- Author: Ben Faber
Natural enemies have significantly suppressed Asian citrus psyllid populations in southern California
Ivan Milosavljevi, Department of Entomology, University of California, Riverside, CA
Christina D. Hoddle, Department of Entomology, University of California, Riverside, CA
David J.W. Morgan, California Department of Food and Agriculture, Mt. Rubidoux Station, Riverside CA
Nicola A. Irvin, Department of Entomology, University of California, Riverside, CA
Mark S. Hoddle, Department of Entomology, University of California, Riverside, CA
Is California facing a citrus apocalypse?
Asian citrus psyllid (ACP) (Diaphorina citri) is an invasive pest of citrus first discovered in urban citrus in San Diego County California (CA) in 2008 . ACP presents a significant economic threat to CA's citrus industry because it vectors a bacterium, CLas, which causes a citrus-killing disease, huanglongbing (HLB). There is currently no cure for HLB which kills susceptible commercial citrus varieties in as little as 5 to 8 years. Since HLB was first discovered in Florida (FL) in 2005 (ACP was discovered in FL in 1998), citrus production in that state has fallen by 80% . In CA, the first case of HLB was detected in 2012 in Los Angeles County and infestations of ACP-CLas are largely restricted to urban-grown citrus in southern CA. Should HLB spread north into the San Joaquin Valley, where 75% of CA's citrus fruit is grown, it would jeopardize ~262,000 fruit-bearing acres, which generates over $3 billion annually and provides over 26,000 jobs. Because ACP-HLB poses such a significant threat to CA's citrus industry, ACP population suppression is key to slowing the spread of CLas into CA's commercial citrus groves.
Biocontrol suppresses ACP populations
In CA, ACP has been the target of a classical or introduction biological control program with two tiny parasitic wasps or parasitoids, Tamarixia radiata and Diaphorencyrtus aligarhensis, sourced from Pakistan, a part of the ACP's presumptive native range. CA's biocontrol program against ACP began with the release of T. radiata in December 2011, and in December 2014, D. aligarhensis was added to the release program with the intent of establishing a complementary set of parasitoids that specifically attack ACP nymphs. To date, >24 million parasitoids (T. radiata and D. aligarhensis combined) have been mass-produced and released at >19,500 sites in southern CA by the Applied Biocontrol Lab at UC Riverside and the California Department of Food and Agriculture. Of these two parasitoids, T. radiata established readily and rapidly spread into sites in CA where it was not released. Conversely, D. aligarhensis failed to establish following release in CA and mass production and release of this parasitoid was subsequently discontinued in 2019.
The ongoing biocontrol effort: So far, so good, but there is room for improvement
Since the inception of the ACP biocontrol program in CA in 2010 and the first release of T. radiata in 2011, densities of ACP infesting urban citrus have declined by ~70%. Two different multi-year and multi-site studies in urban citrus in southern California has clearly demonstrated that the proximate causes of these widespread population declines are due to natural enemies, specifically parasitism of ACP nymphs by T. radiata and predation by generalist predators, of which syrphid fly larvae are key predators of ACP nymphs . Consequently, reduced ACP densities may have slowed the spread of CLas in CA and subsequent development of HLB in infected citrus trees. However, the efficacy of natural enemies attacking ACP eggs and nymphs has been reduced by the presence of another invasive pest, the Argentine ant (AA) (Linepithema humile). Field work on ACP biocontrol in CA identified AA as a significant impediment to natural enemies. When present on trees, AA reduced the abundance of natural enemies interacting with ACP and suppressed the efficacy of T. radiata and syrphids by over 50 percent. When AA is excluded from ACP colonies, natural enemy abundance and attack rates increase significantly, particularly impacts by T. radiata and syrphid fly larvae.
Why are Argentine ants problematic, and what can we do about them?
AA aggressively protect >55% of ACP from natural enemies. In return for this protection, AA is rewarded with food, honeydew, which is a sugary waste product excreted by ACP nymphs. Consequently, AA protection exacerbates infestations of ACP and other honeydew producing pests in citrus (e.g., brown soft scale and mealybugs). This results in a positive feedback loop – more pests survive due to AA protection and their populations increase which in turn produces more food for AA which results in increasing ant populations. An undesirable outcome of these population increases is greater applications of insecticides to control sap sucking pests and AA. Ironically, sprays of contact insecticides targeting AA (and ACP) kill natural enemies needed for “free” pest control and this disrupts IPM programs which aim to reduce insecticide use.
To ameliorate this problem of increased insecticide use, biocontrol ACP (and other sap sucking pests) can be enhanced through three management practices: (1) monitoring AA activity with infra-red sensors to determine when ants have reached densities that need controlling, (2) controlling AA with highly targeted applications of ultra-low concentrations (i.e., 0.0001%) of insecticide delivered to foraging inside of biodegradable hydrogel beads that are infused with 25% sucrose water and insecticide, and (3) floral resources that provide food and shelter to natural enemies, especially hover flies, that attack ACP nymphs. This three-pronged management approach for controlling AA and the pests ants protect (e.g., ACP) is undergoing field evaluation in commercial citrus orchards in southern California. The outcomes of these large, replicated field trials will be discussed in an upcoming article in Topics in Subtropics: “Maximizing IPM of Argentine ant and sap sucking pests with biodegradable hydrogels, infra-red sensors, and cover crops in commercial citrus orchards.”
