It is becoming increasingly evident that monitoring for spotted wing drosophila (SWD) is an integral part of a program to manage this fly to non-damaging levels. Growers who are aware early on that SWD has entered their field maintain a wider range of options than those who suddenly find themselves confronting a very heavy infestation with a lot of infested fruit.
Recall from the previous post that an interest in alternatives to GF120 Fruit Fly bait being used as a trap, either because of a lack of efficacy, cost or just plain curiosity in what will work sparked an effort to test other materials.
A further screen of materials has recently been pursued also by my private industry colleague. He made purees of peach, nectarine, orange, pineapple, beer + banana and added an amount of yeast to each one. One further treatment was a mixture of yeast, sugar and water (one packet baker’s yeast 0.25 oz, 4 teaspoons sugar and 12 fl oz water distributed among 5 or 6 Mason jars). Each of these was placed in a field having significant SWD activity.
While the fruit and yeast mixtures did catch flies to a certain extent, the mixture of yeast, sugar and water was by far the most successful. The yeast, sugar and water mixture is also very attractive to people because it is very clear and allows for easy viewing of trapped flies. Indeed in a further test comparing yeast, sugar and water mixture to GF120 traps (1 part GF120 to 4 parts water), found that it was far more attractive to SWD and vinegar flies. This success has been replicated by several area growers as well.
Some conjecture and comment about the yeast, sugar and water mixture.
While the GF120 trap mixture worked in the winter and to a certain extent in the spring, it may be that during the fruiting season the smell of this material is being masked by the abundance of other food scents coming from fruit and flowers. As anglers well know, fish change their preferences over the course of the season, and it the possibility that SWD is doing the same is difficult to deny.
Why the strong attraction to a yeast and sugar bait? There has been some amount of thinking determining that the yeast consuming the aqueous sugar is releasing plumes of carbon dioxide which is attracting SWD. While this may be true for the first 24 hours, it appears that the trap is most effective AFTER most of the sugar has been consumed, so it is apparent that the smell of fermentation is what is pulling the flies.
One positive addition to the yeast, sugar and water bait trap would be a material to keep the flies there once they have flown within the confines of the jar or bottle. There have been some comments from users noting the high level of attractiveness of this material is partly undone by the ability of flies to escape. Unlike GF120, there is no poison or surfactant which breaks the surface tension of the water to the point where flies alighting upon it fall on down into the liquid rather than still be able to walk on the surface. This is something worth looking into at a later date.
There has been significant discussion as of late as how to best trap for spotted wing drosophila, Drosophila suzukii. As a very small and mobile insect, detection and estimates of population can be challenging.
Although spotted wing drosophila lays eggs in yet unripe fruit, it is attracted to fermenting fruit to feed and thus this can be a useful medium for trapping. Additionally, there are several commercial formulations of fly trapping solutions.
This past week, the following materials were tested as a comparison for spotted wing drosophila detection and enumeration:
1. Strawberry puree, two days old.
2. Plantain + beer, two days old.
3. One part molasses mixed with 4 parts water.
4. Filth fly trap mix (AgBio Inc.)
5. Methyl eugenol plug over soapy water to trap flies
6. Apple vinegar straight
7. One part GF120 Fruit Fly Bait mixed with 4 parts water
8. Sweep netting one side of 32 feet of raspberry hedgerow, canes tapped with stick to dislodge flies. After sweeping is complete, net is inverted into a solution of soapy water and flies counted.
Most materials were tested twice in a heavily spotted wing drosophila infested raspberry field. 100 ml of the first seven treatments were placed in a 500 ml Nalgene bottle with four 7/16 inch holes drilled into the lid and hung about 3 feet from the ground, well into the hedgerow (see photo below). Food containers such as Mason jars and so on should not be used. Bottles were placed in the morning of day 1, and collected for evaluation the following morning.
In summary, the methyl eugenol and the molasses mix were useless and captured no flies in two tries.
Plantain + beer, apple vinegar and the filth fly mix, while capturing flies, caught fewer in comparison to the best treatments. On average they captured 3 flies in 24 hours in a heavily infested field. The beer and plantain mix is time consuming to make, is not worth the effort and have better use elsewhere, in my opinion. The apple vinegar and filth fly mix are easier to use.
GF120 and the strawberry puree were consistently the most effective of the baits, capturing on average 9 flies in 24 hours in a heavily infested field. The strawberry puree was more effective the second time around, since it had been around a little longer and was more fermented. Flies do not drown in the puree, thus the trap must be handled carefully prior to counting.
Finally, the sweep netting was far more effective in evaluating numbers of flies in the field. On average, 40 flies were captured over the 36 feet of hedgerow in a matter of minutes in a heavily infested field.
In conclusion, while the baits will continue to be effective as sentinels to detect initial infestations of spotted wing drosophila, it appears that they will be clearly inferior to sweep netting when making evaluations of already established populations in caneberries.
There will be a meeting held in San Martin, California concerning the spotted wing drosophila. The meeting is open to all who are interested, please refer to the following agenda:http://awqa.org/attachments/spotted_wing_09.doc
Several strawberry growers have identified a consistent pattern to spotted wing drosophilid infestation in strawberry.
Spotted wing drosophilid apparently lays eggs beneath the skin of the fruit, and after a few days the eggs hatch. As the larvae develop, a soft area at the area of infestation develops. There is no hole apparent in the fruit, see close up in first photo below, but the bruise and softening are quite apparent, and many pickers should be able to identify these fruits as culls.
On taking several samples of these fruit, and on cutting open the bruise, one finds from one to two larvae as in the second photo below.