Sericulture Biological Control Part One: The Background

The production of silk (also known as sericulture) is a huge worldwide business which supplies millions of people with jobs every year and produces billions of dollars worth of imports and exports every year. In 1997, the US imported about $ 2 billion in silk products. Although silk makes up about a fifth of a percent of the global textile trade, its importance cannot be understated because raw silk fetches about twenty times more per weight than raw cotton.

Silk is unique among the textile world in that it’s a naturally produced fabric that comes from an animal. Many animals produce silk, mostly larvae of various moths as well as some arachnids (most notably spiders, but also mites). The vast majority of silk on the market comes from the silkworm moth Bombyx mori which is a species which is believed to have been cultured from Bombyx mandarina sometime about 5,000 years ago. It’s an interesting evolutionary story, but one we’ll have to explore another time.

B. mori was first domesticated by the Chinese sometime around 5,000 years ago and as the new millennium dawned, sericulture began to spread around the world as a result of the demand for silk prices and industrial espionage. This meant many new opportunities for people trained in the art of sericulture, and unfortunately those new opportunities for people also meant new opportunities for parasites. If we’ve learned anything from cockroaches it’s that if we give insects an inch in the form of a semi-habitable habitat with strongly reduced predation they’ll take a mile and refuse to let it go no matter how many nerve gasses we bomb them with. Unfortunately, the types of pest sericulture attracts are a lot more bothersome-and gruesome than cockroaches.

Life as a caterpillar is tough. You rarely make it to adulthood. If you’re lucky, you get eaten by a bird. If you’re not lucky, you fall prey to a parasitic wasp or a tachinid fly. Sure it sounds odd that I’d consider being ripped apart by a bird a decent way to die, but if you’re eaten by a bird at least it’s quick. If you happen to be attacked by a parasitic wasp or fly, you’re eaten from the inside out over a period of about two weeks. Remember the movie Alien? Yeah.

Exorista bombycis (also known as ujifly or uzifly) is a tachinid parasitoid which attacks either Bombyx species. The female who finds her victim by smelling it’s poo (or frass, as entomologists call it) lays eggs on the outside of the silkworm, usually in folds of skin. The thing to remember is that if you’re a silkworm, you’re pretty much a bag of fat reserves with a pair of mandibles. You can try to whack the fly as hard as you can, but the fly is a whole hell of a lot more mobile than you are so unfortunately, you can put up a hell of a fight if you want…you’re still pretty much defenseless. Fifteen hours to four days later (development depends on temperature), the egg hatches and the fly maggots begin to burrow into you and eat you from the inside out for about a week or so. The larvae are actually pretty big…about half an inch or so and there are more than one per host. They burrow out of you and turn into a pupa about half a day later in some nice, secluded area.

This fly is actually a pretty serious pest. Each female can lay between 100 and 1,000 eggs, so as you can imagine, things will get out of control very quickly during an infestation. Unfortunately, people don’t inspect their silkworms as much as they need to and that’s how this pest spreads. E. bombycis was first recorded as a pest in the north-eastern sericultural regions of India and was best known in Bengal and Assam. As a result of poor inspection of transported animals, the pest was carried all the way to the Karnataka province and first recorded in 1980. Two years later, an across the board average of 40% losses were recorded and many silk producers faced almost an 80 to 90 percent drop in silk production. Many were also put out of business by repeated destruction of their silkworm crops.

Silk producers face a unique challenge from the standpoint of pest management because this environment is unique to pest control. Their main product is an insect which is reared indoors, so they can’t exactly spray an insecticide on their field like my neighbor occasionally does. Insecticides aren’t entirely out of the picture…ovicides have been developed and used with success and there’s a bacterium (a strain of Baccillus thuingiensis) which is pathogenic to the ujifly. Sericulturalists do their best to fly-proof rooms with nylon screens and install fly-proof window screens to block the flies from rearing facilities. Sterile releases (remember this post?) have also been tried, and workers try to remove infected silkworms, as well as any E. bombycis maggots and pupae they find. They also try to use various attractants to lure the flies to their deaths, but they still keep on coming.

