Research by Wageningen Plant Research shows that a new isolate of the entomopathogenic fungus Metarhizium robertsii reduces reproduction of virgin suzuki-fruitflies very effectively. After 4 weeks in cages with a small dish of a blueberry-fungus mix there was virtually no reproduction of the fruit fly compared to a control treatment (94% reduction). Two parallel tested fungal products had less or no effect on reproduction.
The suzuki-fruit fly (Drosophila suzukii) is a serious pest of soft fruit such as cherries, blueberries and raspberries. It has settled in southern Europe in the last decade and arrived in the Netherlands approximately 5 to 6 years ago. In contrast to other fruit flies this species is able to infest young, undamaged fruit on the plants before harvest. The options to control this pest with chemical insecticides are limited. Spraying of fruit has a short time window for application because of the residue risk of the chemicals on the fruit at harvest. Biological control options for this fruit fly are in research. Some promising parasitic wasps are found in the area of origin (Japan/China) but it is unlikely that these non-native natural enemies will get a registration in Europe. Local parasitic wasps found in Europe and tested on the suzuki-fruit fly are so far not effective enough to control the pest. Entomopathogenic fungi as a biocontrol option could be an alternative for the wasps. Several entomopathogenic fungal products are registered in Europe for different pests but so far the application as a spray on berries and flies appears to be not effective enough.
Entomopathogenic fungal products
There are a number of products on the market containing entomopathogenic fungi used to control a range of pest insects above-and belowground. Three registered products in the Netherlands are Botanigard (Beauveria bassiana), BIO1020 (Metarhizium brunneum) and PreFeRal (Isaria fumosorosea). So far,application of these fungi appear not effective enough to prevent economic damage by the suzuki-fruit fly. Even though some of these products kill part of the population in the lab they are unable to prevent reproduction of the flies due to the slow mode of action. Before the flies die from the fungi, they still lay a considerable number of eggs in the fruit. It needs therefore faster working fungal strains and a different application strategy whereby young/virgin flies will be exposed to higher doses of the killing fungus.
‘Lure and Infect’ strategy
A different strategy to expose insects to entomopathogenic fungi is to lure them to baits. A high concentration of fungal spores is present in the baits and is picked up by the attracted flies after landing on the bait. The advantage is that no large volume of spores has to be sprayed on the crop and no residues of the fungi on the plants and fruit will occur. Furthermore, the attracted insects will pick up higher doses of the spores via the baits than possible via crop spraying, increasing thereby the chance of a successful infection of the insect. This ‘Lure and Infect’ strategy, however, can only be effective if the baits are highly attractive, the amount of baits per field are economically affordable and the fungi have a fast mode of action preventing in an early stage the reproduction of the flies.
Fungal strain of Metarhizium robertsii near 100% effective
Wageningen UR has a large collection of entomopathogenic fungi and several isolates appear to be highly effective against difficult to control pest such as grubs of the cockchafer (Melolontha melolontha) and larvae of the crane fly (Tipula sp.). We tested several of these promising fungal strains on their efficacy to infect and kill the suzuki-fruit fly. One of the best strains (Metarhizium robertsii) was then tested for its value in a ‘Lure and Infect’ strategy next to two commercial products (Botanigard and PreFeRal).
Proof of concept
The ‘proof of concept’ experiment for the‘Lure and Infect’ strategy was performed in cages. In each cage we placed a bait consisting of blueberries mixed with rice and spores of the entomopathogenic fungus. We released freshly hatched fruitflies in these cages and determined reproduction of the flies during a period of 4 weeks. For M. robertsii and B. bassiana we also determined how effective the two types of fungi were picked up by the attracted fruitflies in these cages.
The results show that the fruitflies picked up 5 times more spores of M. robertsii than B. bassiana spores (product Botanigard). In the cage experiment where we studied the influence of the fungus-berry bait on reproduction of the fruitfly we found a 94% reduction in reproduction with the blueberry-fungus M. robertsii bait compared to the control. Botanigard reduced the reproduction not before 2 weeks after exposure to the bait resulting in 71% reduction compared to the control and no significant reduction was found with the PreFeRal-berry bait. Visual observations indicate that the lower reproduction is strongly correlated to the faster death of the flies in the cages and not by rejection of the baits (multiple landings, walking and feeding on the berries were observed in all treatments). Although Botanigard did not perform bad overall a two week time window before the flies stop laying eggs may be too slow to have an effect on the population in the field. In the field there is a mix of virgin and older female flies that are already laying eggs when exposed to the fungus-bery baits. The fungi should block egg-laying within days to be effective in a Lure & Infect strategy we expect.
A future solution for pest control?
Although the ‘Lure and Infect’ strategy could be a good alternative for the classic field spray, there are still some hurdles to tackle before it becomes practicable. The bait with the entomopathogenic fungus has to be very attractive and able to compete with the fruit in the field. But the strategy could work if used in an early stage of the season before ripening of the fruit. What kind of bait is attractive enough in the field is still unknown and is part of the follow-up research. In 2017 we plan to move from the cages out to the field.
This research is being carried out in the EU project DROPSA.