A Wasp Wonder

I went looking for cocoons of Looper caterpillars to photograph, and found this.  It is a caterpillar  that has had wasp eggs laid in it.  The parasitised looper curls into an S shape after spinning a cocoon, but will fail to pupate itself.

This wasp, Copidosoma floridanum, has an interesting story that almost made me feel sorry for the caterpillar….Its a a polyembrionic parasitoid that lays its egg in the egg of the caterpillar which then serves as a host for thousands of wasp larvae that will feed off and pupate themselves inside its body.

Copidosoma genus (Copidosoma ratzeburgi)Fig 194. Pg 472. Fauna Iberica. Ricardo Garcia Mercet. Madrid, 1921

The lifecycle of the Copidosoma floridanum wasp.

The parasitic Copidosoma wasp lays one or two eggs in the egg of the Chrysodeixis Moth.  Unlike most eggs, these don't have any yolk or shell, as they will be protected and fed by the Moth egg and later larvae.

Day 1: Wasp lays 2 eggs in newly laid moth egg

When the looper larva emerges from its egg about about 3 days after it was laid, and begins to feed, the Copidosoma wasp eggs begin to reproduce mitotically until there can be thousands of eggs genetically identical to one of the original pair laid inside the caterpillar.  The collection of wasp embryos, called a polymorula is implanted near the caterpillars trachea so they can use the oxygen the host breathes. 

Day 3: Caterpillar hatches and wasp embryo starts to multiply

Over the next 8 days the caterpillar grows and moults 3 times (referred to as being in its 4th “Instar”). The wasp embryos keep multiplying all that time. A small proportion of wasp embryos will hatch and become soldier larvae during this time and they protect the other embryos.

Days 3 to 10: Wasp eggs are multiplying, some hatch as soldiers

Before the last moult of the host caterpillar (in the second half of the fourth instar) most of the wasp embryos begin to undergo morphogenesis, developing into larvae. On the second day after the caterpillars final moult, in the 5th instar (11 or 12 days since the egg was laid) the wasp eggs all hatch and the larvae start to feed on the host - its fluids first, obtaining oxygen from them, and then they eat the entire inside of the caterpillar, once they can breathe for themselves.

Day 11 or 12: Wasp larvae emerge inside the caterpillar and begin to consume it.

The chemicals produced due to the presence of wasp eggs and larvae caused the Looper caterpillar to live longer as a larva (it is a caterpillar for 5 instars instead of the normal 3), and then eventually to build a cocoon, by day 14, just before it dies. 

Day14: Only the skin of the caterpillar remains, full of wasp larvae.

The wasp larvae pupate themselves inside the hosts caterpillars now empty exoskeleton, protected by the cocoon it built.  2 weeks later, adult wasps emerge.  

The shape of the caterpillars head and some legs can still be seen.

Biological Caterpillar Control

The wasps are harmless to humans and are often given as examples of helpful parasitoids that control the population of Looper caterpillars organically. (But they would probably have been killed if I had sprayed with insecticide to control the caterpillars).  However, studies suggest that they may cause more crop damage because they prolong the life of the caterpillar. 

Caterpillar skin broken open, showing white wasp larvae. The wire on the right is about 1mm wide.

Record numbers of Offspring

A Copidosoma floridanum wasp has the record for having the largest recorded brood of any parasitoidal insect - 3,055 individuals.  They are able to have so many offspring in one batch because the embryos multiply themselves mitotically. The wasp only has to carry one or two eggs and they take up a minimum of space in their host by using the moth eggs for food and protection rather than each having their own yolk and “shell”.

Soldier Spite Behaviour

The lifecycle of the C. floridanum is of interest as a model that can be studied to learn more about the evolution of families. The soldier larvae of the C. floridanum are also studied as examples of spite behaviour.

Up to 24% of larvae are soldiers, who are infertile and will die when their siblings develop into pupae. They serve to protect the eggs and larvae from those of other wasps, and to reduce competition in its own siblings.  If the original egg was fertilised, the offspring are all female, and if not they are male. If one of each gender is laid, the female soldiers emerge first and work to destroy male embryos, keeping the female to male balance high.

Unlike other social insects such as bees and ants who show "altruistic" behaviour (because they forgo reproduction in order to help their queen), C.floridanum soldiers show "spite" behaviour in forgoing reproduction and longer life in order to kill others.

Genome Sequenced

The genome of the Copidosoma floridanum is being sequenced by the Human Genome Sequencing Centre as part of the i5K project.  It was chosen because it has a number of special qualities.

• it is found world wide, 
• it is of interest to agriculture for biological pest management, 
• it is already studied as the the best known example of insect polyembrony and its unique caste system, 
• the nature of its relationship to other important insects like the honey bee in the evolutionary tree,
• it produces thousands of genetically identical offspring so that there is plenty of identical DNA to work with

What a big story for such a tiny creature found in my garden!

