Invasive Plants Threaten Our Bees!

Hysteria over disappearing bees is widespread: declines in domesticated and wild insect pollinator populations have been documented worldwide. Insect pollinators, particularly bees, are responsible for the vast majority of plant pollination services in wild landscapes and agricultural systems. Without effective and sufficient pollination, the persistence of native plants and various fruit and vegetable crops is in jeopardy. The suspected drivers of pollinator decline include habitat loss, habitat fragmentation, agrochemicals (e.g. pesticides and fertilizers), invasive species, emerging pathogens, and climate change. Herein, the threats posed to insect pollinators by invasive plants are explored.

Invasive species are those transported to novel habitats, often across great distances, which demonstrate negative impacts on recipient communities. Invasive plants often dominate vast contiguous areas of land, in effect generating a form of habitat loss or fragmentation. This feature of invasive plant ‘behavior’ has severe consequences for native insect pollinators and their access to food. Bees and other insect pollinators feed on nectar and pollen harvested from flowers. Invasive plants with large, showy and rewarding flowers are often very attractive to bees – they tend to draw in more species, and more individual pollinators than neighboring native plant species. At first glance, it might appear that invasive plants are in some way “better” than their native competitors – the invasive plants are associated with greater insect diversity and abundance. However, closer inspection reveals quite the opposite story. Invasive plants tend to reduce the number of pollinator species and the abundance of pollinators in a landscape, and they also reduce the number of pollinator visits that native plants receive.

How and why does this happen? What is it about invasive plant that is so destructive to native insect pollinator communities? Scientists are still figuring this out, and there is a lot of work left to be done, but here’s a bit of what we know so far:

Invasive plants are visited a lot by native pollinators, especially by native generalist pollinators who visit many species and types of flowers. This weakens the relationships between generalist pollinators and native plants. When invasive plants are present, neighboring native plants generally receive fewer pollinator visits, and are thus less likely to be pollinated! In addition, the pollen transported by visiting pollinators is overwhelmingly that of the invasive plant, native plants can’t use this to create their own seeds, and it may even be harmful to early seed production. This double-whammy leaves native plants with reduced reproductive success: they produce fewer fruits and fewer seeds. These circumstances present a dismal prognosis for the native plant community: the few flowers present produce few seeds, so fewer native plants grow the following year. As this continues season after season, the native plants might disappear altogether, becoming locally extinct. Clearly, this is a threat to the native plant community; it also has cascading consequences for entire communities of native pollinators. Along with the generalist pollinators already described, many types of wild bees and other insects are specialist pollinators that rely on specific native plant species for their survival. When those native plant species are difficult to find, or even non-existent, specialist pollinators are faced with starvation.

The scenario described above is bleak. Likewise, there are a lot more ways that ecologists know invasive plants could be harming native pollinator communities, they just haven’t been investigated thoroughly yet. Ecologists around the world are working to understand how invasive plants affect pollinators, and how to mitigate negative impacts to support biodiversity conservation, agricultural productivity and ecosystem stability. This research inspires hope for the future of bees and other insect pollinators, as well as all our favorite insect-pollinated foods.


A List of Some Important References

Bartomeus I, Vila M, Santamaria L. 2008. Contrasting effects of invasive plants in plant-pollinator networks. Oecologia 155:761–770.

Brown BJ, Mitchell RJ, Graham SA. 2002. Competition for pollination between an invasive species (Purple Loosestrife) and a native congener. Ecology 83:2328–2336.

Brown BJ, Mitchell RJ. 2001. Competition for pollination: Effects of pollen of an invasive plant on seed set of a native congener. Oecologia 129:43–49.

Chittka L, Schürkens S. 2001. Successful invasion of a floral market. Nature 411:653.

Dietzsch AC, Stanley DA, Stout JC. 2011. Relative abundance of an invasive alien plant affects native pollination processes. Oecologia 167:469–479.

Hanula JL, Horn S. 2011. Removing an invasive shrub (Chinese privet) increases native bee diversity and abundance in riparian forests of the southeastern United States. Insect Conserv. Divers. 4:275–283.

Kearns C a., Inouye DW, Waser NM. 1998. Endangered mutualisms: The conservation of plant-pollinator interactions. Annu. Rev. Ecol. Syst.:83–112.

Lopezaraiza-Mikel ME, Hayes RB, Whalley MR, Memmott J. 2007. The impact of an alien plant on a native plant-pollinator network: an experimental approach. Ecol. Lett. 10:539–50.

