“How low is too cold for my caterpillars?”

It’s the time of year that the straggler caterpillars, in the more southern US, are outside and may face freezing.  Monarchs, for example, that didn’t go into diapause and migrate, may still be laying eggs if the area hasn’t had a freeze.  But once a freeze is predicted, it’s the time that many of us begin asking that question.  “How low is too cold for my caterpillars?”

Onset of winter
Onset of winter

First, the simple answer.  For Monarchs and those that do not go into diapause as caterpillars, if the lows are above freezing and the day temperatures are above 65-70 F (18.33-21 C), they’ll be fine.  As cold-blooded critters, if temperatures drop too low, they’ll literally freeze.  If temperatures don’t climb high enough in the day, they can’t eat and will either contract disease from their weakness and die or will die from starvation.

Next comes the question of bringing them inside when it is too cold outside.  When they emerge as adults, what if it will be too cold to release them outside?  They can be fed indoors and kept contained.  We recommend Gatorade (not low calorie) as food.  If it isn’t too cold, they can be released.

But the bigger question is this.  Should we bring them in?  That is a personal choice.  There are two basic sides to this question.

1. Some people will save every single one they can save.  They simply do not want one to die.  Period.  They are willing to do whatever it takes to keep them alive.

Feeding Monarch butterflies Gatorade
Feeding Monarch butterflies Gatorade

2. Some people worry that there may have been a genetic glitch that kept them from going into diapause and migrating.  They do not want to pass on that genetic flaw (if there is one) to their offspring to be passed on further, causing this to happen more often.  They will allow nature to make the decision about the caterpillars’ lives.

Frozen Monarch
Frozen (now dead) Monarch with icicles on it

It is a personal choice.  Neither side should judge the other.  Instead, we should all be thankful for all who care about butterflies, whether we agree or not.

From us, we have two words to say to all of you who plant host plants, use less pesticides, teach others, and care about butterflies, whether we agree with your method or not.  Thank you.

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