DEEP: The Gypsy Moth - Outbreaks and Natural History

The Gypsy Moth
Outbreaks and Natural History
The gypsy moth first came to North American around 1868.  Its natural range is in Eurasia.  In Europe, the moth has been known to be a problem since at least the 1600's.  Here in North America, it has found a niche and is established in the northeastern parts of the United States and adjacent parts of Canada.  It continues to spread west and south - states such as Minnesota have ongoing programs that seek to prevent the insect from invading their woods. 
However, this is a battle that Connecticut lost in the 1910's and 20's, after it was first found in Stonington in 1905.
{Chart showing defoliation in Connecticut}
If the gypsy moth stayed at low population levels, it would be of little more than passing note among naturalists. At low population levels, natural controls work to keep the gypsy moth population in check. This includes a range of predators and parasites. Wasps parasitize the egg masses, while beetles, birds and mammals consume each of the various gypsy moth life stages.  Diseases such as that caused by the nuclear polyhedrosis virus (NPV) are also a factor.
However, the gypsy moth has the ability to go into outbreak mode at regular intervals.  In Connecticut, 1971 and 1981 are remembered as particularly bad years.  In 1971, defoliation topped 650,000 acres, while in 1981, that number reach 1.5 million acres.  That is nearly 80% of the state's 1.9 million acres of forestland.
{Deer Mice consuming gypsy moth caterpillars}
white-footed mice eating larvae
It is not known for certain why the gypsy moth goes into outbreak mode.  One hypothesis ties the start of an outbreak to a low year of mast production.  'Mast' refers to the accumulated seeds of forest trees such as acorns, beech nuts and hickory nuts.  In the forest, mast is the primary food for a variety of wildlife, including white-footed mice (Peromyscus leucopus).  When the gypsy moth population is at a reduced level, these mice are able to keep the moth population down by feeding on a proportion of larvae and pupae.  While this feeding by the mice helps keep the gypsy moths in check, it does not substantially influence the number of white-footed mice, as the gypsy moth is not a critical food source for the mice. 
The amount of mast available in the forest is more important in regulating the mouse population.  When the mast crop declines, the mouse population also drops in number.  When this happens, the gypsy moth population, freed from being consumed by mice, begins to increase in numbers.  
Because gypsy moths are not a favorite food, the rise in the gypsy moth population does not necessarily translate into an increase in the number of mice.  Once the gypsy moth population gets past a point at which mice are able to keep them in check, the growth of the gypsy moth population tends to accelerate, until it achieves outbreak status. 
The pattern of collapse of the gypsy moth population on the other end of an outbreak is better known.  At low moth population levels, a virus (NPV) is often persistent within the population, also at low levels.  After the moth population reaches outbreak status, the NPV virus tends to spread widely within the moth population.  However, there is usually a lag to this occurring, of 2 or 3 years.  Once NPV catches up with the moth population, it infects and kills moths in great numbers.  This infection occurs during the caterpillar stage.  It is this epidemic of NPV that, for most of the history of the gypsy moth in North America, has caused the gypsy moth population to collapse. 
{Dead caterpillars on a tree trunk}
most of these caterpillars are dead,
killed by the maimaiga fungus
The emergence of the maimaiga fungus (Entomophaga maimaiga) in 1989 changed this dynamic greatly.  This fungus is originally from Japan.  It was released several times in the US as a potential biological control agent, going back to 1910.  After each trial, its effects on gypsy moths appeared to be minimal.  That is, until 1989.  In that year, to the surprise of everyone, this fungus emerged from oblivion and immediately began killing gypsy moth larvae on a wide scale. 
The first known occurrence of this happening was in Connecticut.  The spring of 1989 was exceptionally wet in the state.  Since that year, in Connecticut, the fungus has re-emerged each year on its own, proving that it is able to sustain itself in the wild.    
The maimaiga fungus is effective against gypsy moth caterpillars because the annual life cycles of the two organisms interconnect very well.  The maimaiga fungus persists in the soil and leaf litter as a resting spore.  In the spring, when there is an adequate amount of rain, humidity in the soil and leaf litter activates the fungus.  
As it turns out, starting around the 3rd instar, the gypsy moth caterpillar spend a significant amount of time on the ground, as they seek shelter during the hotter parts of the day.  When these larger caterpillars begin descending to the ground, these activated spores have no problem finding and infesting them.
After being infested with the maimaiga fungus, a certain number of the caterpillars begin to climb back up a tree before succumbing to the fungus.  From this perch, as the dead caterpillar decomposes, the fungus releases a second, wind-borne type of spore that spreads of the disease even further among the surrounding gypsy moth caterpillars.  Once it gets going, the fungus tends to move quickly through the population.  (See a video of the fungus in action.)
At the end of the gypsy moth season, the fungus produces a resting spore that then accumulates in the soil and leaf litter, to await the next year.  These resting spores have been shown to remain viable in the soil and leaf litter for at least ten years. 
This cycle of infection by the maimaiga fungus has kept the gypsy moth population in Connecticut at a low level for the past two-and-half decades.  Each time the population of the insect began to grow, this fungus has managed to prevent it from becoming anything more than a localized outbreak.
That is, until the recent drought.
{Chart of rainfall by month in Hamden CT}
Note that rainfall for the spring months is below average in both 2015 and 2016.
Chart created using data collected by the CT Agricultural Experiment Station.
With much of Connecticut suffering under drought conditions in 2015 and 2016, the number of localized gypsy moth outbreaks have increased and begun to spread.  This is attributed in large part to the maimaiga fungus not being activated.  There is concern that, if 2017 is as dry as the previous two years, the situation regarding the gypsy moth could intensify. 
There is one other aspect of the forest mast, mice and gypsy moth story that deserves mention.  White-footed mice are known to be a main reservoir for the bacterium that causes Lyme disease.  While the number of gypsy moth larvae or pupae tend not to affect the mice population directly, the gypsy moth does tend to influence how well the oaks and other forest trees produce their seed crops.  A sufficiently large outbreak of the gypsy moth can lead to a decreased forest mast crop in the subsequent years, which will then lead to a decrease in the number of deer mice in the forest.  As a result, gypsy moth outbreaks have been associated with declines in the number of local cases of Lyme disease.