Montero-Castaño A, Vilà M. 2012. Impact of landscape alteration and invasions on pollinators: A meta-analysis. J. Ecol. 100:884–893.

Morales CL, Traveset A. 2009. A meta-analysis of impacts of alien vs. native plants on pollinator visitation and reproductive success of co-flowering native plants. Ecol. Lett. 12:716–728.

Potts SG, Biesmeijer JC, Kremen C, Neumann P, Schweiger O, Kunin WE. 2010. Global pollinator declines: trends, impacts and drivers. Trends Ecol. Evol. 25:345–353.

Powell KI, Krakos KN, Knight TM. 2011. Comparing the reproductive success and pollination biology of an invasive plant to its rare and common native congeners: A case study in the genus Cirsium (Asteraceae). Biol. Invasions 13:905–917.

Pysek P, Jarosik V, Chytry M, Danihelka J, Kuhn I, Pergl J, Tichy L, Biesmeijer JC, Ellis WN, Kunin WE, et al. 2011. Successful invaders co-opt pollinators of native flora and accumulate insect pollinators with increasing residence time. Ecol. Monogr. 81:277–293.

Schweiger O, Biesmeijer JC, Bommarco R, Hickler T, Hulme PE, Klotz S, Kühn I, Moora M, Nielsen A, Ohlemüller R, et al. 2010. Multiple stressors on biotic interactions: How climate change and alien species interact to affect pollination. Biol. Rev. 85:777–795.

Stout JC, Morales CL. 2009. Ecological impacts of invasive alien species on bees. Apidologie 40:388–409.

Thijs KW, Brys R, Verboven H a F, Hermy M. 2012. The influence of an invasive plant species on the pollination success and reproductive output of three riparian plant species. Biol. Invasions 14:355–365.

Vanparys V, Meerts P, Jacquemart A-L. 2008. Plant–pollinator interactions: comparison between an invasive and a native congeneric species. Acta Oecologica 34:361–369.


Entomologists convene in Minneapolis, MN: I co-organized a symposium!

The annual conference of the Entomological Society of America (ESA) just wrapped up in Minneapolis, MN following four days of symposia, oral and poster presentations, keynote speeches, workshops, committee meetings, special breakfasts and lunches, social mixers and receptions, and so much mingling and networking! #entsoc15

I particularly look forward to this conference each year for the great diversity of people that attend – from academic, government, industry, non-profit organizations – and the resulting opportunities for networking! I met scientists from large and small, public and private universities, from government agencies like USDA and county-wide extension, and from the private sphere of corporations and advocacy groups. The ESA meeting is great for students at all stages for exploring future prospects and networking with potential collaborators and employers.



This year, I co-organized a member symposium with C. Scott Clem of Auburn University (Scott just defended his M.S. thesis, woohoo!) on Arthropod-Mediated Associational Effects Among Native & Non-Native Plants. Our symposium was held on Wednesday morning and was a fantastic success! We hosted eight presentations from student and professional scientists from around the globe. Check out our speakers below:


Scott Clem presented “Can interactions between native and non-native trees in urban landscapes influence herbivore abundance and diversity?”

I, Sarah O’Neill, presented “Invasive annual promotes spillover of invasive herbivore on to native perennial”


Doug Tallamy presented “The impact of non-native plants on insect herbivore alpha and beta diversity”


Matt Greenstone presented “Plant provenance and natural enemy diversity: Parasitoid and spider data from a residential-scale experiment”


Andrew Merwin presented “Resource density, resource frequency and herbivore density: Assembling associational effects from behavioral choices”


Andrea Litt presented “Influence of patch size and neighborhood composition on arthropod communities in the face of plant invasion”



Phil Hahn presented “Plant frequency generates associational effects by altering grasshopper foraging behavior”


Martijn Bezemer presented “Disentangling the aboveground and belowground associational effects of native plants on aboveground insects associated to ragwort”


Many sincere thanks to Scott Clem for co-organizing, and to all our speakers for their contributions. See you all next year!

Oh, the places you’ll go: Anza-Borrego Desert State Park

Earlier this year, I completed a survey of the plants and arthropods in a weed management area of Anza-Borrego Desert State Park.

The University of California Steele-Burnand Anza-Borrego Desert Research Center in Borrego Springs, CA.

The University of California Steele-Burnand Anza-Borrego Desert Research Center in Borrego Springs, CA.

