Monitoring Results
Rates of Mortality
One of the first things that most MLMP volunteers notice about their monitoring numbers is that they see many more eggs than larvae, and more 1st and 2nd instar larvae than the larger 3rd though 5th instars.
See, for example, data collected by Ray Sullivan at his Wauwatosa WI site in 2001 (below); his bars have lots more black, representing eggs, than the five colors representing caterpillars.
This is an extreme example--Ray's 2003 data for June show slightly higher survival--but almost all of our volunteers have similar patterns.
Monarchs are eggs for about 4-5 days and larvae for about 9-12 days, so without mortality, we should expect to see about 2-2.5 times more larvae than eggs. The fact that we don't reflects the high amount of mortality that occurs in the milkweed patch.
One way to visualize monarch survival is to compare the numbers of eggs and late instar caterpillar observed. Dividing the number of 5th instars observed by the number of eggs observed is an approximate measure of survival; since monarchs are eggs and 5th instars for about the same amount of time, a given monarch has an equal chance of being observed in either stage if it stays alive until it becomes a 5th instar. We have done this for all of the years of the MLMP in the upper Midwest, where we have the most volunteers and thus the most reliable data.
The graph below shows the proportion of 5th instars relative to the number of eggs observed, and thus provides an estimate of monarch survival from the egg to the 5th iinstar.
We're not sure what causes the variation from year to year, but we can conclude that only about 10% of monarch eggs make it to the 5th instar. This mortality can result from several sources: monarchs' interactions with their milkweed hosts, predation and parasitism, and environmental factors such as extremely dry or hot conditions.
References