Month: March 2020

Hope for eastern hemlocks – IF funding can be obtained

eastern hemlocks in Great Smoky Mountains National Park

As we all know, eastern (Tsuga canadensis) and Carolina (T. caroliniana) hemlocks have suffered huge losses due primarily to the introduced hemlock woolly adelgid (Adelges tsugae – HWA). In New England, there has been more than a 60% decrease in total hemlock basal area since 1997 and a virtual absence of hemlock regeneration in HWA-infested areas. HWA continues to spread – most recently into western Michigan and Nova Scotia (all information, unless otherwise indicated, is from Kinahan et al. 2020; full citation at end of this blog). [However, Morin and Liebhold (2015) found that hemlock basal volume continued to increase for the first 20 years or so after invasion by the adelgid, due to ingrowth of immature hemlocks. See “results” in Morin et al., full citation at the end of the blog.]

This loss deprives us of a gorgeous tree … and unique habitats. Hemlock-dominated forests were characterized by deep shade, acidic and slowly decomposing soil, and a cool microclimate. They provided unique and critical habitat for many terrestrial and aquatic species.

A team of scientists based at the University of Rhode Island has carried out an experiment comparing cuttings from eastern hemlocks apparently resistant to HWA to susceptible ones. Matching sets of resistant and susceptible trees were planted at eight sites in seven states – Ithaca and Bronx, NY; Boston; southern CT; Lycoming County, PA; Thurmont, MD; southern WV; and Waynesville, NC. All plantings were within or adjacent to forests containing HWA-infested hemlocks.

After four years, 96% of the HWA-resistant hemlocks had survived, compared to 48% of the control plants. The HWA-resistant plants were 32% taller, put out 18% more lateral growth, had 20% longer drip lines, and were in 58% better condition. HWA was found on trees at only three out of the eight plots. HWA density on resistant eastern hemlocks was 35% lower than on HWA-susceptible hemlocks, although this difference was not statistically significant.

Trees in all eight plots were infested with elongate hemlock scale (Fiorinia externa – EHS), a second insect damaging hemlocks in eastern North America. However, the HWA-resistant hemlocks had EHS densities 60% lower than those of the controls.

Kinahan et al. note that identification and use of host tree populations’ potential for pest resistance has played a role in other programs managing non-native pests and pathogens, including Dutch elm disease and chestnut blight.  

The same scientists note that significant effort has been put into biocontrol or insecticides for management of hemlock woolly adelgid, but without achieving the desired improvement of forest health. Attempts to cross eastern hemlocks with HWA-resistant hemlocks unfortunately produced no viable offspring. However, Kinahan et al. were inspired to explore possible genetic resistance within natural populations of eastern hemlocks by the 1) evidence of resistance in Asian and western hemlocks; 2) the different foliar terpene profiles in those species; and 3) the presence of apparently healthy mature hemlock trees growing in proximity to heavily infested trees.

They asked forest managers and other concerned groups to help locate stands with trees that were mature and apparently completely healthy, were located within HWA-devastated hemlock stands, and had not been chemically treated. They chose a small stand of eastern hemlocks growing within the Walpack Fish and Wildlife Management Area in northern New Jersey. This stand was called the “Bulletproof Stand”. They evaluated HWA resistance in five of these trees, then chose two for propagation and planting in the test.

New Jersey’s “bullet-proof stand” on the left
photo by Richard Casagrande

The trees were planted in September 2015. Due to funding gaps, they were not revisited for four years. Thus, Kinahan et al. re-evaluated the resistant and vulnerable trees in Autumn 2019 – with the results I reported above.

Does this study prove that clonal propagation of apparently resistant hemlocks is an effective strategy to restore the species?

It is not that simple.

The difference in survival and condition was striking, but the authors note several caveats:

1) they had not recorded pre-experiment data on plant height or other variables, so they cannot be certain that variation in initial plant height or dripline did not contribute to current treatment-level differences in these variables.

2) they cannot distinguish between the impacts of HWA and EHS on plant growth.

3) since they could not monitor the planting sites for four years, they cannot definitively link increased mortality of HWA-susceptible trees to higher pest densities. However, the lower pest densities and higher survival of HWA-resistant hemlocks are consistent with herbivore-driven tree mortality.

They also cannot assess the impact of other environmental stressors (drought, cold, etc.) on their results.

4) The small number of trees planted at each site prevented detailed site-level analyses.