So what’s a silkworm farmer to do? You can’t use pesticides…you’ll kill your crop. You have some luck with the ones you can use, but they’re definitely not enough. You’ve done your best to block the pest from your facilities…but let’s face it. They’re flies. No matter how hard you try to keep them out of your house during the summer, you eventually find one dead on the window screen or running into a window, trying to get out. Tachinids are good at finding their hosts. They’re parasites…it’s what they’ve evolved to do. You need something which is good at finding tachinids. You need a parasitoid that parasitizes your parasitoid and nothing else. You need a hyperparasitoid.

T.K. Narayanaswamy, R. Govindan (2000). Mulberry Silkworm Ujifly, Exorista Bombycis (Louis) (Diptera: Tachinidae) Integrated Pest Management Reviews, 5 (4), 231-240 DOI: 10.1023/A:1012982030848

Environmentally Friendly Pest Control: The Vedalia Beetle and Cryptochaetum iceryae

Biological control is exactly what it sounds like in a nutshell...that is, the practice of using organisms to control organisms. It's cheap and effective quite a bit of the time. Biological control depends on the density of the pests to be effective. The more dense the pest population, the easier time that whatever control organism used (predator, parasite, pathogen, etc.) will have finding it's victim. We have quite a few exotic pests that reach incredible population densities quickly such as the soybean aphid, the gyspy moth and the fire ant. All of these are susceptable to biological control (and I'll be working on articles detailing methods to control them with bacteria, nematodes, parasitic flies, wasps and ants at a later date ;)). However to explore biological control fully in context, we first need to go back to the beginning. The very first case where biological controls were investigated, implimented and showed results. We must also investigate a spectacular failure. We'll do both in the coming weeks.

Agriculture has always been important to the US economy. I live in Iowa and my hometown is surrounded by countless miles of cornfeilds. Currently, I really do live next to a cornfeild. Many of my classmates grew up on farms. Agriculture isn't just a cornerstone of the US economy, it's a cornerstone of the world's economy. After all, we need food to live.

Unfortunately for us, many insects love agriculture as well. And why wouldn't they-we grow big plants and pump as many nutrients into them as we possibly can. There aren't a whole lot of insects that can substist in an environment such as farms that almost completely dissapear every year and begin anew the following year. So most insects aren't actually pest species. However, there are about 600 (out of a few million or so) which become regular problems in the US.

Some of the most major pests are in the Homoptera. These insects have mouthparts which peirce the plant and suck sap out of it. Aphids are an example. It's not exactly the damage which most people associate with insects...most people think of insect damage as the skeletonization and bite marks we associate with japanese beetles or catterpillars. The damage isn't exactly dramatic or even obvious, but large numbers of these insects will remove enough nutrients to stunt growth and reduce yields.

So...back to agriculture. Back to the olden days of the 1850s. The California gold rush really helped to populate California. However, as the gold mines stopped producing people switched to agriculture. They experimented with nearly everything, including pesticides in an era when pesticide development was still in it's infancy.

Unfortunately as mentioned earlier, when you start growing large amounts of crops insects aren't far behind at all. And due to the fact of international trades, sometimes we have pests show up and we have no clue where they came from. These are the worst types of pests because many native pest levels are controlled by so-called natural enemies. Things like predators and parasitoids.

The specific pest which showed up was a bad one. Not just pain-in-the-ass bad...I'm talking large, prolific, armored and potentially apocalyptic for the then-fledgling citrus industry in California. The pest was called Icerya purchasi. It's a kind of scale insect...kind of like an aphid, but not.

Like an aphid, scale insects have the sucking mouthparts and damage plants in a similar manner. Unlike aphids, they're hermaphrodites...but they self-fertilize so males aren't really neccessary. The adults have a thick, waxy covering which can make control with pesticides difficult. Their young are mobile and the adults are sedentary. The young, known as crawlers, look for a nice place to hunker down and suck sap. The adults keep producing crawlers who can then spread by wind or by crawling from tree to tree.

The scale insects were devastating and threatened to wipe out the California citrus industry in it's entirety because they were so difficult to control. By this time, many entomologists were well aware of the fact that many of our pests were exotic. As far back as the late 1850s, Asa Fitch and Benjamin Walsh suggested using natural enemies to control exotic pests, however they weren't exactly vocal about the practice.