- Author: Ben Faber
A lot of work has been put into monitoring the Brown Marmorated Stink Bug (BMSB), Halyomorpha halys (Hemitpera: Pentatomidae). It has been a major problem in many areas of the country, as well as California. Hoddle and crew did extensive trapping in the state and found some serious populations in some areas and fewer in others. Read their great story here - https://cisr.ucr.edu/blog/2013/06/17/pheromone-trapping-program-brown-marmorated-stinkbug
A recent study by Illán et al suggests a more predictive method of assessing the threat of spread across the US and California:
Evaluating invasion risk and population dynamics of the brown marmorated stink bug across the contiguous United States
Abstract
BACKGROUND; Invasive species threaten the productivity and stability of natural and managed ecosystems. Predicting the spread of invaders, which can aid in early mitigation efforts, is a major challenge, especially in the face of climate change. While ecological niche models are effective tools to assess habitat suitability for invaders, such models have rarely been created for invasive pest species with rapidly expanding ranges. Here, we leveraged a national monitoring effort from 543 sites over 3 years to assess factors mediating the occurrence and abundance of brown marmorated stink bug (BMSB, Halyomorpha halys), an invasive insect pest that has readily established throughout much of the United States.
RESULTS; We used maximum entropy models to estimate the suitable habitat of BMSB under several climate scenarios, and generalized boosted models to assess environmental factors that regulated BMSB abundance. Our models captured BMSB distribution and abundance with high accuracy, and predicted a 70% increase in suitable habitat under future climate scenarios. However, environmental factors that mediated the geographical distribution of BMSB were different from those driving abundance. While BMSB occurrence was most affected by winter precipitation and proximity to populated areas, BMSB abundance was influenced most strongly by evapotranspiration and solar photoperiod.
CONCLUSION: Our results suggest that linking models of establishment (occurrence) and population dynamics (abundance) offers a more effective way to forecast the spread and impact of BMSB and other invasive species than simply occurrence-based models, allowing for targeted mitigation efforts. Implications of distribution shifts under climate change are discussed.
https://doi.org/10.1002/ps.7113
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- Author: Ben Faber
So what happens to an avocado tree when it runs out of water? The stomata close and stops transpiring. When water no longer evaporates from the leaf surface, it heats up. If it gets hot enough, and it will, the leaf sunburns and dies. It stopped photosynthesizing long before that, because the process is temperature sensitive. Avocado is an “upper story, late successional” tree which is more affected by drought than faster growing, weedy species. So read about what happens to a forest as temperatures increase.
by: Olof Lönnehed
University of Gothenburg
olof.lonnehed@science.gu.se
Trees get overheated in a warmer rainforest
The ability of rainforests to store carbon can decrease in pace with climate change. This is due to photosynthesis rates in the leaves of rainforest species falling at higher temperatures and the trees' natural cooling systems failing during droughts. Increased heat threatens especially the species that store most carbon. This has been shown in a new thesis from the University of Gothenburg.
Some species of trees are able to handle rising heat in the tropics by sucking up large quantities of water to their leaves and transpiring through wide-opened pores in their leaves. These are mainly fast-growing trees that establish themselves early as a rainforest grows up. The same cannot be said for the trees that make up the canopy of rainforests in old growth forests. They grow slower, but get bigger and taller, and their leaves do not have the same ability to cool themselves via transpiration.
Water powers the ‘air conditioning' of the leaves
“The tropics have not experienced Ice Ages and have thus had a relatively stable climate historically as well as seasonally. With climate change, it has started to get warmer and then we have seen that some species of trees are showing increased mortality rates, but we have not really known why before,” says Maria Wittemann, who wrote the thesis.
She has studied several species of tree that can be roughly divided into early successional species, which establish themselves early in a new rainforest, and late successional species, which grow slower but grow considerably bigger, and are thus a larger carbon sink over the long term. A clear difference is how the trees in the two groups handle heat. The early successional species open the pores wider in their leaves, through which they transpire large amounts of water, thus keeping down the temperature in their leaves – similar to an air conditioning system. The late successional species do not open their pores as much, and therefore it's more difficult for them to stay cool.
More sensitive to drought
“We found large temperature differences in the leaves in our measurements. There could be a difference of 10 degrees Celsius between late successional species and early successional species growing in the same location. The late successional species had more difficulty coping with abnormally high temperatures. These trees had a higher mortality rate,” says Maria Wittemann.
However, the early successional species' profuse transpiration through their leaves also requires a lot of water. During a period of drought, the researchers noted that early successional species became more vulnerable to the heat and dropped their leaves. Their reduced consumption of water meant that late successional species were more resistant to drought.