The scientists conclude that their work is most appropriately viewed as a ‘proof of concept’ experiment highlighting the need for future research exploring how HWA-resistant eastern hemlocks might best be integrated into existing HWA management.

Unfortunately, the Rhode Island researchers report they cannot persuade the US Forest Service to support continuing this effort. Will these promising hints not result in action?

Kinahan et al. stress the importance of the reduced pest densities (both HWA and EHS) on the putatively resistant hemlocks. They think this might be a result of the higher terpene concentrations in the twigs and needles. Finally, they note that lower densities of sap-feeding herbivores may also indirectly provide protection against other consumers, including gypsy moth (Lymantria dispar) and hemlock looper (Lambdina fiscellaria).

SOURCE

Kinahan, I.G., G. Grandstaff, A. Russell, C.M. Rigsby, R.A. Casagrande, and E. L. Preisser. 2020. A four-year, seven-state reforestation trial with eastern hemlocks (Tsuga canadensis) resistant to hemlock woolly adelgid (Adelges tsugae). Forests 11: 312

Morin, R.S. and A.M. Liebhold. 2015. Invasions by two non-native insects alter regional forest species composition and successional trajectories. Forest Ecology and Management 341 (2015).

Posted by Faith Campbell

P.S. I have been working with colleagues to promote a more coordinated and well-funded program to combat non-native forest pests – including much greater reliance on identifying and breeding resistance to the pest. Visit here to see this effort.

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

Another New Pest Detected in California; Possible Threat to Native Shrubs

Rhus integrifolia

The California Department of Food and Agriculture (CDFA) is seeking comments  on the appropriate pest rating for Leptosillia pistaciae, a recently discovered fungus that causes pistachio canker.

The Department’s draft pest ranking assigns the highest Economic Impact score – three. It assigns a medium Environmental Impact – two. This is because the pathogen can kill an important native shrub, with possible follow-on consequences of reduced biodiversity, disrupted natural communities, or changed ecosystem processes.

CDFA states that there is no uncertainty in its evaluation, but I see, and describe here, numerous questions about the possible true extent of the invasion and possible host range.  

Comments are due on April 4, 2020.

The pathogen was detected in June 2019, when a habitat manager from an ecological reserve in San Diego County noticed multiple dead lemonade berry shrubs (Rhus integrifolia) in one of the parks. This is the first known detection of Leptosillia pistaciae in the United States and on this host. USDA APHIS has classified Leptosillia pistaciae as a federal quarantine pest. Rhus and Pistacia are in the same family, Anacardiaceae (cashews and sumacs).

According to the CDFA, Leptosillia pistaciae is the only member of this fungal genus known to be associated with disease symptoms on plants. Other species are endophytes or found in dead plant tissues. [It is not at all unusual for fungal species to be endophytes on some plant hosts but pathogenic on others. A California example is Gibberella circinata (anamorph Fusarium circinatum), which causes pitch canker on Monterey pine (Pinus radiata) but is an endophyte on various grass species (Holcus lanatus and Festuca arundinacea).]

(Reminder: this is the second new pest of native species detected in California state in 2019; I blogged about an ambrosia beetle in Napa County here. )

Rhus integrifolia (lemonade berry or lemonade sumac) is native to California. It grows primarily in the south, along the coast – from San Diego to San Luis Obispo. However, some populations are also found in the San Francisco Bay area. This and other sumacs are also sold in the nursery trade.

On pistachio trees in Italy, symptoms are observed in the winter and late spring. During the winter dormant season, trees had gum exudation and cracking and peeling of bark on trunks and branches. On trunks and large branches, cankers appeared first as light, dead circular areas in the bark; subsequently they became darker and sunken. Under the bark, cankers were discolored with necrotic tissues; in some cases, these extended to the vascular tissues and pith. During the active growing season, the symptomatic plants also showed canopy decline. Inflorescences and shoots, originating from infected branches or twigs, wilted and died. When the trunk was girdled by a canker, a collapse of the entire tree occurred.

range map for Rhus integrifolia

On lemonade berry, large clumps of dead adult shrubs were observed on the edge of hiking trails. Some shrubs that had completely dead foliage were re-sprouting from their bases. Trunks of shrubs that were not completely dead were copiously weeping sap and fluids and showed foliage browning and die back with symptoms of stress.