I have not been able to find any copyright free images of the Copidosoma floridanum species; instead here is one of Encyrtus Vitis, a relative in the same family Encyrtidae.

Male (left) and Female (right) Encyrtus Vitis syn

Plate 395. Volume 3. British entomology Published by John Curtis

In searching for a vintage illustration, I found a gem of a book The Life of the Caterpillar, by J.Henri Fabre, published in 1916.  In it he writes of the Pieris rapaea caterpillar (Cabbage White Butterfly) that is parasitised by Microgaster glomeratus.  It is a similar relationship as between C. floridanum and Chrysodeixis eriosoma, except that the wasp larvae leave the dying caterpillar, but remain attached and feed off it until they pupate.  He writes in such poetic language that I have included an excerpt about the synchronicity of the development of both host and parasite.

..."They are stoics who do not forget their duty in the hour of death.".......

Looper Caterpillars

This is one of the culprits responsible for the recent damage to my veggies. They are Vegetable or Green Loopers (Chrysodeixis eriosoma)*.  Their “loopy” name is because of the way they move -  they lack legs in the middle of their bodies so when they walk they hunch up in the middle, forming a loop.

When small they are green all over, but develop stripes as they get older, and can grow up to 4cm long. The amount of time they spend as larvae or caterpillar form, varies from 2 to 4 weeks, depending on the temperature, and the quality and quantity of food available.

Vegetable loopers feed on a wide range of plants, and mainly consume the leaves, but they will also attack  fruits, like tomatoes or beans, and flowers.  When small the larvae feed on only one side of the leaf, leaving translucent feeding windows, but as they develop, they chew holes in the leaf, and along the leaf margin, down to the central ribs. 

Control of Green Loopers (Chrysodeixis species)

Commercial crops, especially ones that are not used for their leaves, such as beans or peas, are often not treated as the amount of damage due to defoliation can be tolerated. 

For home gardeners the best method is manual removal of caterpillars, which is easiest when they are active in the cooler parts of the day.  To save your crop, early detection, before they can destroy too many leaves is going to be needed. (although its fine to eat the remainder of the leaves they have chewed holes in).  The removal of cocoons and eggs also helps.

The caterpillars cleverly lie along the veins of leaves and can blend in amazingly well. Look for holes, or the windows that the smaller caterpillars make. Piles of their droppings can be another clue to track them down. They spin loose cocoons on the back of leaves which are easy to remove.

Most general insecticides will kill them, but because they have a lot of natural predators, such as parasitoids, some bugs, spiders, lacewings, ladybird beetles and other predatory beetles, that may also be killed in the process, using them may not reduce the presence of caterpillars in the long run.  Products such as Dipel (Bacillus thuringiensis (Bt)) while being a better choice, because it is more selective to caterpillars, will only work when Vegetable Loopers are small (less than 12mm).

Other Organic Controls

• Keep plants healthy with fertilisers and Seasol (seaweed liquid) - Vigorously growing plants will be better able to compensate for flower and fruit damage, and damaged leaves will be replaced more quickly.  
• On healthy plants caterpillars will need to eat less and move on from the larvae stage more quickly, so the damage is less (but presumably once they become moths they will come back and lay their eggs again more quickly)
• Good plant spacing (between the same species, or others that the loopers prefer) can reduce the transfer of caterpillars (and make spraying easier if necessary).
• Removal of unhealthy or finished plants and leaves to limit transfer of caterpillars.
• Trap crops - sacrificial plants that the loopers prefer.  The caterpillars eat them instead.  They must be removed once infested before caterpillars start to look elsewhere. Gardening Australia have a Fact sheet about these and dead end trap crops here.
• Reduce access by moths so they can't lay eggs in the first place, or by caterpillars, with plant netting or fruit bagging.
• Plant Resistant varieties - different forms, colour, natural chemicals, cell thickness or leaf surface coverings can make a difference to plant susceptibility or how much is consumed if attacked.

Links for more details and further advice about Looper control:

• Gardening Australia Fact sheet: Friends and Foes
• Gardening Australia Fact sheet on Caterpillar Control
• Loopers in field crops - Queensland Department of Agriculture, Fisheries and Forestry
• Ausveg article on Caterpillar Control
• T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network - Chrysodeixis eriosoma (Green garden looper)

Note: No caterpillars were harmed in the taking of these photographs. (However they were humanely squashed soon after)

* I think these are (Chrysodeixis eriosoma), but they might also be Tomato Loopers (Chrysodeixis chalcites). Entymologists, who would be qualified to judge the difference, are unsure whether the two species might actually be allotropic and conspecific - which would mean they have developed differences as a result of having bred in isolation from each other, but they are the same species. 

And now, a vintage technical illustration to end with: