Do food and egg density levels have an effect on intraspecific interactions that could lead to cannibalism among monarch (Danaus plexippus) larvae?

The North American monarch (Danaus plexippus) butterfly is an iconic species. The eastern and
western populations have declined dramatically over the last two decades. Scientists have
proposed that billions of additional milkweed stems are needed to maintain viable populations.
Larval development begins with an egg and progresses through 5 stages (instars). Cannibalism
has been observed between monarch larvae; first instars cannibalizing monarch eggs has also
been observed. We wanted to know if cannibalism would occur while rearing larvae of the same
cohort on different quantities of food. We also wanted to know which instar would be most
likely to cannibalize monarch eggs. We hypothesized that larval interactions and cannibalism
would increase as the food quantity decreased; and that first instars would cannibalize more
eggs. For the food quantity study, we reared five larvae starting at instar one, in five different
treatments with decreasing food quantities, with approximately four replicates each. We made a
total of 20 observations of their interactions, rating the intensities. Our results showed the
intensity of observed interactions increased as the food quantities decreased. For the egg
cannibalism study, we had five treatments, each instar, with thirty replicates each. We provided a
leaf with ten eggs to a single larva for twenty-four hours. Second instars were observed
cannibalizing the most eggs. This study helps to understand monarch behavior when more than
one larva is present on the same milkweed stem. These results suggest a need to reduce
intraspecific interactions by adding more milkweed stems to the landscape.