It is thought that spores could be spread by wind, rain splashing, and the movement of dead or dying trees, greenwaste, and infected nursery stock. Contaminated pruning tools might also transport the spores. The possibility of a latent phase – or perhaps asymptomatic hosts – adds to the probability of anthropomorphically assisted spread.

I question how much effort has been put into detection surveys, especially in natural systems with native Rhus species. California has three other native sumacs: R. ovata, R. aromatica, and Malosma laurina (CNPS; full citation at the end of the blog). In addition, there are numerous other species in the family, including poison oaks (Toxicodendron spp.) and the widespread invasive plant genus Schinus.

Furthermore, some plants in the family (other than pistachios) are grown for fruit or in ornamental horticulture, including two of the native sumacs and two non-native species, Rhus glabra and R. lanceolata, cashew, mango, and smoke trees (Cotinus spp.).

Yet CDFA confidently states that there are only two hosts and that it has been detected in only one population – that in San Diego. This is because CDFA considers only official records identified by a taxonomic expert and supported by voucher specimens.

CDFA states that the pathogen is likely to survive in all parts of the state where pistachios are grown – primarily in the Central Valley. California supplies 98% of the pistachios grown in the United States; the remainder is raised in Arizona and New Mexico. California production occurred on 178,000 acres in 2012. A map is included in a flyer on production available at the url listed at the end of this blog.

In discussing spread potential, no mention is made of possible human-assisted spread.

The CDFA document includes instructions for submitting comments; the deadline is April 4.

Sources:

Rhus and related species native to California: California Native Plant Society

https://calscape.org/loc-california/Rhus(all)/vw-list/np-1?

Rhus species used in horticultural plantings in California: CalFlora https://www.calflora.org//cgi-bin/specieslist.cgi?where-genus=Rhus 

Pistachio production information: https://apps1.cdfa.ca.gov/FertilizerResearch/docs/Pistachio_Production_CA.pdf

Posted by Faith Campbell.

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

Progress – Now Threatened – On Protecting Our Cacti

prickly pear cacti in Big Bend National Park
photo by Blake Trester, National Park Service

The cacti that are such important components of desert ecosystems across nearly 2 million square miles straddling the U.S.-Mexico border are under threat from non-native insects – as I have noted in earlier blogs. Of course, cacti are important in other ecoregions, too – I wrote recently about the columnar cacti in the dry forests of Puerto Rico.

Flat-padded prickly pear cacti of the genus Opuntia are threatened by the cactus moth, Cactoblastis cactorum.

In 1989, the cactus moth was found in southern Florida, to which it had spread from the Caribbean islands (Simonson 2005). Recently, the moth was found to have spread west as far as the Galveston, Texas, area and near I-10 in Columbus, Texas, about 75 miles west of central Houston (Stephen Hight, pers. com.) Two small outbreaks on islands off Mexico’s Caribbean coast have been eradicated.

In Florida, the cactus moth has caused considerable harm to six native species of prickly pear, three of which are listed by the state as threatened or endangered.

When the cactus moth reaches the more arid regions of Texas, it is likely to spread throughout the desert Southwest and into Mexico. In the American southwest, 31 Opuntia species are at risk; nine of them are endemic, one is endangered. Mexico is the center of endemism for the Opuntia genus. In Mexico, 54 Opuntia species are at risk, 38 of which are endemic (Varone et al. 2019; full citation at end of this blog).

The long-term effects of the cactus moth on these North American Opuntia are unknown because there may be substantial variations in tolerance. The attacks observed in the Caribbean islands have shown great variability in various cactus species’ vulnerability (Varone et al. 2019).

The Opuntia cacti support a diversity of pollinators as well as deer, javalina (peccaries), tortoises, and lizards. Prickly pears also shelter packrats and nesting birds (which in turn are fed on by raptors, coyotes, and snakes), and plant seedlings. Their roots hold highly erodible soils in place (Simonson 2005).

While scientists have been concerned about the possible impacts of the cactus moth since it was detected in Florida 30 years ago, a substantial response began only 15 years later. The U.S. Department of Agriculture began trying to slow the spread of the cactus moth in 2005 (Mengoni Goñalons et al. 2014), with a focus on surveys and monitoring, host (cactus) removal, and release of sterile males. This program was successful at slowing the moth’s spread and eradicating small outbreaks on offshore islands of Alabama, Mississippi, and Mexico.

