FOLIAR spraying with the biological control agent Bacillus thuringiensis serotype kurstaki (Btk), a bacterial insecticide, is a long-established technique for the control of lepidopteran insect pests in agriculture, horticulture and forestry.

It is clearly more targeted than approved chemical insecticides and as such has rapidly become the mainstay control for oak processionary moth (OPM) in the United Kingdom. However, there is an alternative biocontrol agent currently used in countries like the Netherlands, but apparently yet to catch on in the UK where OPM control has become increasingly monopolised by foliar spraying with the Btk bacterial insecticide.

The technique is based on an entomopathogenic nematode (microscopic roundworm) also applied by foliar spraying to target the larva stage of OPM. However, further investigation reveals the control mechanism is not as simple as it first seems, because the nematode operates in tandem with a specific bacterium which actually kills the OPM larvae and with which the nematode has a symbiotic relationship.

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In the spotlight is the nematode Steinernema feltiae found naturally in the soil but used commercially as a biological control agent against a wide range of insect larvae including lepidopteran (butterfly and moth) insect pests. On contact with target larvae these microscopic roundworms penetrate the caterpillar through one of its natural body openings. The S. feltiae nematodes actively seek out their prey and enter the larvae via natural openings in the mouth and respiratory system. Once inside they will begin to feed on the larval tissues and secrete a specific bacterium from their digestive tract. The bacterial cells infect the OPM larvae, multiplying and spreading rapidly and typically killing the host larva within a matter of days.

The nematode acts essentially as a vector to carry and transmit a lethal entomopathogenic bacterium into the target prey. This three-way relationship is clearly complicated. That between the nematode and the target larvae is essentially one of predator and prey, while that between the bacterium and the larva is one of parasite and host. The relationship between the nematode and its bacterium is one of symbiosis with the bacterium in the role of a symbiont.

The nematode/bacterium mechanism is most effective against OPM when targeted against larvae in the first and second instar stages (L1 and L2 stages) because these early instar stages are easier to infect. Effective control at this stage prevents larvae from moving into the later L3 to L5 stages when the urticating (stinging) hairs develop.

So, what are the pros and cons of this novel nematode/bacterium control mechanism and particularly in comparison with Btk? Unlike Btk, the nematode does not have to be actively ingested with oak leaf and bud biomass, which means less restriction on the timing of spraying in relation to oak tree foliar leaf cover. However, this novel nematode/bacterium duo will presumably kill non-target lepidopteran larvae such as those of green oak tortrix moth and winter moth which feed on the oak tree during the same window as OPM. Thus, native birds like blue tits which rely on the April/May flush of these larvae to feed their young will suffer in the same way as they apparently do from sprays of Btk insecticide.

What’s more, the nematode/bacterium model and mechanism is active against a much wider range of insect larvae. Indeed, commercial formulations of the S. feltiae nematode are also used to control larvae of a wide range of soil-dwelling and foliar-feeding insect pests including fungus gnats (sciarid flies), thrips, vine weevils and leaf miners.

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