“Our results show that photosynthesis rates in rainforest trees falls when the temperature rises in their leaves, which occurs mainly in late successional species. The proteins and membranes in their leaves, that are essential for photosynthesis, fail, and eventually the trees die due to carbon starvation because they cannot convert enough carbon dioxide from the air. This affects the entire ecosystem. We know, for example, that some animals eat the fruits of the late successional species,” says Maria Wittemann.
Co-operation with a local university
Previous research shows that the situation is worst in the Amazon. It is estimated that this carbon sink will be transformed into a carbon source by as early as 2035. In African rainforests, climate change has not gone as far.
Research at the University of Gothenburg is being conducted in high-elevation forests in Rwanda in collaboration with the University of Rwanda. The trees have been studied in situ, but seeds have also been planted in climate chambers in Gothenburg to study their development at different temperatures.
“We are working with various stakeholders in Rwanda. There is not much rainforest left in Rwanda and when they plant new trees, they want to know which indigenous species will be able to survive in a warmer climate,” says Maria Wittemann.
Facts about the study: The sensitivity of trees to climate change was studied by planting tree species adapted to a cooler climate in Rwanda's elevated tropical rainforests at three locations with different climates at different altitudes. One step down the elevation gradient corresponds to a possible future climate. The field experiment is called Rwanda TREE (TRopical Elevation Experiment) and consists of 20 species and 5,400 trees. To learn more about Rwanda TREE, visit the website www.rwandatree.com or watch the film https://www.youtube.com/watch?v=EkDvbwisqlQ.
Ruth Hohl Borger, U of Florida Communications Specialist
LAKE ALFRED, Fla.—Americans' love affair with sugar can be a deadly attraction that sometimes leads to major health problems, including obesity and type 2 diabetes.
Finding natural, non-caloric sugar substitutes is desirable but challenging. However, researchers at the University of Florida Institute of Food and Agricultural Sciences have made a breakthrough — discovering new, natural sweeteners in citrus for the first time.
This finding opens opportunities for the food industry to produce food and beverages with lower sugar content and lower calories while maintaining sweetness and taste using natural products.
Yu Wang, associate professor of food science at UF/IFAS, managed the multi-year project that found eight new sweetener or sweetness-enhancing compounds in 11 citrus cultivars.
“We were able to identify a natural source for an artificial sweetener, oxime V, that had never been identified from any natural source previously,” said Wang, a faculty member at the UF/IFAS Citrus Research and Education Center in Lake Alfred, Florida. “This creates expanded opportunities for citrus growers and for breeding cultivars to be selected to obtain high yields of sweetener compounds.”
Replacing and reducing sugar in processed foods is a long-term goal of both the healthcare system and food and beverage industry. Consumers want a sweet-tasting orange juice, but they're also concerned about too much sugar consumption. Identifying the sweeteners and sweet-enhancing compounds could provide a solution for the “Sugar Bias” for the citrus industry.
To date, reducing sugar in food without compensating for sweetness can reduce the taste of most food. Replacing sugar with artificial, non-caloric sweeteners such as saccharin, sucralose and aspartame can negatively impact flavor profiles by leaving a bitter and metallic aftertaste. Consumers have shown increasing preference for naturally derived sweeteners that more closely resemble the sensory profile of sugar. To date, even natural, non-caloric sweeteners still possess some licorice-like and bitter aftertastes. While natural sweeteners are currently derived from fruits, some fruits are difficult to cultivate.
In addition to trying to find actual sweeteners in citrus, researchers looked to find sweetness enhancers that can significantly reduce the amount of sugar required to achieve the same level of perceived sweetness. To date, only six synthetic and two natural sweeteners/sweetness enhancers have been created and used by the food industry that are approved by the U.S. Food and Drug Administration. These also have the negative side effects of unpleasant aftertaste and are expensive to produce.
Eleven selections from the UF/IFAS citrus breeding program were selected for unique and exceptional flavors. These cultivars included UF 914 (a grapefruit hybrid), and EV-2 and OLL-20 (both sweet oranges). Mandarins, including Sugar Belle®, Bingo, 13-51, 18A-4-46, 18A-9-39, 18A-10-38, were also included in the research project.
Wang's research could lead to increased opportunities for the food industry to produce food and beverages with lower sugar content and calories while maintaining sweetness and taste using natural products, The research methodology also suggested that efficiencies of identifying flavor metabolites could be improved. This research was recently published in the Journal of Agricultural and Food Chemistry
And can you name these new sugars?
https://pubs.acs.org/doi/10.1021/acs.jafc.2c03515
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- Author: Ben Faber
TOPICS IN THIS ISSUE – P. Rolshausen Editor
- Mycorrhizae: An Underground Support Network for Trees.
- Notes on Applying Gibberellic Acid (GA3) to Navel Orange and other Citrus in the San Joaquin Valley of California
- Saline Waters - A Growing Problem
- 35th Anniversary of the Nematode Quarantine Facility at the University of California Riverside