Charles Valentine Riley, a student of Walsh's was the Chief of the Division of Entomology at the USDA. He wasn't exactly a popular figure because of his aggressiveness and ambition. He traveled quite a bit, taking frequent trips to Europe which resulted in congress passing appropriations legislation to prohibit him from traveling.

So you need to find a solution to the exotic pest threatening the extinction of an entire industry, but you can't travel because you can't get the $2,000 to fund your expedition. What do you do?

Well, first you need some ecology 101. You need to know where to look and what to look for. Everything has parasites. Usually multiple parasites. Everything. Grasshoppers are parasitized by nematodes and tachnid flies. Aphids are commonly parasitized by parasitic hymenopterans. However, most parasites/parasitoids are pretty specific; they have evolved to feed on either one species or a narrow range of species. There are also predators that eat a narrow range of insects. Ladybird beetles (Coleoptera: Coccinellidae) are an example. But if you want to find these guys, you need to look for them in their native habitat because that's where these types of relationships will evolve. Many of our exotic pests here in America are not native to our country. They evolved elsewhere, got imported and then began to spread. C.V. Riley was almost certian I. purchasi came from either Australia or New Zealand because it was described in 1878 feeding on Acacia (yay taxonomy-I'm currently eyeing taxonomy as my feild of study...either that or insect evolution).

So...you've hit the books and you know that the populations of this pest are kept in check by natural enemies. You need to get around this whole travel thing. How do you do it?

Well, you ask help from the people you're trying to help. In that day and age, the citrus growers were wealthy, powerful and understandably desperate. Riley knew that he would need about $2,000 for his venture-and back in the day, this wasn't a small sum of money. But thankfully when rich people are desperate, they'll do almost anything to keep their lifestyles intact.

The fruit growers adopted a resolution in favor of sending someone of Riley's choosing to Australia to look for predators and parasitoids. They put pressure on their representatives, who used an international exposition in Melbourne as a way around the troublesome appropriations legislation.

Riley sent Alfred Koeble to look for I. purchasi in it's natural habitat and he eventually found two promising natural enemies-a tachnid fly called Cryptochaetum iceryae as well as a coccenelid which we now know as Rodolia cardinalis that fed exclusively on I. purchasi.

Back in those days, overnight USPS shipping didn't exist. I can tell you from experience (I raise tarantulas) that you should expect casualties when shipping things over long distances even when you're only using overnight to three day shipping. Most invertebrate dealers won't guarantee three day shipping. In this instance we're not talking about a three day journey on an airplane, we're talking about a cross-ocean voyage on a ship. Koeble constructed mini-insectaries on the deck of the ship and housed citrus trees infested with I. purchasi with the vedalia beetles to get them across.

Koeble sent about 12,000 of the tachnids and only 129 R. cardinalis. The tachnid flies proved to be less effective in controlling the beetles everywhere except in cooler climates. R. cardinalis, however turned out to be a voracious and effective predator. Those 129 specimens turned into about ten thousand, and then exploded to millions within a few years. Imports of citrus out of California nearly tripled, from 700 carloads per year at the peak of the infestation to 2,000 carloads per year. Not too bad for $1500.

To this day, the beetles and flies still do a pretty good job of keeping I. purchasi in check despite the fact they're competitors. The flies don't parasitize the beetles and the beetles don't eat scales infested with flies. The immature stages of C. iceryae are resistant to cold weather, which means they become active sooner. This also means that they can control I. purchasi in places like coastal California, where the vedalia beetle doesn't go through as many generations in a year due to the cooler temperatures. In the hot desert areas, the vedalia beetle is the dominant predator and in interior areas like Riverside, the vedalia beetle and C. iceryae switch off between spring and summer with the fly being the dominant natural enemy during the cooler parts of the year and the vedalia beetle being dominant during the warmer seasons.