Cactus moth damage to native cacti in Florida
photo by Christine Miller, UF/IFAS

However, the moth continued to spread west and the program never received an appropriation from Congress. The primary funding source was a US – Mexico Bi-National Invasive Cactus Moth Abatement Program. Both countries contributed funds to support the research and operational program to slow the spread in the U.S. Funds were provided through USDA Animal and Plant Health and Inspection Service (APHIS) and the Mexican Secretariat of Agriculture, Livestock, Rural Development, Fisheries and Food (SEGARPA). Unfortunately, funding was reduced by both entities and became inadequate to maintain the Bi-National Program.

Therefore, in 2012, APHIS abandoned its regional program and shifted the focus to biocontrol. This is now considered the only viable control measure in the desert Southwest where vulnerable cacti are numerous and grow close together. The biocontrol project has been funded since 2012 through the Plant Pest and Disease Management and Disaster Prevention program (which receives funding through the Farm Bill). It has received a total of slightly more than $2 million over seven years. More than half the funds went to the quarantine facility to support efforts to rear non-target hosts and verify the biocontrol agent’s host specificity. About a quarter of the funds supported complementary work of an Argentine team (both the cactus moth and the most promising biocontrol agent are native to Argentina). Much smaller amounts have supported U.S.-based scientists who have studied other aspects of the cactus moth’s behavior and collected and identified the U.S. moths being tested for their possible vulnerability to attack by a biocontrol wasp.

Here are details of what these dedicated scientists achieved in just the past seven years at the relatively low cost of roughly $2 million. Unfortunately, the project now faces a funding crisis and we need to ensure they have the resources to finish their work.

Some Specifics of the BioControl Program

After literature reviews, extensive collections, and studies in the cactus moth’s native habitat in Argentina (Varone et al. 2015), a newly described wasp, Apanteles opuntiarum (Mengoni Goñalons et al. 2014), has been determined to be host specific on Argentine Cactoblastis species and the most promising candidate for biocontrol. Wasps were collected in Argentina and sent to establish a colony in a quarantine facility in Florida to enable host specificity studies on North American Lepidoptera (Varone et al. 2015).

Quarantine host specificity studies and development of rearing technology has not been straightforward. Initially, it was difficult to achieve a balanced male/female ratio in the laboratory-bred generations; this balance is required to maintain stable quarantine laboratory colonies for host range testing. This difficulty was overcome. A second challenge was high mortality of the cactus-feeding insects collected in the Southwest that were to be test for vulnerability to the biocontrol wasp. These desert-dwellers don’t do well in the humid, air-conditioned climate of the quarantine facility! For these difficult-to-rear native insects, scientists developed a molecular genetics method to detect whether eggs or larvae of the cactus moth parasitoid were present inside test caterpillars after they were exposed to the wasps. For easy to rear test insects, caterpillars are exposed to the wasps and reared to adulthood. Host specificity tests have been conducted on at least five species of native U.S. cactus-feeding caterpillars and 11 species of non-cactus-feeding caterpillars (Srivastava  et al. 2019; Hight pers.comm.).

To date there has been no instance of parasitism by Apanteles opuntiarum on either lepidopteran non-target species or non-cactus-feeding insects in the Florida quarantine or in field collections in Argentina (Srivastava et al. 2019; Varone et al. 2015; Hight pers.comm.).

The scientists expected to complete host-specificity testing in the coming months, then submit a petition to APHIS requesting the release of the wasp as a biocontrol agent. Unfortunately, the project’s request for about $250,000 in the current year was not funded. This money would have funded completion of the host specificity testing, preparation of a petition to APHIS in support of release of the biocontrol agent into the environment, and preparation of the release plan.

Meanwhile, what can we expect regarding the probable efficacy of the anticipated biocontrol program?

Some of the wasp’s behavioral traits are encouraging. The wasp is widely present in the range of the cactus moth, and persisted in these areas over the years of the study. The wasp can deposit multiple eggs with each “sting”. Multiple wasps can oviposit into each cactus moth without detriment to the wasp offspring. Unmated wasp females produce male offspring only, whereas mated females produce mixed offspring genders. In the field, female wasps attack cactus moth larvae in a variety of scenarios: they wait at plant access holes to sting larvae when they come outside to defecate; they attack larvae when they are moving on the surface of the pads; they can sting the youngest cactus moth larvae through the thin plant wall of mined the pads; and they enter large access holes created by older larvae and attack larger larvae. The wasps are attracted by the frass (excrement) left on the outside of the cactus pads by cactus moth larvae (Varone et al. 2020).

