As part of keeping up to date with pests affecting grapegrowing areas overseas, Vinehealth Australia’s Technical Manager Suzanne McLoughlin joined Washington State Wine Commission’s WAVEx1 webinar on a new grape leaffolder species found in Washington vineyards, delivered by Washing State University research entomologist, Dr David James.
White-headed grape leaffolder (Desmia maculalis) is a new pest assumed to have been in Washington state for approximately five years. It has not been recorded as a significant pest in north American viticulture before and has not become a widespread pest of concern despite the potential ability of the moth stage to fly quite far.
It is similar but not the same as the Californian grape leaffolder (Desmia funeralis) which has been a sporadic pest in California for approximately 100 years.
Previously Washington has had no lepidopteran pests of any note, unlike most other grapegrowing regions.
Research started on this pest in 2018 looking to investigate pest biology, impact and potential management options after growers began to notice the characteristic folded leaves in the vineyards which the caterpillars use to hide from predators.
When present in high enough numbers, the caterpillars can cause severe defoliation of young vines, reduced leaf function as leaf photosynthesis is compromised, potentially delayed grape ripening in mature vineyards and sunburning of grapes.
In severe outbreaks, albeit rare, the larvae can also end up inside the bunches – worse for table grape situations. Damage is often noted to be restricted spatially and is therefore very intense where it occurs. This is thought to be because successive generations tend to stay in the same location as previous generations.
About the pest
- Larvae emerge in late autumn from an overwintering pupae on the vineyard floor and go through five instar stages before the adult stage; the pace of instar development being driven by temperature;
- They are active feeders both day and night but only use 2-3 leaves during their whole development;
- First instar larvae feed openly on the leaf surface and don’t roll leaves. This is very important as this is the lifecycle stage most exposed to predators and chemical control. Instead of leaf rolling, this instar does produce silk strands as an attempt at protection. Instars 2-5 are leaffolders but this action is more like ‘rolling’ than ‘folding’ whereby the larvae use silk to form the folding of the leaf. Once the roll has been created the larvae feed on either side of the leaf and overtime eat the whole leaf. The larvae remain inside a roll for their whole lifetime and only perform 2-3 leaf rolls in their life:
- Larvae are very wriggly, such that opening a folded leaf causes the larvae to fall to the ground.
- At the end of the season only 20-30% of folded leaves actually have larvae left in them.
- Adults lay very small, flat, eliptical eggs which are very hard to see and photograph
- Each season there are 2 or 3 generations; in warmer seasons they’ve had fewer generations. The first generation has tended to show low numbers, being indicative of poor overwintering capability of the pest.
Some alternative hosts have been identified for the Californian leaffolder, so suspect hosts may not be restricted only to grapevines.
Growers are using bait traps set up as bucket traps with fermenting molasses which attract the moths at night.
Biological control from a number of wasps, flies and other predators including spiders is happening but not well studied to date.
Chemical control evaluations are underway with the best control achieved from Altacor to date, but further options and timing are to be tested.
The economic threshold for impact on grapevines is still being determined.
In relevance to the Australian situation, currently as part of the requirements set out by the Department of Agriculture, Water and the Environment for the importation of fresh table grapes into Australia from California, the grape leaffolder is one of a number of pests that a consignment must be visually inspected for and remedial action taken if found.
1 WAVE (Washington Advancements in Viticulture and Enology) includes a suite of tools to share research supported by Washington’s wine industry. WAVE research seminars and WAVEx webinars are co-sponsored by the Washington State Wine Commission and Washington State University.