Can adjuvants improve mealy bug biocontrol?

Why use adjuvants?

Previous research led by Dr Antoinette Malan of Stellenbosch University (SU) found that adjuvants boosted the efficacy of EPNs against vine mealy bugs. Adjuvants are compounds added to pesticides, herbicides, and plant growth regulators to facilitate their application or enhance their activity.

The mechanism by which adjuvants increase the efficacy of pesticides, herbicides, and plant growth regulators varies for different adjuvants:

• Wetters and spreaders reduce droplet surface tension, allowing them to adhere to and spread on the target surface.
• Stickers improve droplet adhesion.
• Penetrants improve the uptake of active ingredients.
• Extenders keep active ingredients effective for longer.
• Humectants slow drying.

In addition to the humectants and sticker-spreaders tested by Malan and co-workers, the current project includes three others: a sticker-spreader and two organosilicone-based products. Organosilicones act as wetters, spreaders, and penetrants.

“The primary purpose of the adjuvants in our trials is to give the EPNs time to infect the mealy bugs,” says Carelse. “When you apply the EPNs, not all of them will land directly on a mealy bug. The adjuvants can allow them to make their way to the mealy bugs without drying out.”

“The primary purpose of the adjuvants in our trials is to give the EPNs time to infect the mealy bugs.”

EPNs versus mealy bugs

Before adding adjuvants, the project team tested four EPN species (Heterorhabditis bacteriophora, H. zealandica, Steinernema jeffreyense, and S. yirgalemense) obtained from SU against vine mealy bugs in the laboratory.

The researchers exposed individual adult female mealy bugs to suspensions of juvenile EPNs in water and checked for mealy bug mortality 24 and 48 hours later. Each EPN species was tested on 60 mealy bugs.

Juvenile EPNs kill insects by penetrating their bodies and releasing bacteria that liquefy the host tissues, usually resulting in death within 48 hours. The nematodes and bacteria feed on and multiply in the resulting soup. Eventually, the juice runs out, and a new generation of infective juveniles swarms from the cadaver, in search of fresh victims.

To confirm that EPNs caused mortalities in their trials, the researchers conducted post-mortem examinations under a microscope. They scrutinised each dead insect for nematodes in its body cavity. If none were present, the mealy bug died from other causes.

Of the four EPNs tested, three gave good results. The average mortality of the mealy bug females was approximately 82% for S. yirgalemense, 65% for H. zealandica, 62% for H. bacteriophora, and 10% for S. jeffreyense.

Mortality in the control mealy bugs, which were exposed to water without EPNs, was about 2%.

The effect of adjuvants

As S. yirgalemense killed the most mealy bugs, it was selected for the next round of laboratory tests, in which the researchers compared juvenile nematodes suspended in one of five adjuvant formulations with nematodes in water.

Each treatment was applied to 12 grape-vine leaves, each of which held five adult female mealy bugs. The control mealy bugs were exposed to water (without adjuvants or EPNs).

The researchers removed the mealy bugs after 48 hours, keeping each individual isolated for up to 96 hours to monitor mortality. As before, they conducted mealy bug post-mortems to confirm the cause of death.

None of the control mealy bugs died. Mortality in the mealy bugs exposed to EPNs suspended in water was approximately 27%. In contrast, the adjuvants were associated with significantly higher mortality:

1. EPNs plus a combination of a humectant and a sticker-spreader: ± 75% mortality
2. EPNs plus either of two sticker-spreaders: ± 95% mortality
3. EPNs plus either of two organosilicone products: 100% mortality.

Although all the mealy bugs exposed to EPNs formulated with organosilicone adjuvants died, the insects had no EPNs in their bodies, indicating that the adjuvants, rather than the EPNs, caused the deaths.

The insecticidal properties of organosilicones have been demonstrated against several pests, including mealy bugs. For example, Dr Christopher Tipping and colleagues at the University of California, Davis, reported that an organosilicone surfactant could cause 100% mortality in mealy bug (Pseudococcus maritimus) crawlers.

The team also observed high mortality (above 90%) in immature and adult Western flower thrips (Frankliniella occidentalis) and Pacific spider mites (Tetranychus pacificus) exposed to organosilicones.

Next steps

Based on the laboratory results, the next phase of the project is field trials comparing S. yirgalemense suspended in water with a sticker-spreader formulation and an untreated control. The plan is to apply treatments to selected vines in a commercial vineyard with a natural mealy bug infestation.

“I’m excited about the fieldwork, to see whether we get similar results as in the lab,” says Carelse. “We want to move toward more sustainable and biological pest control and be less reliant on chemicals.”