Good Parasites II

In Good Parasites I, we asked you to "Imagine what would happen if a destructive caterpillar had no natural parasites. 

Gypsy Moth- Wikimedia

Consider the success of the gypsy moth, Lymantria dispar.  It has spread across Eastern North America in the last hundred years and seems unstoppable.  During periodic population surges of the caterpillar, they cause the death of thousands of acres of trees by defoliation.  Most birds avoid the hairy caterpillars.  More importantly, they left all their normal parasites back in Europe and Asia when they made the trip here so they have no consistent parasites to control their numbers.*

In the absence of an effective native parasite, more than 45 foreign species have been introduced in a futile and occasionally desperate attempt to control the gypsy moth.  One tachinid fly,  Compsilura concinnata, was introduced as a known parasitoid of the gypsy moth and other introduced species such as  the satin moth and brown-tail moth.

Compsilura concinnata

  The  gypsy moth is univoltine (undergoes one generation per year), but the flies are multivoltine, having three to four cycles a year.  This means that only one cycle will feed off gypsy moth larvae and to survive the year, they have to find other caterpillar larval food sources, attacking species which pupate later.  Also, their larva must overwinter in a larval host.  Since the gypsy moth overwinters in the egg, Compsilura has to find another host. 

Some of the alternate hosts are garden pests such as the cabbage looper, Trichoplusia ni and the imported cabbageworm, Pieris rapae L.   Unfortunately, other hosts include swallowtails, Nymphalidae (brushfooted butterflies) and Saturniidae (giant silk moths).  Parasitism rates can be as high as 81% in the cecropia moth and 68% in the promethea moth.**

The lesson of all this-
"A Good Parasite is hard to find,
you always get the other kind.
Just when you think he is your pal,
You look for him and find him fooling 'round some other gal"
                  - with apologies to Eddie Green.

*    Missouri Natural Areas Newsletter Vol. 11 No 1 2011
**  University of Minnesota