mites on bugs

Uncovering the infamous shroud over biological control

By Dhaval Vyas, Sustainability Leadership Fellow and PhD Candidate in the Department of Bioagricultural Sciences and Pest Management.

The next time you’re peeling an orange, squeezing a lime over some tacos or enjoying a glass of lemonade, take some time to thank the vedalia beetle (Rodolia cardinalis).  This little insect was the hero in one of our country’s first successful attempts at introducing one organism to control another organism (i.e., biological control).  Since then, land managers have used an assortment of animals and germs to control unwanted visitors.  Some of these efforts succeeded, others were disastrous; nonetheless, biological control remains a valuable tool for ensuring food security, public health and conservation of endangered species.

Why pay homage to the vedalia beetle?  In the late 19th century, hordes of people moved into California with dreams of striking it rich in the gold mines.  As the demand for food increased, several crops were brought from the eastern US and grown, for the first time, in California.  Citrus fruits were one of many imported crops and, like these other crops, the citrus trees were attacked by insects that the plants had never experienced.  Any defenses that the plants had evolved to combat insect pests from the eastern US were useless in California.  One particular insect, the half-inch long cottony-cushion scale (Icerya purchasi), decimated citrus groves and brought California’s fledgling citrus industry to its knees.  The cottony-cushion scale’s origin was traced to Australia and researchers traveled down under to see what kept this insect from destroying crops in its home range.  They found that the vedalia beetle, a type of ladybug beetle, devoured cottony-cushion scales and helped prevent the scale from overcoming plants1.  Unbeknownst to most people, ladybug beetles and their young are voracious predators!  The vedalia beetles were brought by boat to California and these beetles saved the citrus industry, which today gives us tasty breakfast beverages and helps makes a certain Mexican beer drinkable.

 

An adult vedelia beetle next to a female cottony-cushion scale with her white egg sac (A) and a young (larval) vedalia beetle next to cottony-cushion scales without the egg sacs (B).

Thanks to the domestication of plants, humans must intervene to save crops from their enemies. Domestication does two things to plants: it makes them tasty and it weakens their defenses2.  The two outcomes can be intertwined because many of the plant chemicals used for defense against being eaten are the same chemicals that produce nasty tastes.  For example, let’s take the beloved cabbage.  The round heads of red and green cabbage are all domesticated types of wild cabbage (Brassica oleracae), which is in the family of plants known as the Brassicaceae.   Nearly every type brassica plant is armed with glucosinolates, a family of chemicals that can be toxic and disgusting when eaten3.  Glucosinolates are the plant’s way of saying, “The first bite’s free, but afterwards, you’ll pay dearly!”.   With the power of domestication on our side, we bred out these nasty chemicals so that the plants could please our taste buds.  Domestication of cabbage gave rise to crop varieties including broccoli, cauliflower, Brussel’s sprouts and kale; all of which are actually different parts of a cabbage plant!  There still exist some brassicas that have kept their chemical defenses and you can actually taste them, just grab a mouthful of some raw mustard greens!

An infographic showing six varieties of vegetables that are all derived from domesticated wild cabbage (Brassica oleracea). [Image adapted from Business Insider; minor modification for clarity]
The majority of our food comes directly or indirectly from plants that have had their chemical defenses weakened from domestication.  This is one of the reasons why we rely so heavily on chemical pesticides.  If we’re going to feed six billion (and growing) humans, we can’t share our food crops with pests!  However, we’re also concerned with the harm caused by pesticides to environmental and human health.  Integrated Pest Management (IPM) is becoming a more common alternative to “spray and pray” approaches for reducing food loss from crop pests.  The key to IPM is that it aims to control the pests, not necessarily to eradicate them from the environment.  Social, ecological and economic outcomes are all in consideration for managers that use IPM, which is often guided by ecological and evolutionary principles (e.g., food web interactions and evolved resistance to pesticides).  One of the IPM strategies is biological control, which reduces the harm caused by pests by introducing the pests’ natural enemies.  Enemies can be predators, pathogens (viruses, fungi or bacteria) and parasitic insects.  Ecologists are often tasked with finding the most effective and safest natural enemy based on its natural history, biology and potential for pest control.  Once one or more biological control agents have been identified and tested for effectiveness, the species are released in the area affected by the pests.

