The two final summaries examine what happens when biocontrol agents are exposed to real-world conditions.
A local biological agent offers potential as an exciting new tool to control woolly apple aphids
Woolly apple aphids are important apple pests globally and key pests of apple production in SA. They have developed resistance against several chemical insecticides and showed natural resistance to trialled entomopathogenic nematodes. Fortunately, entomopathogenic fungi hold promise as biological control agents for these insect pests.
A survey of entomopathogenic fungi was conducted in Western Cape apple orchards. Insect larvae were used to bait entomopathogenic fungi from soil samples. The susceptibility of woolly apple aphids to the isolated entomopathogenic fungi was assessed under laboratory conditions.
The soil samples yielded six entomopathogenic fungi. Laboratory bioassays showed that local isolates of Metarhizium were the most effective when tested against woolly apple aphids. On average, M. brunneum, M. pinghaense, and M. robertsii killed more than 88% of the exposed aphids within five days.
The results provide insight into the diversity of entomopathogenic fungi across apple orchards in the Western Cape, and show that the Metarhizium isolates are potential biological control agents of woolly apple aphids.
Adapted from: Mathulwe LL, Malan AP and Stokwe NF. 2023. The occurrence of entomopathogenic fungi in apple orchards and their biocontrol potential against Eriosoma lanigerum. African Entomology 31: 1-9.
Can local entomopathogenic fungi and nematodes control mealy bugs?
Eight local fungi and six local nematodes were screened for their effect on obscure mealy bugs (Pseudococcus viburni). Two commercial isolates of the fungus Beauveria bassiana were also evaluated. The nematodes were all multiplied in insect hosts, and two were also mass-cultured in an artificial medium.
Trials were done in the laboratory. Of the entomopathogenic nematodes tested, Hetero-rhabditis indica was the most effective, killing, on average, more than 70% of the mealy bugs.
Of the fungi, Metarhizium pinghaense and M. robertsii were the most effective, killing 91% and 96% of the mealy bugs, respectively. This was the first time entomopathogenic fungi were tested as biocontrol agents for obscure mealy bugs.
The laboratory success of H. indica, M. pinghaense and M. robertsii as biocontrol agents for the obscure mealy bug should be followed up with field trials. Doing so could lead to successfully incorporating these biocontrol agents in an integrated pest management system for mealy bugs.
Adapted from: Mathulwe LL, Malan AP and Stokwe NF. 2022. Laboratory screening of entomopathogenic fungi and nematodes for pathogenicity against the obscure mealy bug, Pseudococcus viburni (Hemiptera: Pseudococcidae). Biocontrol Science and Technology 32(4): 397-417.
Can combining local entomopathogenic nematodes and fungi enhance the control of obscure mealy bugs?
The combined application of entomopathogenic nematodes and fungi against the obscure mealy bug was evaluated in laboratory trials. Three combinations of entomopathogenic nematodes and fungi were applied simultaneously or sequentially to mealy bugs at different intervals.
Combining the nematode, Heterorhabditis indica, with either of the two fungi, Metarhizium pinghaense or M. robertsii, resulted in higher mealy-bug mortalities than when the biocontrol agents were applied individually.
Simultaneous application of H. indica and M. pinghaense killed 95% of the mealy bugs after five days, primarily due to nematode infections. After five days, the simultaneous application of H. indica and M. robertsii killed 85% of the mealy bugs.
Sequential applications of biocontrol agents were more effective than using the agents individually. When the fungi were applied 48 hours before the nematodes, the combination of M. robertsii and H. indica killed 85%, and the combination of M. pinghaense and H. indica killed 78% of the mealy bugs on average.
When the nematodes were applied 24 hours before the fungi, both combinations killed 88% of the mealy bugs on average.
By comparison, the application of only H. indica resulted in 76% mortality of the mealy bugs after two days, while the application of only M. pinghaense or only M. robertsii resulted in 74% and 83% mortality, respectively, after seven days.
H. indica and the two fungal isolates were shown to have an additive effect when used simultaneously and sequentially. Combining entomopathogenic nematodes and fungi could be an alternative strategy for managing the obscure mealy bug.
