Effect of Heat Stress on Grape Production in the San Joaquin Valley
By Karl Lund, UCCE Viticulture Advisor for Madera, Merced, and Mariposa Counties
The Spotted Lanternfly (SLF) (Lycorma delicatula) is an invasive plant hopper native to Northern China. It was first identified in the United States in Berks County, Pennsylvania, back in 2014. From there, it spread along the eastern seaboard with known infestations from Massachusetts down to North Carolina, and west as far as Chicago, Illinois, and Nashville, Tennessee, with 17 states currently having known infestations (Image 1). Since the publication of image 1, an additional identification of a SLF infestation in Fulton County, Georgia (which includes Atlanta) brings the total up to 18 states. Given SLF's quick spread across the eastern United States, it is crucial to understand where else SLF can spread.
Image 1. September 2024 Spotted Lanternfly Infestation Map. Red is Counties with infestations under quarantine, Yellow is Counties with infestations not under quarantine, and Purple is Counties with at least one sighting but no known Infestation. Map and data from New York State Integrated Pest Management webpage, part of Cornell University College of Agriculture and Life Sciences (https://cals.cornell.edu/new-york-state-integrated-pest-management/outreach-education/whats-bugging-you/spotted-lanternfly/spotted-lanternfly-reported-distribution-map)
Scientists from the USDA and Xinjiang Institute of Ecology and Geography in China have already conducted work to see what portions of the United States have the right climate for SLF. The analysis shows that the Atlantic coast from the Carolinas up to New Hampshire and Maine is moderately to highly suitable for SLF. On the western side of the Appalachian Mountains, a belt forms starting in Western New York along the Great Lakes, spreading along the lakes into Ohio before it spreads out and covering most of Ohio, Indiana, Illinois, and Missouri, along with portions of Southern Michigan, Iowa, Nebraska, and Kansas with high suitability climate for SLF infestation (Image 2). Comparing the current (Image 1) and climatic (Image 2) ranges of SLF we can see that SLF has already colonized much of its climatic range along the East Coast and is spreading into the belt west of the Appalachian Mountains.
Image 2. Potential distribution of Spotted Lanternfly across the United States using climatic data. Image from https://entomologytoday.org/2019/10/03/invasive-spotted-lanternfly-large-potential-range-united-states-beyond/
These two incidents occurred in July of 2022 and March of 2024. The July 2022 incident occurred when CDFA Border Inspection Agents identified Spotted Lanternfly egg masses on firewood at the Truckee Border Protection Station. The firewood, originally from New Jersey, was confiscated and destroyed before entering California. The March 2024 incident again occurred when CDFA Border Inspection Agents identified Spotted Lanternfly egg masses at the Truckee Border Protection Station. In this case, 11 viable egg masses were identified on a 30-foot-tall metal artwork being transported to California. The infested artwork was refused entry until it was decontaminated. In Nevada, 30 additional egg masses were found during decontamination. Once the artwork reached its final destination in Sonoma County, the local AG Commissioner’s staff reinspected the load and found an additional three egg masses, which were promptly destroyed.
These two incidents are a great example of the value that the CDFA Border Inspection Agents and local AG Commissioner’s staff can have in preventing invasive species from getting into California. They are also two critical examples of how quickly and far humans can move an invasive species. Lastly, it shows us in California that we need to understand the threat that Spotted Lanternfly poses to California's viticulture and agriculture in general. To do this, we will go through SLF reproduction, identification, host range, feeding damage, and control methods being used in Pennsylvania.
Image 3. Spotted Lanternfly life cycle. Image from: https://extension.psu.edu/spotted-lanternfly-what-to-look-for
In Pennsylvania, SLF has an annual lifecycle (Image 3). Eggs are the overwintering portion of the lifecycle, hatching in spring (May to June). The first instar that hatches from the eggs has a black body with white spots and can be up to 1/8 inch long (Image 4B). The second and third instars also have black bodies with white spots only differing from the first instars by their larger size. The fourth instars appear in Summer (July to September). They have red bodies with black lines and white spots measuring up to ½ inch in length (Image 4C). Adult SLFs appear in summer and live until the end of the growing season. The adults have an upper and lower set of wings. The upper wings, which are visible when they are resting are light grey with black stripes at the wing tips and black spots closer to the shoulders (Image 4D). The lower set of wings are exposed when the adults fly/glide and are red wings with black spots that are easily identified (Image 4E). Adults lay eggs in the fall to early winter (September – December). Egg masses are covered in a white to light-grey putty-like substance. This substance hardens, cracks, and resembles mud or tree bark once dry (Image 4A). Each adult female can lay at least two egg masses, and each egg mass will include 30 – 50 eggs.
All SLF instars are great jumpers, and jumping is their primary mode of movement and defense. They are adapted to jump from plant to plant and away from potential enemies. All instar stages continually migrate while feeding, rarely staying on a single plant for more than 2-3 days. Instar stages 1 - 3 prefer to feed on young (still green) shoots. The fourth instar stage has grown large enough to feed on mature (lignified) plant materials. Adult SLFs are also good jumpers and still use this to get around. The wings of adult insects are strong enough to fly, although not very well. Their dominant mode of movement is to jump and then use their wings to glide from location to location. This jump and glide technique allows SLF adults to have a feeding range of 2-3 miles. The adult SLF is strong enough to feed through the bark of many different tree species.