This particular area along Henderson Canyon Road in Borrego Springs, right in the center of Anza-Borrego Desert State Park, has been used as a demonstration plot for the effects of hand-weeding Brassica tournefortii (Sahara Mustard) over the past ~10 years. Every winter, a team of volunteers sweeps the area removing all of the seedling plants before they grow, flower, and reproduce. I visited 3 times over the growing season – check out the dramatic change in the landscape in these photos each 1 month apart!

Similarly to my other field survey experiment (click here) I use pitfall and pan traps to collect arthropods that walk, crawl, fly and otherwise move through the study site.

The traps are left out for 24 hours, then all the specimens captured are collected and brought back to the lab.

In the course of studying the flora and fauna of this area, I had the great pleasure of viewing flowers upon flowers, and insects galore!

The field season is long over for Anza-Borrego – it’s a very short field season! The organisms in this harsh desert come and go quickly while conditions are favorable, and wait out the long, hot summers as seeds, eggs, dormant live stages, or even migrate somewhere else. Here’s hoping for a good rain year in 2015-2016 to bring up lots of desert wildflowers next winter and spring!

*This research was supported by the Anza-Borrego Foundation Howie Wier Memorial Conservation Grant (2014).

How many bugs does it take…?

Started a new experiment last week, with the gracious help of two research assistants, Stephanie and Bryan. To be honest, this is not the first time I’ve tried to do this experiment. It’s the 3rd or 4th time now…

Why has it not worked in the past? Mostly, because it requires so many bugs! Seriously, thousands of Bagrada hilaris adult bugs. The goal is to determine threshold levels of herbivory – the number of bugs feeding, and for how long – required to kill a native plant, Atriplex canescens.

This is one hardy plant, I might add. It grows throughout western North America, in the hottest and driest deserts, and even in salty dry lake beds. Indeed, it’s often one of the few (or only) plants found growing in salty soils!

Add to that, Bagrada hilaris doesn’t particularly care for it. It’s not it’s favorite or the most appropriate food for its survival, reproduction and development.

To make this experiment work, we have to continually replace dead Bagrada hilaris with new live individuals. This adds up, fast.

Here’s how it all works:

"Bugdorm" cages holding plants, and hundreds of Bagrada hilaris bugs feeding on them

“Bugdorm” cages holding plants, and hundreds of Bagrada hilaris bugs feeding on them

1. Grow thousands of Bagrada hilaris in captivity. I use bugdorm cages to contain a few live plants, and lots of Bagrada hilaris that feed on the plants, and reproduce inside the cage.

2. Search each bugdorm cage thoroughly and remove every adult Bagrada hilaris with a handheld aspirator (see pictures above). Aspirators work by vacuum pressure generated through suction applied to the long clear tube, sucking up the targeted bug through the metal tube inlet and into a small plastic vial. Debris, bugs and other unwanted material are trapped behind filters in the vial so that they aren’t sucked into my mouth!

Carefully counting each Bagrada hilaris adult into containers of 100 adults each

Stephanie is carefully counting each Bagrada hilaris adult into containers of 100 adults each

3. Count all of these adult Bagrada hilaris, and place them into smaller mobile containers of 100 bugs each.

4. Transfer the appropriate number of carefully counted Bagrada hilaris adults onto their experimental plants.

5. Wait! Once everything is set up, my job is to watch and wait. I check on the plants and bugs everyday, recording the response of the plants to the bugs feeding on them. Every week, all the dead Bagrada hilaris are counted and removed, and replaced with new live Bagrada hilaris adults.

Many, many dead Bagrada hilaris

Many, many dead Bagrada hilaris

Fingers crossed everything goes as planned, and we’ll have some exciting results from this experiment in a few months time!

Oh, the Places You’ll Go: Oasis de Los Osos Reserve

Today’s post is all about the work I’ve been doing at a University of California Natural Reserve in eastern Riverside County, Oasis de los Osos. For nearly 2 years, I’ve been visiting Oasis de los Osos regularly to monitor plants and arthropods.

Oasis de Los Osos Reserve Sign

Welcome to the Oasis de los Osos Reserve! Source:

Oasis de los Osos is a small reserve (65 hectares) adjacent to the Snow Creek community of Palm Springs, CA. A perennial stream “Lamb’s Creek” runs through the reserve and supports dense riparian vegetation, amphibians, and many other critters seeking reprieve from the desert heat. The remains of a former home sit alongside the stream near the southwestern edge of the reserve; it was allegedly built in the 1930s by Denver and Lucy Ellen Lamb (see this site for more details).

