Primary leaf mowers

Greetings all,
Annually, I try to grow two pumpkin plants (giant variety) in my small yard garden. This year some predator has chewed threw the base of both primary leaves, killing both of my plants. I am saying chewed threw because the base of the leaf is serrated not a clean break. The stalk is in perfect shape as if the top was mowed off. 
I see no evidence of the predators at all.

Does any body know what causes this kind of early damage?

Regards,
Curt Mearns
Albuquerque NM

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