Image 4. Spotted Lanternfly Identification Images. A. Egg mass, B. Instar 1- 3, C. Instar 4, D. Adult with closed wings, E. Adult with exposed underwings. Image from Penn State Extension: https://extension.psu.edu/spotted-lanternfly-what-to-look-for
The host range of SLF is extensive. SLF has been found feeding on soy, corn, basil, cucumber, roses, Limonium flowers, and even grass for basic survival. None of these are preferred hosts, but they allow the SLF to feed and survive while looking for its preferred hosts. The most preferred host is the invasive Tree of Heaven (Ailanthus altissima). This is a Chinese weedy tree species that attracts SLF. Spotted lanternfly feeding on Tree of Heaven reproduce readily. Grapes, black walnuts, hops, and maple trees also see high levels of feeding and reproduction, although not as high as the Tree of Heaven. Less preferred hosts include willow, apple, blueberry, mulberry, fig, stone fruits, birch, sycamore, lilac, poplar, staghorn sumac, and Virginia creeper. Spotted lanternfly will feed on these hosts but move on more quickly to find better hosts.
Luckily, SLF feeding is not extremely detrimental to most plants. SLF are phloem feeders that emit large amounts of honeydew while feeding. That honeydew can act as a substrate to initiate the growth of sooty molds or as an attractant to other insects. Due to the SLF's large size and mobile, the cooperation between ants and other honeydew-producing insects is not likely to be established. The production of honeydew is indicative of a large amount of carbohydrates (energy) being removed from the plant, and the effects this loss has on the plants can be seen in several studies. Studies done on Black Walnut, Red Maple, Silver Maple, and Tree of Heaven found that SLF feeding lowered carbon assimilation, stomatal conductance, and transpiration (Lavely et al. 2022). Long-term, this feeding does lead to lower starch levels, total nonstructural carbohydrates, and overall tree size (diameter) (Hoover et al. 2023), but it does not kill mature trees. The only tree deaths attributed to SLF are young saplings of Black Walnut and Tree of Heaven after heavy infestation.
Studies done on grapevines show a similar lowering of carbon assimilation, stomatal conductance, and transpiration, which leads to lower starch levels in the root system at the end of the growing season. It also leads to lower nitrogen levels in both leaf and root tissues. The carbon deficit is enough that grape samples taken from moderately to heavily infested vines showed lower Brix levels at harvest (Harner et al., 2022). Additional work that has yet to be published shows that SLF infestation leads to lower berry and cluster weight, lower numbers of clusters per vine, and an overall lower yield. Vineyards with high SLF concentrations also see higher levels of overwinter vine mortality.
As a phloem-feeding insect that moves regularly between vines, SLF is also a possible vector of viruses and microbial diseases. Thus far, testing has shown that SLF is not able to vector any grape (or other host plant) viral diseases. However, recently, SLF has been implicated in the vectoring of Pierce’s Disease (PD). Despite being a phloem-feeding insect and PD being a xylem-limited bacteria, recent work has shown that SLF can vector PD (Islam et al. 2024). The findings do indicate that SLF is a poor vector of PD. However, given that SLF feeds in large numbers, regularly moves between plants while feeding, and has an extensive feeding range, even if SLF is a poor vector, it could still be a devastating PD vector.
Given the damage that SLF can cause to grapevines, what control options are available? Extensive testing has been done in the Eastern US, primarily in Pennsylvania, to control SLF in vineyards. As these tests have been conducted in different states, the results may vary due to differences in climatic and cultural practices. More importantly, due to regulatory differences, availability and application concentrations may differ in California (or other western states). For systemic control, dinotefuran has shown excellent control, while imidacloprid has shown variable control. Beta-cyfluthrin, bifenthrin, carbaryl, zeta-cypermethrin, malathion, and natural pyrethrins have demonstrated excellent contact control. At the same time, neem oil, insecticidal soaps, horticultural oil, paraffinic oil, and botanical oil have all shown good control. Overall, this gives growers many different options to choose from.
Despite many promising chemical controls, there are a few general problems with them. The first is the timing of applications near harvest. SLF doesn’t start laying eggs until September and continues through December. They are still actively feeding and moving into new locations during harvest. An uninfested vineyard could become infested the week or even the day before harvest and in large numbers. This means you will be limited by the pre-harvest interval as to what controls are available.
Another problem is the reinfestation of your vineyard after a successful spray. Given the mobile nature of SLF, reinfestation of a vineyard after a successful spray is high. Of all previously mentioned chemical controls, only dinotefuran, beta-cyfluthrin, and bifenthrin had good residual activity. The remaining chemicals had limited or no residual control effects on SLF. This means growers must regularly spray their vineyards to combat the reinfestation cycle. Even regularly spraying the vineyard will not prevent the negative impact on the grapevine. The previously mentioned work showing lower carbon assimilation, stomatal conductance, transpiration, and Brix due to SLF feeding (Harner et al. 2022) was conducted by only allowing the SLF to feed in 4–10-day feeding cycles.
