Friday Cat-erpillar Blogging: Compatible Caterpillar Conditions

Elm Span Worm

Elm Span Worm Caterpillars Rest in a Parking Spot in Winnipeg
Photo: CBC

My friends in Manitoba, Canada are experiencing outbreaks of not one but two tree defoliating caterpillars this summer. After experiencing smaller but increased numbers of Forest Tent Caterpillars in 2013, populations in 2014 are creating notable defoliation. Forest tent caterpillars are cyclical. Outbreaks often are separated by a decade or more. An outbreak can last for multiple years (usually 1-2). Outbreak populations of forest tent caterpillars end when viral disease such as NPV skyrockets in the population or levels of fungal diseases or parasitoids increase. Virus, which only affects caterpillars and not people, can remain in the environment suppressing caterpillar populations for years.

Elm Spanworm (Ennomos subsignaria) defoliation makes the forest tent caterpillar defoliation pale in comparison. The spanworms are now fifth instar larvae and are defoliating trees, dropping from the branches and massing on objects in downtown Winnipeg and other areas of Manitoba. Outbreaks occur when weather conditions are favorable and parasitoid populations are low. Elm Spanworm populations are usually limited by mortality from egg parasitoids. The egg parasitoid, Ooencyrtus ennomophagus is credited with ending an outbreak in Connecticut in the 1970s. In Pennsylvania, the 1993 outbreak of elm spanworm completely defoliated over a million acres of forest.

The city will spray with the BT bacterial to kill caterpillars, but many spanworms have already formed their pupa stage or stopped eating and will not be affected by the spray. Moths will emerge later in the summer to mate and lay eggs. Residents hope the parasitiod populations will be high enough to stop the outbreak.

About jjneal

Jonathan Neal is an Associate Professor of Entomology at Purdue University and author of the textbook, Living With Insects (2010). This blog is a forum to communicate about the intersection of insects with people and policy. This is a personal blog. The opinions and materials posted here are those of the author and are in no way connected with those of my employer.
This entry was posted in by jjneal, Caterpillar Blogging, Environment, Pest Management. Bookmark the permalink.

1 Response to Friday Cat-erpillar Blogging: Compatible Caterpillar Conditions

  1. Pingback: Friday Cat-erpillar Blogging: Friendly? Flies | Living With Insects Blog

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