False codling moths damage table grapes and many other crops. Chemical crop protectants are important to false codling moth control but environmentally friendly alternatives must be identified.
In the laboratory
This project focused on locally occurring entomopathogens because these are adapted to South African conditions. Local entomopathogens are more likely than exotics to survive and thrive in our vineyards.
The team screened the Stellenbosch University entomopathogen collection to identify the most effective fungi and nematodes. The researchers placed larvae and pupae of false codling moths in bioassay trays and inoculated them with different entomopathogenic fungi and nematodes. Mortalities in the treatments were compared to untreated controls.
The researchers also gathered data on the larval and pupal mortality at different entomopathogen doses. Their general conclusion was that pupae of false codling moths are more resistant than larvae to entomopathogens even when the pupae are exposed to higher numbers of entomopathogens.
Entomopathogenic fungi
The highest mortality – 90% – occurred in the larvae exposed to Metarhizium pinghaense. M. anisopliae came second with 71%, and M. robertsii third with 64%. Significant numbers – 24% – of the untreated larvae also died. These trials should be repeated to reduce the control-group mortality.
Over 80% of the pupae exposed to most Metarhizium strains or Beauveria bassiana died within 14 days. The largest number died in the group treated with M. pinghaense. The mortality in the untreated pupae was below 7% after seven days but increased to 48 – 75% after 14 days.
Entomopathogenic nematodes
Heterorhabditis noenieputensis killed 100% of the larvae, making it the most effective of the nematodes tested. The green variant of H. zealandica took second place with a 97% mortality rate, and Steinernema yirgalemense came third with 93%. Only 1% of the untreated larvae died.
Heterorhabditis baujardi was the most effective against pupae, killing 28%. H. noenieputensis took second place at 26%, and H. indica came third at 23%. None of the untreated pupae died.
The researchers were puzzled by the initial poor performance of the nematode S. jeffreyense in killing only 54% of larvae. They modified their culture method and retested with better results – 98% of larvae died. It seemed that the trick was to starve the nematodes before setting them loose.
In the orchard
The team tested the fungus Metarhizium brunneum and the nematode Steinernema yirgalemense separately and in combination in late summer, in an apricot orchard near Ceres.
The researchers made cages by glueing mesh to the open sides of sections of PVC pipe. Larvae of false codling moths were placed in orchard soil in the cages and buried about two centimetres deep in the orchard. Treatments were watered in at dawn, and the cages were retrieved the following day.
Mortality rates were assessed in the laboratory for the next week and turned out to be disappointing. Malan ascribes this to cold soils – average soil temperatures were below 15 °C. Low temperatures retard fungal growth and inhibit the bacteria on which entomopathogenic nematodes depend for killing their hosts.
Subsequent incubation at 25 °C increased larval deaths in the nematode but not the fungal treatments.
The research team further explored the effect of temperature by treating groups of false codling moth larvae with nematodes at 14, 20, and 26 °C and comparing their mortality to control groups at the same temperatures.
They found significant larval deaths within 48 hours at 20 and 26 °C, and most of these larvae were dead by day six. Very few of the larvae held at 14 °C died within one week.
Survivors from all groups were subsequently moved to 25 °C. The mortality in the 14 °C groups increased to 33 – 70% within 48 hours. Fewer than 12% of the controls died, regardless of temperature.
This highlights the importance of considering environmental conditions when applying biocontrol agents. Entomopathogens are living organisms, and – like all living organisms – they function best within specific temperature and moisture ranges.
Mass culture
Larger-scale trials and commercialisation of entomopathogens require mass production. One aspect of this study was developing techniques for generating significant quantities of M. pinghaense spores. M. pinghaense was chosen because it was the most promising entomopathogenic fungus in the initial laboratory screening trials.
Previous work by Dr Letodi Mathulwe found that this fungus grows and sporulates well on parboiled rice and yeast extract. The current study tested rice, wheat, barley, and four different types of yeast extract.
Fungi grown on liquid brewer’s yeast and barley produced the highest spore numbers – the researchers harvested about 9 x 1011 spores per kilogram of substrate. To put this in context, the spores from one kilogram of substrate could treat an area of about 90 m2 at the application rate used in the field trial or kill about 80 000 larvae at the application rate used in the laboratory screening trials.
Unfortunately, fewer than 15% of the spores grew when tested for viability on artificial media. One of the challenges when dealing with biocontrol agents is that the organisms must be alive to infect pests, but many variables during production, formulation, storage, transport, and application can affect viability and efficacy.
What comes next?
In addition to entomopathogenic fungi and nematodes, parasitoid wasps can help control false codling moths. The team conducted a Western Cape survey for parasitoids but found none. This work continues in a project based at the University of Cape Town, in which Addison remains involved – it recently revealed at least one parasitoid that may benefit our growers.
The commercialisation of entomopathogenic fungi and nematodes is no different to that of conventional treatments in requiring substantial investments from crop-protection companies. Pressure on synthetic chemicals is shifting the focus of these companies to biologicals, so growers can expect to see more options becoming available.
Biologicals containing some of the species tested in this project – the fungi Beauveria bassiana and M. anisopliae and the nematode H. bacteriophora – are already available in SA. For the best results, growers should seek assurances that products contain the number of organisms claimed on the label and that these are alive.
Larger-scale orchard trials are needed to evaluate commercially available products and locally isolated entomopathogens, and to develop formulation and application strategies for the latter. The problem is finding experimental sites with supportive management and sufficient pests.
Biocontrol agents are unlikely to replace synthetic crop protectants entirely, but research shows that they can play a role in integrated pest management and help growers meet the evermore stringent demands of their markets.