Adapted from: Mathulwe LL, Malan AP and Stokwe NF. 2023. Combined effect of an entomopathogenic nematode, Heterorhabditis indica, with entomopathogenic fungi, Metarhizium pinghaense and M. robertsii, against the obscure mealy bug, Pseudococcus viburni. International Journal of Pest Management 2023:1-11.
The susceptibility of weevils to entomopathogenic nematodes and fungi
Two weevils in the genus Phlyctinus are pests of economic importance to the South African deciduous-fruit industry. The researchers tested two strains of the entomopathogenic fungus Beauveria bassiana and two species of entomopathogenic nematodes, Heterorhabditis indica and Steinernema yirgalemense, against two Phlyctinus species, P. callosus and P. xerophilus.
Adult weevils were exposed to the ento-mopathogens in the laboratory. The nematode H. indica killed 23 – 73% of the weevils, while the nematode S. yirgalemense killed 23 – 80%. Both weevil species appeared equally susceptible to the nematodes.
In contrast, P. callosus was more susceptible to entomopathogenic fungi than
P. xerophilus. Both Beauveria strains killed 18 – 83% of P. callosus and 8 – 54% of
P. xerophilus.
Adult weevil mortalities due to the entomopathogens were low overall, and the results obtained with different bioassay batches were variable.
A second study examined the susceptibi-lity of weevil larvae and pupae to the entomopathogenic nematode S. yirgalemense. Trials were again done with two Phlyctinus species. One set of trials was performed in the laboratory, while a second set involved the exposure of caged larvae buried in an orchard – weevil larvae are soil-dwellers.
In laboratory trials, the larvae and pupae of both weevil species appeared equally susceptible to S. yirgalemense. The nematodes killed 49 – 78% of the weevil larvae and 36 – 100% of the pupae by 96 hours after exposure.
Exposure to S. yirgalemense in the orchard killed about 48% of P. xerophilus and 25% of P. callosus larvae.
Adapted from: Hansen SP, Malan AP, Haran JM and Addison P. Susceptibility of adult Phlyctinus (Coleoptera: Curculionidae) to entomopathogens: A first look at potential differences in a newly revised species complex. (In preparation.)
Adapted from: Hansen SP, Malan AP, Haran JM, Dunn MD and Addison P. Steinernema yirgalemense (Steinernematidae) to control Phlyctinus callosus and Phlyctinus xerophilus (Curculionidae). (In preparation.)
Implementation of biological control options against false codling moth in laboratory and field trials
False codling moths are key pests of stone and other fruit in SA. The effect of different entomopathogenic nematodes and fungi on larvae and pupae of false codling moths was assessed.
Of the nematodes, Heterorhabditis noenieputensis performed the best against larvae, causing 100% mortality. H. zealandica killed 97%, and Steinernema yirgalemense 93% of the larvae.
Against pupae, H. baujardi performed the best, causing 28% mortality. H. noenieputensis killed 26% and H. indica 23% of pupae.
Metarhizium pinghaense performed the best of the fungi against larvae, causing 90% mortality. M. anisopliae killed 71%, and M. robertsii 64% of the larvae.
Against pupae, M. pinghaense consistently performed the best, killing more than 97% by 14 days after exposure.
Field trials investigated the effect of S. yirgalemense and Metarhizium brunneum on caged larvae of false codling moths that were buried in an orchard – the larvae would typically drop from the fruit into the soil to pupate.
M. brunneum did not cause significant mortality, but S. yirgalemense killed 77% of the larvae. A combination of M. brunneum and a smaller number of S. yirgalemense killed 72% of the larvae.
The researchers also investigated techniques for the mass production of M. pinghaense spores.
Adapted from: Addison P. 2023. Final report for Hortgro.
South African entomopathogenic fungi can easily be mass-cultured for field application against insect pests of deciduous fruit
Two South African isolates of entomopathogenic fungi, Metarhizium pinghaense and M. robertsii, were found to be promising biological control agents of economically important insect pests of fruit crops under laboratory conditions. This study focused on developing a method for the mass production of these isolates.
The results showed that fungal spores can be mass-produced by growing the fungi on agricultural grains, specifically rice. The rice can be inoculated by fungal spores called blastospores. This discovery will facilitate the large-scale production of fungal spores for commercial use.
