Insect-pathogenic nematodes

Biological pest control and industrial production

Like many parasitoid insects, insect-pathogenic nematodes also use insect larvae to multiply in. However, they do not lay their eggs in the larvae – they enter the host through the mouth, anus or spiracles or even directly through the skin. The entire development cycle including mating takes place inside the insect larvae.

Like insects, nematodes are widespread. They have conquered almost all habitats: from the deep sea to the highest mountains. No other organism is more abundant than the nematode. The majority of these are harmless soil dwellers, some cause severe damage to plants, and there are a number of dangerous animal and human parasites.

Despite the variety of habitats colonised, the organism's structure is very uniform between species. It is simple and efficient – a model of success comparable perhaps to the bicycle or the VW Beetle. The organism's body is so robust that it hardly needs any repair mechanism. The development from the egg to the fully-grown nematode is predictable for every stage of cell division and is the same for every organism. These characteristics have made the nematode, Caenorhabditis elegans, the most important research object of genetic scientists and developmental biologists. In fact, it was the first multicellular organism to have its genetic code fully deciphered.

Insect-pathogenic nematodes: conquering new habitats through symbiosis

The insect-pathogenic nematodes of the genera Heterorhabditis and Steinernema have seemingly independently entered into a symbiotic relationship with bacteria. The bacteria belong to the family of Enterobacteriaceae and hence are closely related to the Escherichia coli bacterium which lives in our intestine. They are harmless to people and die at temperatures above 35 °C. Thanks to this symbiosis, the nematodes can quickly kill the insect they enter. There are a number of closely related nematodes that do not carry symbiotic bacteria which still enter the insect larvae but have to wait until the insect dies – and there are other species that only enter dead insects. Thanks to this symbiosis, the genera Heterorhabditis and Steinernema are able to conquer the living insect as a new habitat and nutrient source. The same holds true for the symbiotic bacteria, which are unable to reach the insects' blood without the nematodes.

This is how it works: the lifecycle of the nematode

Infective juvenile of the nematode

Fig. 1: Infective juvenile of the nematode

The soil-dwelling infective juveniles of the nematode are approx. 1mm long (Fig. 1) and carry the symbiotic bacteria in the intestine (Fig. 2). The infective juveniles of some species such as the genus Heterorhabditis find their hosts by cruising through the soil, whereas other species such as Steinernema carpocapsae ambush their prey in the upper layers of the soil.

Melon-shaped vesicle containing symbiotic bacteria in the infective juvenile

Fig. 2: Melon-shaped vesicle containing symbiotic bacteria in the infective juvenile

Many species employ a mixture of the two strategies. Once the infective juveniles have found a host, they enter into the haemocoel through the skin or via the insect's mouth, anus or respiratory system. The genus Heterorhabditis is equipped with a tooth which serves as a penetration tool.

Having reached the haemocoel, the nematodes release the bacteria, which propagate rapidly, killing the host within 3 days. The bacteria and the digested insect tissue is the nutrient source for the nematodes, which proliferate enormously; 2 to 3 weeks later, up to 300,000 new nematodes may emerge from one infested grub whereas it only takes 2 to 3 infective juveniles to kill the grub.

An insect weighing 60,000 kg: industrial-scale production of nematodes

An overwhelming number of highly efficient pest-insect antagonists cannot be used on a large scale as mass production is not possible; however, insect-pathogenic nematodes grow and proliferate very well in conventional bioreactors. The trick is to mimic the situation in the infected insect. The nematodes and the symbiotic bacterium are grown in a sterile environment, free from other organisms. One of the bioreactors we use to produce nematodes has a capacity of 60,000 litres.

Reliable service: quality control

Nematodes are living, biological control organisms and, thus, the quality may differ considerably between individual production batches depending on the production process. Therefore, it is essential that quality checks are performed continually. We check every production batch for its ability to kill insects. We only sell batches that can meet these high demands. Our quality control method is simple and accessible to anyone who wants to find out more about the quality of different batches of nematodes.