However, I wonder about the extent to which the cactus moth is controlled by parasitoids in Argentina. Cactoblastis eggs are killed primarily by being dislodged during weather events (rain and wind) and by predation by ants. First instar larvae are killed primarily by the native Argentine cactus plants’ own defenses – thick cuticles and release of sticky mucilage when the young larvae chew holes into the pads where they enter and feed internally. As larvae feed and develop inside the pads, the primary cause of mortality is natural enemies.

Of all the parasitoid species that attack C. cactorum, A. opuntiarum is the most abundant and important. When the larvae reach their final state (6th instars), they leave the pads and find pupation sites in plant litter near the base of the plants. It is at this stage that the parasitism from A. opuntiarum is detected in the younger larvae that were attacked while feeding inside pads. As the moth larva begins to spin silk into which to pupate, larvae of the wasp erupt through the skin of the caterpillar and pupate within the silk spun by the moth. Predation by generalists (ants, spiders, predatory beetles) accounted for high mortality of the unprotected last instar and pupae (Varone et al. 2019).

Finally, the cactus moth has three generations per year when feeding on O. stricta in the subtropical and tropical coastal areas of the Americas and the Caribbean. In Argentina, on its native host, the moth completes only two generations per year (Varone et al. 2019).

 How to Get the Program Support Needed

Opuntia in Big Bend National Park
Photo by Cookie Ballou,
National Park Service

To date, no organized constituency has advocated for protection of our cacti from non-native insect pests. Perhaps now that the Cactoblastis moth is in Texas, the threat it represents to our desert ecosystems will become real to conservationists and they will join the struggle. The first step is to resolve the funding crisis so that the agencies can complete testing of the biocontrol agent and gain approval for its release. So now there is “something people can do” – and I hope they will step forward.

I hope Americans are not actually indifferent to the threat that many cacti in our deserts will be killed by non-native insects. Many are key components of the ecosystems within premier National Parks, and other protected areas. Cacti also are beautiful treasures in botanical gardens. I hope conservationists will agree that these threats must be countered, and will help to ensure funding of the final stages of the biocontrol tests.

Sources

Mengoni Goñalons, C., L. Varone, G. Logarzo, M. Guala, M. Rodriguero, S.D. Hight, and J.E. Carpenter. 2014. Geographical range & lab studies on Apanteles opuntiarum (hymenoptera: braconiDae) in AR, a candidate for BC of Cactoblastis cactorum (Lepidoptera: Pyralidae) in North America. Florida Entomologist 97(4) December 2014

Simonson, S.E., T. J. Stohlgren, L. Tyler, W. Gregg, R. Muir, and L. Garrett. 2005. Preliminary assessment of the potential impacts and risks of the invasive cactus moth, Cactoblastis cactorum Berg, in the U.S. and Mexico. Final Report to the International Atomic Energy Agency, April 25, 2005 © IAEA 2005

Srivastava, M., P. Srivastava,  R. Karan, A. Jeyaprakash, L. Whilby, E. Rohrig, A.C. Howe,  S.D. Hight, and L. Varone. 2019. Molecular detection method developed to track the koinobiont larval parasitoid Apanteles opuntiarum (Hymenoptera: Braconidae) imported from Argentina to control Cactoblastis cactorum (Lepidoptera: Pyralidae). Florida Entomologist 102(2): 329-335.

Varone, L., C.M. Goñalons, A.C. Faltlhauser, M.E. Guala, D. Wolaver, M. Srivastava, and S.D. Hight. 2020. Effect of rearing Cactoblastis cactorum on an artificial diet on the behavior of Apanteles opuntiarum. Applied Entomology DOI: 10.1111/jen.12731.

Varone, L., G. Logarzo, J.J. Martínez, F. Navarro, J.E. Carpenter, and S.D. Hight. 2015. Field host range of Apanteles opuntiarum (Hymenoptera: Braconidae) in Argentina, a potential biocontrol agent of Cactoblastis cactorum (Lepidoptera: Pyralidae) in North America. Florida Entomologist — Volume 98, No. 2 803

Varone, L., M.B. Aguirre, E. Lobos, D. Ruiz Pérez, S.D. Hight, F. Palottini, M. Guala, G.A. Logarzo. 2019. Causes of mortality at different stages of Cactoblastis cactorum in the native range. BioControl (2019) 64:249–261

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.