This all leads to the ultimate limitation of chemical control of SLF: growers cannot spray the source of the SLFs. Given SLF's extensive host range and large feeding range, they are just as likely to find a home in agricultural, urban, abandoned, or riparian/wild settings and move back and forth between them. To prevent the infestation cycle, we need to control SLF in all these settings, which is outside of growers' control. Even with urban spraying from the local agricultural commissioner’s office or other government agencies, we won’t be able to curb their numbers everywhere once a population is established in enough locations.
The best option to stop SLF is to either prevent it from entering California or eliminate it before it can establish and start spreading. CDFA Border Inspection Stations have blocked SLF from entering CA twice (with an assist from the Sonoma County Agricultural Commissioner's office). Continued and increased support for the border inspection stations will hopefully prevent California agriculture from ever dealing with SLF. However, given the size of California, the number of ways into California, and the amount of people and products that come into California, there is a strong likelihood that, eventually, SLF will find itself in California.
If (when) SLF does find its way to California, the only chance to stop it is before it can establish and spread. That can only happen if we catch SLF before they can do that. When going back to the initial infestation site in Pennsylvania and the most recent site in Georgia, SLF had been in those sites for an extended time before official discovery. In the case of the initial infestation site in Pennsylvania, local workers commented on trying to catch the 1st, 2nd, and 3rd instars as they jumped away from them. Those workers didn’t know what they were playing around with would become such a problem. Nor did those workers know how and where to report SLF. California cannot have the same thing happen. The first Californian who sees a SLF needs to know what they are looking at and how to report that SLF has made it to California.
Thankfully, CDFA and the Pierces Disease/Glassy Winged Sharpshooter (PD GWSS) Board have seen SLF's threat to California. CDFA has produced a series of handouts and fliers in English and Spanish in their snag it, snap it, report it response plan. These handouts and fliers are free to download from the CDFA website: https://www.cdfa.ca.gov/pdcp/board/spottedlanternfly.html. The handouts and fliers have images to help identify SLF, a phone number, a website, and even a QR code to report any SLF sightings. This is a fantastic start for California’s response to SLF. The problem with the response is its lack of extension to the agricultural community and the general public.
I want your help in spreading the word about this pest. The handouts and fliers linked above are set to print on a standard 8.5 x 11 sheet of paper. Printing out some of the flies to hang your operations’ common areas is a good start. Sharing this information and the link with neighboring operations, labor contractors, and pest management partners will also help spread the information further. A step further would be to hand out these handouts and fliers to anyone spending significant time in your fields. Everyone involved in agriculture, from owners and managers to harvest crews and day laborers, should know how to identify and report SLF. Of course, even if everyone involved in agriculture nationwide knew how to identify and report SLF in 2014, that would not have prevented much of the current situation. From the initial outbreak in Pennsylvania to the furthest western extent of SLF's current expanse in Chicago, Nashville, and Atlanta, have not been agricultural areas. We in the agricultural community need to spread the information internally and get information about SLF to the general public.
This article was original posted to the Vit Tips Newsletter located at https://ucanr.edu/site/fresno-county-viticulture/collection/vit-tips-previously-newsletter-vinelines. Reposted here with permission of author.
Additional Resources:
As they have been on the front lines of this fight for 10 years, Penn State Extension has a vast amount of knowledge available on their webpage:
https://extension.psu.edu/spotted-lanternfly
Their neighbors to the north New York State Integrated Pest Management also have a great webpage to find additional information:
References:
Harner, A.D., Leach, H.L., Briggs, L., and Centinari, M. (2022) Prolonged phloem feeding by the spotted lanternfly, an invasive planthopper, alters resource allocation and inhibits gas exchange in grapevines. Plant Direct Vol 6 Issue 10. https://doi.org/10.1002/pld3.452
Hoover, K., Iavorivska, L., Lavely, E.K., Uyi, O., Walsh, B., Swackhamer, E., Johnson, A., and Eissenstat, D.A. (2023) Effects of long-term feeding by spotted lanternfly (Hemiptera: Fulgoridae) on ecophysiology of common hardwood host trees, Environmental Entomology, Volume 52, Issue 5, Pages 888–899, https://doi.org/10.1093/ee/nvad084
Islam, M.T., Kudla-Williams, C., Harner, A.D., Centinari, M., and Rosa, C. (2024) Infected grapevines are poor hosts but can serve as source of pathogen transmission for SLF. bioRxiv 2024.08.13.607612; doi: https://doi.org/10.1101/2024.08.13.607612
Lavely E, Iavorivska L, Uyi O, Eissenstat DM, Walsh B, Primka EJ IV, Harper J and Hoover K (2022) Impacts of short-term feeding by spotted lanternfly (Lycorma delicatula) on ecophysiology of young hardwood trees in a common garden. Front. Insect Sci. 2:1080124. doi: 10.3389/finsc.2022.1080124