Do growers have to manage for spotted wing Drosophila in processing peaches?

We have demonstrated that processing peaches are not a preferred host for spotted wing Drosophila (SWD), and are now able to answer several common questions about SWD concerns in this crop.

Processing peaches are a niche crop in west central Michigan. These fruits are distinct from fresh market free-stone peaches in several ways, including thicker skin, denser pubescence (“fuzz”) and a firmer texture. While this crop is not necessarily a majority of acreage on most regional fruit farms, it is present on most operations that produce tree fruit crops. Since a sizeable portion of the region’s tree fruit is either sweet or tart cherries, growers are used to having to initiate aggressive spotted wing Drosophila (SWD) management programs once these fruits begin to ripen and soften.

These considerations have led to confusion on the status of SWD management in processing peaches. This has been further complicated by recent reports of SWD showing up in fresh market nectarines and fresh market peaches. Well aware of the increased cost of production for cherry that has accompanied the need for intensive SWD management inputs, growers are reluctant to make a similar, high investment to manage this pest in peaches without evidence demonstrating it is necessary. At the same time, growers are all too aware of what can happen when SWD is not managed in a susceptible crop.

To address these concerns, Michigan State University Extension conducted a study on SWD susceptibility in processing peaches during the 2017 growing season. We measured adult SWD emergence after harvest in all major processing peach cultivars from the west central region. The cultivars Vinegold, Virgil, Venture, Caterina, Arkansas 9, Johnboy, Babygold5 and Redhaven were all sampled at peak field maturity. Both split pit and non-split-pit fruits were collected. Firmness (in pounds) was recorded for the split pit and non-split-pit groups.

After harvest, fruits were briefly dipped in a bleach solution to disinfect them and reduce the incidence of rot during the study. Split pit and non-split fruits were isolated in mesh capture tents designed to trap any emerging insects for identification. All Drosophila flies that emerged were identified and counted. Scentry cup traps were used at a density of five traps per field at three processing peach sites to record numbers of adult SWD in the field during the period leading up to harvest.

The questions we addressed with this work, and the answers we can provide based on our findings are summarized below.

Are processing peaches a preferred host for SWD?

No. While it is possible for SWD to infest peaches with physical damage (suture cracking, split pit, bird pecks, etc.), field infestations in non-damaged fruits were so rare in our trials that we consider them insignificant.

Are peaches with split pit going to cause me problems with SWD?

Sometimes, but this was also uncommon. There was a slight increase in SWD captures from split pit fruits, but it was unusual and not statistically different from catches in intact processing peaches. Peaches that are harvested at the typical firm, early maturity stages should not be an issue for most growers. We did find a couple of instances of infestation in split pits in west central Michigan this season, but it was in split pit fruits with obvious rot setting in. These fruits would be rejected at the processing plant anyway.

Does having problems with brown rot increase my chances of getting SWD in my processing peaches?

Not necessarily. While this is a complicated interaction, work from Brazil, “Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) in Peaches: Is it a problem?” by Adreazza et al. and published in Scientia Agricola, indicates increased brown rot is not necessarily associated with an increase in fruit infestation by SWD. However, growers should generally assume any steps that can be taken to reduce physical damage to their processing peaches are the best possible management strategies for avoiding SWD infestation.

Are we sure there were enough SWD out in the processing peach fields to cause problems if they were susceptible?

Yes. To demonstrate that SWD adults were present in adequate numbers to cause fruit infestations in a susceptible crop, we trapped for SWD adults using Scentry cup traps for the three weeks leading up to harvest at three different peach sites that we sampled from. Five traps were placed at each site. The first cup trap was placed at the edge of the field, and additional traps were placed every five trees, moving in towards the center of the block. SWD captures comparable or greater than those typically occurring in tart and sweet cherry fields were recorded in all three weeks.

In summary, SWD adults were present in sufficient numbers to cause infestation in a susceptible crop and growers at these sites were not managing for the pest. Based on the high catches, we are confident the reason no SWD problems occurred in these fields was due to the processing peach fruits not being susceptible to the pest. These findings are consistent with recent findings reported from Brazil.

Is there a particular cultivar of processing peach that would be more or less susceptible to SWD?

Any cultivar prone to split pit is more likely to be a problem, although none of the cultivars we tested are a preferred host for this pest. Growers already do their best to manage split pits, but there may be increased pressure on local processors to accept a smaller size of peach in the future to avoid issues with SWD contamination. Typically, split pit occurs when peaches over-thinned and become too large. This often occurs due to a push from a processor for a larger size of peach. Allowing a larger crop to stay on the tree will achieve slightly smaller size and reduce split pit, thus reducing the risk of contamination for both the grower and the processor. 

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