SA Graan September 2015

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Integrated pest control Special
September 2015
42
Integration of insect-pathogenic
fungi with chemical insecticides
NOKULUNGA MZIMELA and JUSTIN HATTING, ARC-Small Grain Institute, Bethlehem
Awareness of environmental damage caused by chemical
insecticides, several cases of insecticide resistance being
reported annually against different groups of insecticides
and strict regulations associated with chemical residues on
harvested commodities have prompted a renewed interest in the de-
velopment of more environmentally-sound pest control strategies.
Insects are prone to infection by fungi, known as insect-pathogenic
fungi, and the latter has been assessed as insecticides for more
than 100 years. However, only a few products have reached the
market, regardless of their potential. Biological control of cereal
aphids, such as the Russian wheat aphid ( Diuraphis noxia), oat aphid
(Rhopalosiphum padi) and rose grain aphid (Sitobion avenae), using
insect-pathogenic fungi, holds much potential as these insects are
sap-feeders and fungi (as opposed to viruses and bacteria) do not
need to be ingested to cause infection.
The fungal spore, or conidium, which makes contact with the insect
cuticle, will germinate under favourable conditions and infect the
host by penetrating through the cuticle. Once inside the host, the
fungus can produce toxins and/or destroy internal organs leading to
death of the host, usually within four to five days.
After death, the fungus may emerge from the insect cadaver and
produce new spores on the surface of the insect, thereby increasing
the chances of other healthy aphids becoming infected (Photo 1). A
more detailed overview (in Afrikaans) of insect-pathogenic fungi was
published in SA Graan/Grain, September 2014, Vol 16(9). There have
been no reports of resistance development against insect-pathogen-
ic fungi thus far. These fungi are non-lethal to humans and animals
and leave no unwanted residues on the harvested commodity.
Constraints in implementing insect-pathogenic fungi as a compo-
nent of an integrated pest management (IPM) programme relate to a
relatively slow killing-rate and unpredictable results under field con-
ditions. Aphids also have the ability to rapidly increase in numbers,
thereby “escaping” the disease and successfully establishing pock-
ets of healthy aphids leading to new outbreaks.
Research at the ARC-Small Grain Institute (ARC-SGI) is now focus-
sing on the combined use of insect-pathogenic fungi with sub-lethal
doses of insecticide. Reduced doses of the chemical insecticide act
only as stress factor to render the insect more susceptible to the dis-
ease. Without chemically-induced mortality, selection for resistant
aphids is minimal.
Moreover, in the presence of the fungus, these stressed aphids are
killed more easily, while the diseased insects lose their ability to
withstand even the lower chemical dose they are exposed to. To-
gether, synergism between the chemical and fungus is noted, with
detrimental consequences for the pest. Other stress-related effects
that can be observed following exposure to reduced doses of insec-
ticides include reduced fecundity, i.e. inhibited reproduction, the in-
ability to mate, reduction in body size, and/or antifeedant behaviour.
Research is currently being conducted to test the compatibility of
insect-pathogenic fungi with various contact and systemic insec-
ticides registered against cereal aphids in South Africa. Although
compatibility has been noted, some chemicals have a deleterious ef-
fect on the fungus, inhibiting germination and/or vegetative growth.
On the other hand, some chemicals may actually stimulate fungal
sporulation and general development, thus enhancing overall fungal
performance. Ultimately, effective combinations require an under-
standing of the interactions between the fungal and chemical agents
involved. The fungus Beauveria bassiana has a wide host range and
has been studied extensively as a microbiological control agent
against numerous insect pests.
The ARC-SGI curates >400 indigenous strains of this fungus, ena-
bling wide-scale screening to identify suitable candidates for fur-
ther development. The chemical actives, pirimicarb, dimethoate
and acetamiprid, were found to be compatible with a strain of
B. bassiana and will be used in field trials during the 2015/2016
season. Although immediate “tankmixtures” of the insect-pathogenic
fungi and (sub-lethal) chemical insecticide is the approach that is
now being explored, alternate or staggered applications also hold
potential. If, initially, a full-dose systemic insecticide is applied,
follow-up applications of the insect-pathogenic fungus (only) may
be directed at later aphid populations challenged by the weakened
or “diluted” systemic effect of the chemical.
Such populations would be expected to be only partially affected by
the chemical, now acting in a sub-lethal manner. Again, these pop-
ulations would be stressed by the chemical, rendering them more
susceptible to the fungus. The importance of aphid control within
the small grain industry is underscored by the periodic occurrence
of new resistance-breaking Russian wheat aphid biotypes (four
such biotypes are currently recognised in South Africa; RWASA1 -
RWASA4), as well as the growing problem of aphid-transmitted bar-
ley yellow dwarf virus especially by the oat aphid.
Chemicals continue to play a pivotal role in aphid management, but
the environmental footprint needs to be better managed. Combined
use thereof with insect-pathogenic fungi is a sensible strategy, war-
ranting further research.
This work also forms part of an MSc study (University of Kwa-
Zulu-Natal) by Nokulunga (Lungi) Mzimela and the latest re-
search findings will be published in the popular media from
time to time. For further information, please con-
tact Lungi Mzimela at or
058 307 3442 or Dr Justin Hatting at HattingJ@-
arc.agric.za or 058 307 3468.
Russian wheat aphid
infected with an ento-
mopathogenic fungus.