Questions about spotted wing drosophila (SWD, Drosophila suzukii) continue to come in, and the most recent have been permuations on “how many traps should I put out” or “how many traps should I use per acre”. These questions are somewhat difficult to answer without first discussing the utility of traps for SWD management.
What do spotted wing drososphila traps tell us?
As they are currently deployed, SWD traps tell us presence or absence of SWD in an area, but they are not good predictors of population sizes nor do trap captures necessarily occur before fruit infestation. Therefore, traps are useful in determining if SWD is present on your farm, but at this time, treatments should be timed to host susceptbility (ripening, meaning fruit that is beginning to color, and ripe fruit) rather than to trap captures. This situation is understandably frustrating to growers and to entomologists. Monitoring is the heart of integrated pest management (IPM), and without an effective monitoring program for SWD that predates fruit infestation, unnecessary management action may be taken. There is lots of research on monitoring strategies for SWD underway, and we hope to have better answers soon.
If I want to monitoring, how many traps should I use?
Because traps determine presence or absence, the number placed depends more on how plan to manage your fields than on the number needed to detect population. Remember that the relationship between trap captures and actual populations is unclear. We also do not know how great a distance traps act in, or in other words, how far away they catch flies from, but we suspect that this distance is fairly short. It is impractical to blanket fields with traps, so growers interested in trapping should instead consider the following:
1. Will I manage fields or areas within a field seperately?
If the answer this is yes, then you should consider placing traps in each “management unit”. If you plan to manage fields or your entire farm as a unit, then having lots of traps throughout the farm is less important. This is because traps will trigger management action, so the number of traps and where they are placed should be determined by how that management action will be implimented.
2. Where are flies likely to be?
Traps should be placed in areas where they are likely to catch flies, such as on or very near plants with fruit in shaded areas. If you are using traps to determine if flies are present (their main utility at this point), you should place them in areas where they are likely to catch flies.
3. How many traps do I need to ensure that I have information each week when I check them?
Once you have determined the number of fields or farms that need to be trapped and where those traps should be placed, how many traps should you put out? If traps operated perfectly (were never damaged, lost, flooded, dried out, eaten by a bear, removed by pickers, or poorly labeled and lost) you would only need one. However, I prefer to build redundacy into my trapping systems to control for events like those list. I suggest that folks use three traps per management unit. This allows for the possibility that one or more traps might be rendered useless. This also allow you to calculate average trap captures as we move toward trapping systems that are more IPM friendly.