|Controlling Insect Pests of Stored Products Using Insect Growth Regulators and Insecticides of Microbial Origin (NRI, 1994)|
|Section 4: Microbial control|
Several groups of Protozoa are of interest as agents for the natural control of insect pests.
Adelina tribolii infects several stored product pests and causes epizootics in laboratory and natural populations of Tribolium confusum (Brooks, 1988).
Eugregarines are frequently encountered as commensals in the digestive tract of insects. Of those thought to be potentially pathogenic to their hosts, Ascogregarina spp. have received the most attention (Brooks, 1988). A. bostrichidorum has been isolated from Prostephanus truncatus collected in Tanzania. Examination of the gut of heavily infested larvae revealed that they contained masses of cysts and spores. However, the prevalence of infected larvae in the sample of 2 500 insects was only 2%. (Purrini and Keil, 1989).
Neogregarines occur naturalIy in Lepidoptera, Coleoptera and Orthoptera. Some are highly pathogenic and have been considered as potential control agents against species belonging to these three orders.
Farinocystis tribolii is a parasite of Tribolium destructor, T. molitor, T. castaneum and T. confusum. Farinocystis spp. have also been isolated from Prostephanus truncatus (Schulz and Laborius, 1987). An infection can be spread by the dispersal of spores from dead larvae during the handling or processing of an infested commodity, or by adults feeding on the bodies of the dead larvae. F. tribolii infection results in a slow decline of Tribolium spp. in laboratory cultures. It has also been shown to increase significantly the susceptibility of T. castaneum larvae to the insecticides malathion, chlorpyrifos-methyl, fenvalerate and cypermethrin (Rabindra et al., 1988).
Mattesia trogodermae has been isolated from Trogoderma granarium. This protozoan is cosmopolitan, occurring as a common pathogen of laboratory and natural colonies of Trogoderma spp. (Brooks, 1988). Baits have been used to introduce M. trogodermae into populations of T. glabrum. It has been shown that males surface-contaminated with spores will inoculate females while mating. The efficiency of spore transfer can be increased by releasing a natural female sex pheromone at the site where the males become contaminated.
M. trogodermae is regarded as a potentially useful control agent specific to Trogoderma spp. It is non-pathogenic to vertebrates, and it is relatively easy to produce and isolate in usable quantities (Henry, 1981). Extensive acute oral or acute inhalation tests on M. trogodermae showed no evidence of infection or pathological effects in rats. Tests on non-target species were also negative.
Mattesia spp. have also been detected in Prostephanus truncatus collected from farm-stored maize in Togo. Studies on the distribution and infection rate, which was 1-6% in Togo, indicated that artificial enhancement of the infection source would be necessary for effective control by Mattesia (Leliveldt et al., 1988).
Nosema spp. have been isolated in Prostephanus truncatus (Schulz and Laborius, 1987). Most published articles however, relate to the use of N. whitei in Tribolium spp.
Laboratory efficacy experiments
Al-Hafidh (1985) investigated the toxicity of N. whitei in first instar larvae of T. castaneum and found the LD50 to be 2.41 million spores/g. It also reduced fecundity and fertility and increased adult mortality. Further investigations into the effects of N. whitei on the physiology and behaviour of T. castaneum confirmed the observed reduction in fecundity and fertility in infected insects (Armstrong and Newton, 1985; Armstrong and Bass, 1986; Khan and Selman, 1988).
Onstad and Maddox (1990) created a simulation model of a T. confusum population infected with N. whitei. The model indicated that the infection could suppress the population to less than 10% of the original number in 300 days. Validation trials (over 60 days) showed that the predicted adult population was correct, but other developmental stages were only predicted accurately for the first 30 days of the 60-day trial.
Several pathogenic protozoan species have been isolated and identified from insect pests of durable foodstuffs. However, their use as control agents is in the early stages of development and requires extensive research before they can be recommended for use as grain protectants.