How to kill a fireworm

The blackheaded fireworm lies in wait on the underside of the cranberry’s leaves. It will stay there all winter, secluded in its egg until it emerges in spring to feast on the soft green flesh of the newly sprouting growth.

It will decimate the cranberry plant, and then—after it turns into a moth and lays more than 200 eggs of its own—its progeny will commence with their own destruction.

For decades, a cranberry farmer’s main defence has been synthetic insecticides sprayed from above. But a new saviour could be coming: a virus re-animated from cold storage and brought to the leaves of cranberry shrubs to wage war against the invading fireworms.

It sounds like the start of a science fiction film. It’s actually how BC cranberry farmers could protect their crops in the future from one of the industry’s most common and destructive pests.

In 1992, scientist Sheila Fitzpatrick was trudging through a Richmond cranberry field when she noticed the liquified remains of a small caterpillar. The bug had once been greenish-yellow, with a shiny black head, but by the time Fitzpatrick found it, the caterpillar was oozing apart on the stem of a cranberry plant. She noticed more and more of these dead and diseased insects scattered throughout the field—a massacre of the blackheaded fireworm, the cranberry industry's most common pest.

Fitzpatrick collected several hundred of the caterpillars and brought them back to her lab at the federal agricultural research centre in Agassiz. There, she and fellow scientist David Theilmann worked to figure out exactly what was killing the insects. The pair discovered that 60% of Fitzpatrick’s specimens were infected with a new kind of baculovirus—a rod-shaped virus that infects specific insects. The pair published their findings in 1995, and the virus was put into Theilmann’s freezer for 30 years, waiting for someone else to take an interest in it.

Michelle Franklin did.

Franklin’s entire job involves her studying the bugs that plague berry plants. As a small fruit entomologist at the Agassiz Research and Development Centre, she has studied numerous pests and viruses targeting the Fraser Valley’s crops. (Her research includes work on aphids and blueberry scorch virus and the invasive strawberry blossom weevil, both of which we reported on in the past). She helped create a biopesticide called Loopex, which uses another kind of baculovirus to kill the caterpillars that eat cabbages and broccoli.

When she started at the federal research station in Agassiz in 2020, she applied for funding to take the virus out of Thielmann’s freezer and bring it back into the lab. Now, the virus is thawed and Franklin’s research is underway to figure out how to turn a microscopic enemy into a commercial pesticide.

The fireworms and the baculovirus

Despite the dragon-esque name, blackheaded fireworms are common in cranberry fields.

Like cranberries themselves, Rhopobota naevana are native to North America. Also known as the holly bud moth, the blackheaded fireworm begins its life as an egg on the underside of a cranberry leaf. In spring, an egg laid the previous fall hatches into a worm-like larva that feeds on the new cranberry growth. Those larva and its brethren exist as worms only for the spring, eating the fresh foliage and creating small tents from the plant’s leaves. They then transform into moths, which lay another set of eggs between late May and mid-July. The second generation of larvae continue to eat the plant, this time attacking the ripening fruit as well as the leaves.

When a plant is overrun by fireworms, its tissues can become dried out and look as though it was scorched by fire. In especially warm years, there can even be a third generation of the moths before winter.

Although by no means the only cranberry pest, blackheaded fireworms are widespread across Canada and the United States. And they can be expensive and challenging to manage.

The most common way to get rid of blackheaded fireworms is to spray a field with pesticides. There are a number of chemical pesticides that conventional farmers can use to stop fireworm infestations before they begin. The pesticides can be effective, but are also expensive.

Other options are limited, particularly for organic farmers. More-natural methods of reducing fireworms include briefly flooding a field before eggs hatch, using pheromones to confuse moths during mating, or releasing tiny parasitic worms called nematodes onto the plants. But the methods aren’t very effective. (Researchers in Quebec are currently investigating whether parasitic wasps can be used to kill pests.)

The hope is that the cold-storage baculovirus in Agassiz’s lab could be the basis for a new organic pesticide that targets blackheaded fireworms without affecting other species. So far, that seems to be the case.

The baculovirus that had been in Thielmann’s freezer for decades is specific to the blackheaded fireworm—something that is typical for baculoviruses. They can be picky about which invertebrates they use to replicate. They are also not harmful to people. In fact, a 1973 study of the cabbage looper virus—which Franklin helped turn into a commercial biopesticide—showed that American cabbages in large supermarkets were typically covered in seven million individual viral particles per square inch.

“People are eating them all the time,” Franklin said about the baculoviruses. “They don’t impact us.”

The reason is, partly, in the gut.

A caterpillar first has to eat the virus before it can become infected. The virus is coated in a protein that protects it from the elements. The protein dissolves in places with a high pH, like the gut of a fireworm. A human’s digestive system, on the other hand, is acidic. So in a human stomach, the protein coating remains intact and the virus emerges on the other side whole and unreplicated.

Blackheaded fireworms are not so lucky. With the protein coating dissolved in their digestive tract, the virus begins reproducing. Over the course of a week, the virus takes over the entire caterpillar, turning its internal organs to goo and causing it to climb to the top of the cranberry bush. Once the fireworm is turned into a husk filled with viral ooze, an eventual rainfall will break the body apart. Virus particles slide down the cranberry bush, infecting more unsuspecting larvae.

Farming for pesticides

Since taking the virus out of storage, Franklin and her team have made significant progress towards developing a commercial spray. They first tested the spray at the BC Cranberry Marketing Commission’s Delta research farm over the summer, and although the scientists haven’t had a chance to dig too deep into the data, Franklin says it looks as though there were more dead fireworms on the experiment plots than elsewhere.

(You can learn more about other experiments by Fraser Valley scientists at the cranberry research farm here.)

Franklin plans to test the product again in Delta this year; another research group will do their own experiment in Quebec to see how cranberries there respond. There is even an international pesticide company, Andermatt, interested in potentially bringing the spray onto the market.

But there is a catch.

Franklin’s baculovirus spray is a biopesticide, meaning it is made out of natural materials (in this case, a virus and some diluting solution). And unlike human vaccines, which either mimic a virus, contain a weakened virus, or use a dead virus, the biopesticide needs a full-bodied, fully living viral load to infect the caterpillars. The only way to get that is to farm the virus.

In Franklin’s lab at the Agassiz Research and Development Centre, SFU student Jenelle Breen is now breeding blackheaded fireworms, raising them from egg to moth. When they hit the larval stage, she chooses some to rear into adulthood and others to sacrifice at the viral altar in the pursuit of more virus.

“The blackheaded fireworm is a bit of a tricky insect to rear in the lab,” Franklin said. “A big part of our whole research project … is developing an insect-rearing protocol that can be ramped up to meet commercial-scale production of the virus.”

The moth’s life cycle is one challenge, as the second generation of insects typically overwinters as an egg. “We have to find innovative ways that we can get them through that dormant state in the lab,” Franklin said.

Food is another major challenge. Right now, Breen uses cranberry plants frown at the Agassiz Research Centre to feed the larvae. She hopes to create an artificial diet for the fireworms, to make feeding them more efficient.

Franklin doesn’t know how soon they will be able to scale up their moth-farming operation, and make a commercially-viable virus production line. (Making the biopesticide itself is fairly simple: collect the dead caterpillars, pick out the chunks, dilute the remainder in a solution, and spray.) She said it’s unlikely to be ready in the next year or two, but hopes that in the slightly longer-term, the product will be available to farmers in Canada.

“We’re really excited about the sustainability aspect of this project. It’s safe, and it has a really low environmental risk,” she said. “We really want to see this product be put onto the market for providing a sustainable pest management tool for cranberries.”