At this time, social wasps, multicolored Asian ladybeetles (MALB) and Drosophila flies are likely present in your vineyards looking for sugar sources to feed on (wasps and MALB; Fig 1) or lay eggs in (Drosophila flies). These insects are primarily known to attack previously damaged grape berries and, with the berry splitting you may have experienced last week following the rain events, these insects will likely be feasting on grapes more intensely for the upcoming weeks.
Social wasps are now present in very high numbers in the grape clusters of the vineyard blocks at the West Madison Agricultural Research Station (WMARS). It is important to remember that any wound to the berries provides an entry point for many insects and pathogens and sometimes these two can be linked. Indeed, wasps can not only cause damage to grapes (see previous article here), they can also carry pathogens that they may deposit on berries when foraging. For example, in a recent paper, Madden et al (2017) showed that paper wasps Polistes dominulus harbor portions of the sour rot microbial community and that they are able to transfer viable microorganisms while foraging on grape berries. Wasps also carry yeasts that they disperse in the environment (Stefanini et al 2012) and that could play a role in fermented products such as wine.
We have discussed our research on spotted-wing drosophila (SWD) led by student Emma Pelton several times in previous articles and how SWD females cannot cut the skin of the intact cold climate wine grapes, namely Frontenac, Marechal Foch, Marquette, La Crescent, Concord, St. Pepin, St. Croix and Leon Millot that we tested. However, our research also showed that any of these varieties that were previously damaged were found infested with SWD larvae. Indeed, once there is a wound, such as on split berries, berries become fully accessible to the thousands of drosophila flies, including Drosophila melanogaster which has a weak ovipositor and is ubiquitous, to lay their eggs in. Drosophila flies are also known to carry microorganisms. A recent webinar presented by Megan Hall posted here discusses the interaction between Drosophila flies and grape bunch rot and how to manage bunch rot.
Other interesting insects that were observed feeding on these split berries are striped beetles (Fig 2) which are likely striped cucumber beetles (though western corn rootworm look pretty similar). It is not surprising to see these beetles looking for a sweet treat though it is the first time we see them at WMARS feeding on grapes. They are likely beetles that inadvertently found the grapes and should only be considered a very occasional secondary pest that would not usually be present in vineyards and should not be of concern.
As Amaya Atucha mentioned in her article, as you make the important decision about when to harvest your grapes following berry splitting, you may also consider the insect populations that are present in your vineyard and how these may be carrying pathogens that could lead to sour or bunch rots. If your wasp or Drosophila populations are particularly high this time around, you may consider to harvest sooner rather than later.
Before you harvest, if you are still looking at insecticides to manage these insects, you can refer to Table 1 below for products, pre-harvest intervals (PHI), and efficacy for MALB, social wasps, and SWD. We do not endorse a product over any others and please check the label before using any pesticide.
Table 1. Selected insecticides and active ingredients, pre-harvest intervals (PHI), and efficacy on multicolored Asian lady beetle (MALB), social wasps, and spotted-wing drosophila (SWD). G: Good, E: Excellent, P: Poor, -: no information available.
Thanks to Beth Workmaster, Andi Nelson, and Amaya Atucha for reporting on insects at WMARS.
Madden et al. 2017. PeerJ. DOI 10.7717/peerj.3223
Stefanini et al 2012 PNAS. https://doi.org/10.1073/pnas.1208362109This article was posted in Grapes, Insects and tagged Berries, berry, berry splitting, Christelle Guédot, Grapes, insects, multicolored Asian lady beetle, social wasps, Spotted Wing Drosophila, Wasps, yellowjackets.