An evaluation of protozoal characteristics in relation to biological control of pests
Authors: Canning Elizabeth U. 
Published: June 2009

Of the unicellular eukaryotes, formerly Protozoa, now considered to belong to five separate phyla, only the neogregarines and microsporidia are serious contenders for a role in biological control of invertebrate pests. Ciliates of the genus Lambornella, which penetrate their hosts via cuticular cysts, have potential in mosquito control but have not been investigated in depth.

‘Protozoa’ generally kill their hosts by overwhelming numbers, destroying the normal function of organs or depleting the host of essential reserves. Because they are slow-acting pathogens they cannot be used on their own when pests have already reached a high level of abundance nor can they be relied upon when the damage threshold of pests is low. Their principal use will be as the slow-acting component of a 2-pathogen or pathogen-plus-chemical formulation, which can be used when a degree of damage is tolerable.

A comparison is made between two microsporidia in lepidopteran hosts, Vairimorpha necatrix and Nosema pyrausta. The former causes high mortality in a wide range of hosts, when bacterial septicaemia ensues after disruption of the gut wall by the microsporidian invasion process. Some larvae may survive this period and live to damage crops, but none survives to adulthood. There is no transovarial transmission and the parasite is rarely found in natural populations. V. necatrix could be used as a microbial pesticide for short-term control. N. pyrausta is restricted to a single host, the European corn borer. It has low pathogenicity, causing some larval mortality especially under conditions of environmental stress. Most hosts survive to adults but show reduced longevity and fecundity. The parasite is transmitted transovarially and is highly prevalent in the field. It is not considered pathogenic enough to be used as a microbial pesticide but is an important factor in regulating natural populations. These examples illustrate the inverse relationship between pathogenicity and prevalence and show how cycles of host population abundance may be driven by pathogens of moderate to low pathogenicity.

Two kinds of transovarial transmission mechanisms are discussed. With the microsporidia of winter moth, vegetative stages and spores, even when abundant in egg yolk, do not gain access to larval tissues but are confined to the meconium in larvae just before eclosion. Larvae are not infected when they hatch but the spores are carried over in the eggs to the next period of larval feeding activity. In contrast, some genera of microsporidia in haematophagous diptera, e.g. Amblysopora in mosquitoes, actually infect the cells of developing larvae, which are already infected when they hatch.

The prospects for biological control with ‘Protozoa’ are reviewed for vectors of medical importance and for pests of pasture, field crops, forests and stored products. Particular attention is given to the use of microsporidia in combination with low concentrations of compatible chemical insecticides and with other pathogens (e.g. viruses).

Spores for field application can be produced in natural or experimental hosts by feeding or intrahaemocoelic inoculation. Yields vary according to the species of parasite and host. Examples are Nosema locustae in Melanoplus bivittatus yielding 3·9 × 109 spores/grasshopper enough to treat more than 1 hectare of rangeland, and Vairimorpha necatrix in Heliothis zea, yielding 1·67 × 109 spores per larva, with 2·5 × 1012 spores/hectare required for field application. In vitro culture is at present a laboratory tool only, with yields too low for economic returns.

Spores can be stored, according to species, dry or in distilled water with antibiotics at 4 °C. This gives good survival for months or years. In field applications feeding-bait formulations are more efficacious than sprays because they concentrate the spores for uptake by the target species and give the spores some protection from harmful ultraviolet radiation.

Pheromone lures have been used for the introduction of spores by males into pest infestations in stored grain. Males are lured to sites dusted with spores and return to the grain after removal of the lure, to contaminate females and larvae. The use of these lures, first as traps to monitor pest population increases, then to effect a controlled pest growth curve by introduction of pathogens, is an attractive innovation. Protozoa are considered safe for field application on the limited evidence available.