Snow Creek House

The old Lamb house at Lamb’s Creek. Source:

My monitoring study is set up to assess the plant and arthropod biodiversity that occurs close to and distant from the perennial stream, and changes that occur along its drying portion. In particular, I aim to figure out how plants and arthropods respond to the presence and dominance of the invasive weed, Brassica tournefortii.

Twelve transects are established at every 100 meters along the stream, from the perennially flowing region at slightly higher elevation, to the seasonally wet/dry region below. Within each of these transects, five plots are set up at 0, 4, 8, 16, and 32 meters away from the stream edge. This design allows me to compare the identity and diversity of plants and arthropods that occur at varying positions along the stream, and at varying distances away from it.

Snow Creek Transect Map

A Google Earth view-map of my monitoring study area at the Oasis de los Osos reserve. Each red balloon indicates the twelve transects where plants and arthropods are regularly censused November – June.

At each plot, I have set up one pitfall trap and one platform for a set of pan traps. Pitfall and pan traps are used to passively sample arthropods that occur in the area. Pitfall traps capture critters walking along the ground, and are often used when targeting detritivores, predators, and shy critters. Pan traps capture flying arthropods that come in to land on what they likely thought was a flower; pan traps are generally used when targeting pollinators and other flower-visitors. Because they sample different kinds of arthropod critters, I use both kinds of traps at each plot to assess what kinds of animals live there.

A pitfall trap (left) in the soil, and one set of three pan traps (right) mounted on a platform above the soil.

A pitfall trap (left) in the soil, and one set of three pan traps (right) mounted on a platform above the soil.

Pitfall and pan traps are left open for 24 hours each census period. At this time, the plants growing at each plot are also censused to document variation in which species are present.

Several people help me with each census – 60 plots is a lot! We each set up traps at a few transects on the first day, carefully opening pitfall traps, attaching pan traps, and filling them with soapy water. Hiking with water jugs is made much easier with several helping hands.

On day 2, we each work at a few transects to collect all of the specimens captured in the pitfall and pan traps at every plot. The soapy water and contents are emptied from the trap cups into special baggies to be transported back to the lab, all the cups are collected, and the pitfall traps are closed and covered up to prevent any critters getting in there between censuses.

We’ve found some pretty interesting critters in these traps! There have been 30 different orders of arthropods identified (An order is a taxonomic grouping level above family but below class. For example, ants and bees are in the order Hymenoptera, while butterflies and moths are in the order Lepidoptera).

Back at the lab, all the samples are transferred to glass vials with ethanol and given labels to keep track of where and when they came from.

Once all specimens are transferred into glass storage vials with ethanol, they are ready to be examined under a microscope, identified and counted. This data will be used to understand the arthropod community at Oasis de los Osos, how it changes in response to the plant community, time of year, and other environmental variables.

Stay tuned for updates as more data is generated and analyzed!

A beautiful view of the Oasis de los Osos reserve, Lamb's Creek, and the San Jacinto Mountains

A beautiful view of the Oasis de los Osos reserve, Lamb’s Creek, and the San Jacinto Mountains

Unless otherwise noted, all photos used here are my own. Please feel free to comment or contact me with questions!

Update: Additional information and press on this research and the Oasis de Los Osos Reserve can be found here and here.

Oh, the Places You’ll Go: San Clemente Island

Recently, I had the privilege of visiting San Clemente Island. San Clemente is one of the Channel Islands, off the coast of southern California. It’s south of Santa Catalina and is not visited by tourists.

San Clemente Island, the whole island, is a base of the U.S. Navy. As a military base, and particularly as an island military base, the entry of persons is highly restricted and carefully regulated. So, when the opportunity to visit San Clemente Island presented itself, I took it!

My opportunity came thanks to two friends and colleagues in Dr. Erin Rankin’s lab (

Sheena is a post-doctoral researcher who studies bees and pollination ecology, and has recently begun a suite of studies on the island to identify who the pollinators are, what plants they are pollinating, and how important those pollination services are to the plants. Some of these plants are threatened or endangered, and knowledge like this can make a huge difference for ultimately helping to prevent their extinction. (Check out Sheena on twitter @sheenabees).