L E Caltagirone, R L Doutt (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), 1-16 DOI: 10.1146/annurev.en.34.010189.000245

Environmentally Friendly Pest Control: An Overview of The Sterile Insect Technique

In many cases, we can use the insects themselves to aid us in their elimination. One method, called the Sterile Insect Technique (SIT) involves using sterilized insects to drive the population down.

First, insects are raised in rearing facilities to adulthood. Then, males are singled out with a variety of methods (which I won't go into here...feel free to dig up the sources via google scholar), sterilized and released into the field in droves. This is preferable to pesticides because the technique is generally very environmentally friendly and species specific-no chance innocent bugs getting caught in the crossfire or people accidentally being exposed to pesticide.

This technique has been used with great success against the tsetse fly of Africa and on the screw-worms of North America. The tsetse flies (Glossnia ssp.) are best known for transmitting trypanosomaisis, or sleeping sickness as it's commonly known. It's a horrible disease, and some of the treatments are fatal about 10% of the time. The disease is usually fatal in most people who remain untreated. Fortunately, we can control it's vector. If we control it's vector, it can't pass from person to person. If it can't pass from person to person, the disease dies out. Screwworms aren't disease vectors for humans...and don't really affect us but their effects on cattle are even more horrifying. Adults are attracted to open wounds, where they lay their eggs. The larvae hatch, start eating the living flesh, and the scents released while they feed attract more female screwworm flies. At the very least, the cattle lose value and lose weight. Infestations resulting in death weren't uncommon before the measures were taken to eradicate the pest.

In controlling trypanosomiasis and screwworm flies, scientists use radiation (usually Cobalt-60, but also X-rays) to sterilize male flies and release them on the wild population so they mate with females. Many insects mate only once, and this would cause the female to lay infertile eggs or in the case of tsetse flies whose larvae develop internally, expel dead larvae. This causes a drop in the population of the target insect species, and the process is repeated for the next generation. Sooner or later, the number of sterile males outnumber the wild-type fertile males and eventually the population goes extinct.

Unfortunately, there are some drawbacks to this procedure. For example, one has to know that there will not be much migration into an area. If outside females who have already mated can immigrate into an area on a regular basis and reproduce, this tactic only becomes good for suppression. One of the biggest drawbacks is the fact that it's simply not economical. It has to have backing from a wealthy country like the US or United Nations in many cases and it is very labor intensive. This technique has been used for some agricultural pests (pink bollworm and the Mediterranian fruit fly to name a few), but would be ineffective for parthenogenic sawflies and aphids because they do not rely on males to reproduce. So it's not a silver bullet...but for many species which reproduce sexually, it's definitely effective. Many areas of Africa are now free of tsetse flies because of this technique, and the US hasn't seen an infestation since 1966

Many times, the flies are sexed manually when pupae by trained professionals. Other times, insects may be genetically modified with a gene coding for insecticide resistance on the Y chromosome. After this, they need to be carted into packages, refrigerated and dispersed around the habitat...sometimes by air. Of course, the biggest problem is when the equipment breaks down. In one case, a facility using the SIT had it's irradiation equipment fail and accidentally released millions of fertile male screwworms into the environment. These types of mistakes would simply set the project back awhile, but not render it ineffective.

This technique creates a lot of waste in terms of insects. Only about half the insects produced by any given facility can be used for two reasons. One reason is that in many cases, it's the female and only the female who can transmit disease. A female with non-working reproductive parts can and will take blood meals, which means they can still be disease vectors. The second reason is that mixed releases are simply inneffective. The goal is to get the males to spread throughout the area. Unfortunately, male insects have the 'dude in a nightclub' mentality-they won't spread out if they're released with females.

Why go prowling if you can get laid right at home? I've stayed in many a crappy nightclub for no other reason than because I was having success with the women there...and the same principle applies here.

http://www.ars.usda.gov/is/pr/2002/020828.htm

Alphey, L. (2002). Re-engineering the sterile insect technique. Insect Biochemistry and Molecular Biology, 32(10), 1243-1247. DOI: 10.1016/S0965-1748(02)00087-5

Pedigo, Larry P., Rice, Marlin E. (2009). Entomology and Pest Management (6th ed). New Jersey: Pearson Education

Vreysen, M.B., Saleh, K.M., Ali, M.Y., Abdulla, A., Zhu, Z., Juma, K.G., Dyck, A., Atway, M.R., Mkonyi, P.A., , . (2000). Glossnia austeni (Diptera: Glossinidae) Eridicated on the Island of Unguja, Zanzibar Using the Sterile Insect Technique. Journal of Economic Entomology, 93(1), 123-135.