A training video of the culture technique is available from Dr Letodi Mathulwe at mailto:MathulweL@arc.agric.za.
Adapted from: Mathulwe LL, Malan AP and Stokwe NF. 2022. Mass production of entomopathogenic fungi, Metarhizium robertsii and Metarhizium pinghaense, for commercial application against insect pests. Journal of Visualized Experiments 181: e63246.
Can mass-cultured, stored, formulated spores of South African Metarhizium effectively control the obscure mealy bug?
Entomopathogenic fungal isolates of Metarhizium pinghaense and M. robertsii, isolated from Western Cape apple orchards, can effectively control obscure mealy bugs. Although techniques of mass production of the infective spores of these local isolates were developed recently, formulations to maintain the spore viability and pathogenicity have yet to be evolved.
This study explored the potential of four different oils (liquid paraffin, coconut, canola, and olive) and diatomaceous earth as carriers of the mass-produced spores of the two fungal isolates. The goal is to prolong spore survival during storage, which is necessary to develop a biocontrol product that can be used in the field.
Spore viability and pathogenicity were assessed over eight weeks. Spore survival in the diatomaceous earth formulation was significantly worse than dry spores or spores in oil after eight weeks.
In oil, more than 95% of the spores of both fungi remained viable. The spores in all the oil formulations could also kill 60 – 90% of mealy bugs when tested after eight weeks.
Adapted from: Mathulwe LL, Malan AP and Stokwe NF. 2023. Formulation of Metarhizium pinghaense and Metarhizium robertsii and the infection potential of the formulations against Pseudococcus viburni (Hemiptera: Pseudococcidae) after storage. African Entomology 31:1-7.
The ability of fungal spores sprayed on apple bark to infect woolly apple aphids and codling moths long after application
Entomopathogenic fungi are soilborne pathogens that infect various insects. The fungi Metarhizium brunneum and M. pinghaense have potential as biocontrol agents for pests of South African deciduous fruit. This study examined whether fungal spores remain effective on apple bark.
Pieces of apple bark were sprayed with spore suspensions of M. brunneum and M. pinghaense. Spores were recovered from the bark after one, two, and three weeks and applied to codling moth larvae and females of woolly apple aphids. Insect deaths were assessed every day for 10 days.
The results showed that M. pinghaense spores could remain viable for seven days after application, killing 39 – 82% of the woolly apple aphids throughout 10 days post-exposure. Mortalities of codling moth larvae were 3 – 68% during the seven days after exposure.
Further evaluation showed that up to 63% of the spores of M. pinghaense persisted on apple bark for three weeks after application, compared with only up to 11% of M. brunneum spores. The recovered spores were able to kill insect pests, but persistence declined.
Adapted from: Mathulwe LL, Malan AP and Stokwe NF. 2023. Infection of insects and persistence of Metarhizium (Hypocreales: Clavicipitaceae) species on apple bark. African Entomology 31:1-7.
The effect of visible and ultraviolet light on the pathogenicity of entomopathogenic fungi to false codling moth
Ultraviolet light influences the survival of fungal spores, so natural sunlight could undermine entomopathogenic fungal persistence and pathogenicity.
A study tested the ability of one Beauveria bassiana and five Metarhizium isolates to infect larvae of false codling moths after the fungi were exposed to different light treatments. Spore exposure occurred on either leaf or soil surfaces.
Spores were exposed to either ultraviolet or visible growth light, which emits the same visible light as the sun but excludes ultraviolet light, for 12 hours. The results indicated no adverse effect on the infection potential of the spores of most of the isolates.
The spores of the two M. pinghaense isolates showed the greatest tolerance to visible light and ultraviolet radiation exposure on both soil and leaf surfaces.
Further trials with mass-produced and formulated spores of the Metarhizium isolates should be conducted to test the persistence and efficacy of the isolates under field conditions.
Adapted from: Rossouw S, Mathulwe LL, Stokwe NF and Malan AP. 2023. Effect of visible light and ultraviolet light on the pathogenicity of entomopathogenic fungi to false codling moth, Thaumatotibia leucotreta (Lepidoptera: Tortricidae) larvae. African Entomology 31:1-8.