Most people cringe at the notion of introducing an organism from one place into another.  This reaction is likely from hearing about biological control efforts that failed to resolve the pest problem and ended up creating a worst-case scenario.  The kingpin of such examples is the cane toad (Rhinella marina).  Its original habitat is found along riverways in Central and South America.  However, in the 1930s, the cane toad was introduced throughout the Caribbean and some Pacific islands, including Hawaii, to reduce damage to sugar cane caused by beetles4.  The cane toad introductions were successful, and as the toads gorged on beetle grubs, sugar cane farmers saw the desired reduction in losses from beetle damage.  Australian officials were smitten with the success observed in these smaller islands, so they decided to bring the cane toad to Australia to control beetle problems in their cane fields.  Unfortunately, the Australian beetle grubs fed high atop the sugar cane plant where the toads failed to reach.  The cane toads, hungry and unable to feed on the beetle grubs, satisfied their appetites by turning their attention to a diversity of Australian species, leading to declines in several species found only in Australia.  Mongooses introduced to control rabbits and cats brought to control rats are some additional failures peppered throughout the history of biological control.

The lessons learned from fiascos have guided contemporary biological control efforts, and it continues to be a valuable tool for managing natural resources.  There are numerous success stories where introductions of natural enemies to pests have saved entire agricultural industries and helped conserved endangered species5.  Given the complex process of getting food from seed to our plates, we need creative and responsible solutions that are informed by responsible science.  Federal and state agencies are now in charge of overseeing all organisms proposed for use in biological control.  Gone are the days when you could release animals or other organisms without any regulations.  Strict review procedures, quarantine periods and rigorous testing all precede the release of any biological control agent.  The unforgettable disasters are a constant reminder that successful biological control requires a tempered approach.

Most people are unaware that many economically important crops are highly vulnerable to being wiped out by a single threat.  Inbreeding and monocultures have made many crops defenseless against attack from pests.  Skeptical?  Try stomaching the current state of bananas: http://www.bbc.com/news/uk-england-35131751.  Rubber, coffee and chocolate all share a similar fate.  International traffic allows us to crisscross the globe and this movement guarantees the continued spread of organisms between countries.  One unintentional benefit of biological control is that it allows us to study how organisms react after arrival into a new region.  To make biological controls effective, it’s critical to know what enables an immigrant species to establish or causes it to perish after its arrival.  Controlled introductions can reveal answers to these questions and help us understand the growing phenomenon of transcontinental movement of species.

As I write this blog post, citrus farmers in the US are nervously awaiting the start of the 2018 growing season.  The country’s citrus industry is once again under attack, this time by a bacterium that’s spread by an insect: the Asian citrus psyllid (Diaphorina citri).  The bacterium causes citrus greening disease, also known as Huanglongbing or yellow dragon disease, which makes citrus trees produce green and bitter tasting fruits6.  Once infected, trees die within a few years.  The disease was discovered in 2004 in Brazil, and in 2005, it arrived in southern Florida.  Citrus greening has affected over a million acres of citrus orchards in eight US states and in several African and Asian countries.  The cure continues to evade scientists.

In the meantime, the vedalia beetle has left six big shoes to fill.

A local newspaper in California alerts its readers of the causes and consequences of citrus greening disease.

References:

  1. Caltagirone, L.E. & Doutt, R.L.  1989.  The history of the vedalia beetle importation to California and its impact on the development of biological control.  Annual Review of Entomology, 34: 1-16.
  2. Chen, Y.H., Gols, R. & Benrey, B. 2015.  Crop domestication and its impact on naturally selected trophic interactions.  Annual Review of Entomology, 60: 35-58.
  3. Lewis, J. & G.R. Fenwick.  1987.  Glucosinolate content of brassica vegetables: Analysis of twenty-four cultivars of calabrese (green sprouting broccoli, Brassica oleracea L. var. botrytis subvar. cymosa Lam.).  Food Chemistry, 25: 259-268.
  4. Lampo, M. & De Leo, G.A.  1998.  The invasion ecology of the toad Bufo marinus: from South America to Australia.  Ecological Applications, 8: 388-396.
  5. Fessl, B., Heimpel, G.E. & Causton, C.E.  2017.  Invasion of an avian nest parasite, Philornis downsi, to the Galapagos Islands: colonization history, adaptations to novel ecosystems, and conservation challenges.  In: Parker P. (eds) Disease Ecology. Social and Ecological Interactions in the Galapagos Islands. Springer, Cham., 213-266.
  6. Bové, J.M.  2006.  Huanglongbing: A destructive, newly emerging, century-old disease of citrus. Journal of Plant Pathology, 88:7-37.

 

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