Korie is a graduate student in Dr. Rankin’s lab, and truly the expert among us on all things San Clemente. Korie has worked on San Clemente Island as a field biologist with Soil Ecology and Restoration Group (SERG) for 5 years. She knows the plants by sight, and is primarily responsible for the eradication of argentine ants, Linepethima humile, from the entire island (this is the topic of her thesis).

The process of getting to the island is indeed exactly that. First, Sheena and I drove from Riverside to San Diego, where the navy base is. There are physical checkpoints, my own background check had been completed earlier, and Sheena has a special ID badge that identifies her as an authorized visitor. We park at the airport on base, drop off our luggage, check in and meet up with Korie. When our flight is called, we walk on to the tarmac, up the steps and into a small plane that could hold maybe 24 passengers. I’ve never been on a plane this small and I am anxious about turbulence and my sensitive stomach. Luckily everything stays down.

At the end of the short flight, the first thing that truly strikes me about the island is the caterpillars. There are so many caterpillars, literally everywhere.

On our first day of fieldwork, we surveyed areas throughout the eastern, mesic side of the island for Castilleja grisea through the overcast morning and into the afternoon. Castilleja grisea is commonly known as San Clemente Island Indian paintbrush. It was historically very rare and listed as a federally endangered species.  It was recently downgraded to a threatened species due to increased population size and reduction in threats. Management efforts to support its recovery, including the removal of feral goats and pigs from the island, appear to have been successful. Today, Castilleja grisea is far more common than it once was.

At our first site, we found several clumps of our target plant quite quickly. Our next site proved to be a bit trickier. We walked across ridges, up and down hillsides, around canyons and inlets searching, unsuccessfully. We heard the barks and calls of an endemic Channel Islands fox, and soon after watched it trot along the opposite edge of a canyon from us until it was in clear view. It stopped for pictures and to watch us for a moment before heading off in the opposite direction.

We continued on our search for Castilleja grisea, following in the direction of the fox. Eventually, we found it! Yay!

We even saw large digger bees (Habropoda depressa) visiting the flowers.

We break for lunch at a perfect spot overlooking the ocean and an impressively steep canyon.

Later, Sheena and I visit a few more sites on the opposite side of the island. The western side is noticeably different. It’s drier and rockier, there is much more bare ground and less plant cover, little grass and so much cactus! Despite our best efforts to carefully avoid the cholla cactus all over the place, we end up with spines and brittle cladodes in our shoes, and sometimes on our pants and hands. We found the two populations we expected, and it is evident how challenging this arid environment is for Castilleja grisea. While we found populations of 50 – 200 individuals in the east, we found populations of 2 and 4 in the west.

The next day, we set up vane traps all over the island. Sheena has established a transect that runs north-south along the length of the island, to most effectively sample the diversity of pollinators there. We start at the southernmost end and work our way north all morning, hanging the traps from electric poles and garden hooks. The traps are yellow and blue; these two colors are the most attractive to insect pollinators like bees. The insects fly toward the trap, encounter the blue panels attached to the lid and fall, fly or crawl down into the yellow bowl where they are submerged into soapy water. Vane traps are left up for at least a whole day at a time, in order to ensure that pollinators flying at all times of day and night are sampled.

On my last day on San Clemente, we headed to the southernmost end of the island to visit populations of another of Sheena’s focal plants, Sibara filifolia.

These teeny tiny plants are endemic to San Clemente Island, Santa Catalina Island, and Santa Cruz Island, but they haven’t been found on Santa Cruz Island in about one hundred years.  This species is federally endangered and there are only hundreds to thousands of plants present each year depending on rainfall. Sibara filifolia is thought to have been driven to extremely low population sizes by the trampling impacts of grazing animals, similar to Castilleja grisea. Sheena set up a field experiment using naturally occurring individuals to determine whether pollination increases the fitness of these plants (this can be measured in multiple ways, including greater seed set, greater seed mass or greater seed viability). She used small fabric “tents” to exclude pollinators from being able to visit experimental individuals, while leaving neighboring individuals out in the open.

While we were there, Sheena collected the mature fruits of pollinator-excluded and pollinator-allowed plants to compare them back at the lab.

By late morning, it’s time for me to pack up and get ready to head home. A few days of work on the island was fun and went by quickly, it was a fantastic place to visit. Sheena drops Korie and me off at the airport for our flight; she stays on island for the remainder of the week to continue fieldwork. Once back on mainland California, Korie takes me to a great taco truck in San Diego.

I order too many fish tacos, they are delicious and I am quite full. It has been a great weekend.