Bacterial endosymbiont is a major causative agent of morbidity associated with filarial nematode infections.

Wolbachia is one of my favorite bacteria, I've actually given it it's own label on the blog. It does all sorts of strange things, like dictate the gender of the host’s offspring. Wolbachia lives in the eggs of it’s insect hosts, being passed on from mother to daughter. If they end up in a male, they dead-end because they cannot be passed on through sperm. Instead, they kill any potential male offspring the female has or makes infected males unable to have offspring with uninfected females depending on what species they infect.

However, Wolbachia is actually a somewhat common endosymbiont in insects and nematodes. Although it’s famous for the effects on insects I mentioned earlier, Wolbachia actually has a major role in medicine despite not actually being a human pathogen.

200 million people around the world suffer from a disease called river blindness. This disease is spread by parasitic flies through the bite. When a fly bites a human, it injects nematode larvae in it’s saliva. Once inside the body, these nematodes form nodules under the skin. They mate and then produce larvae which migrate through the skin, looking to be taken up again by a Simulium host.

The larvae when alive don’t really cause a whole lot of damage. Being a parasite, they’ve evolved ways to become invisible to the host’s immune system. The damage from river blindness comes from when the nematodes die. When they die, they become visible to the host’s immune system which subsequently responds, with many different responses but inflammation is the response responsible for the damage that causes the blindness. If the larva happens to be in the eye when this happens, then this can be very serious.

However, the nematode’s body isn’t the only thing that causes the immune response. Scientists studying Onchocerca volvulus have discovered that the effects of the dying larvae are greatly excasterbated by their Wolbachia endosymbiont.

Scientists took extracts from nematodes, nematodes treated with an antibiotic and nematodes that didn’t harbor Wolbachia and injected them into the eyes of mice. Nematodes treated with the antibiotic doxycycline as well as nematodes which don’t harbor Wolbachia triggered less of an immune response than did nematodes who had Wolbachia endosymbionts. To the scientists, this indicated that something in the Wolbachia cells and not the nematode cells caused the inflammation and other immune responses.

But what in the Wolbachia cells could trigger a response?

Wolbachia are a gram-negative bacteria, which means that a major portion of their outer cell wall is formed from a macromolecule called bacterial lipopolysaccaride (LPS). A receptor called Toll Like Receptor 4 (TLR4) is a major part of response to bacterial infection. Since TLR4 responds to LPS, it was a natural candidate for testing.

To test the link between the Wolbachia LPS and the immune response, the researchers used mice with a mutation in the TLR4 gene that made them less sensitive to LPS to determine if this was the cause. Once again, extracts containing Wolbachia were injected into the mice and their immune responses measured.

As one would expect, those mice with an underresponsive TLR4 receptor displayed less severe symptoms than did mice with a normal TLR4 gene. This means that instead of the nematodes causing the inflammation and other immune responses associated with river blindness, it’s actually their endosymbionts that cause those types of problems.

This opens up a whole new avenue of potential research into new treatments for river blindness. Since we now know that it is not the larvae that cause river blindness we can focus on the real culprit, Wolbachia. The Wolbachia are vital endosymbionts-the nematodes don’t properly develop without the bacteria in the system. This means by focusing our efforts on Wolbachia we not only prevent the LPS from the Wolbachia being released when the filarae die, we drastically reduce the numbers of said filarae because without the Wolbachia the nematodes are unable to reproduce.

Saint Andre, A.V., Blackwell, N.M., Hall, L.R., Hoerauf, A., Brattig, N.W., Volkmann, L., Taylor, M.J., Ford, L., Hise, A.G., Lass, J.H. (2002). The Role of Endosymbiotic Wolbachia Bacteria in the Pathogenesis of River Blindness. Science, 295(5561), 1892-1895.