using honeydew need to be assessed. Excessive honeydew can also hamper the movement of natural enemies, which means that honeydew-producers need to be kept below certain thresholds.

“Finally, recent studies show that systemic insecticides are transferred to honeydew excreted by hemipterans that feed on plants treated with these insecticides. This honeydew is then toxic to natural enemies of herbivores,” added Fernández de Bobadilla.

“Therefore, we strongly suggest using insecticides only as a last option within a pest management program.”

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