When tending to the pumpkin, check the leaves regularly for any eggs on the underside. It is likely that these are squash bugs. Established plants can usually cope with these insects, but younger tender plants can be killed off by them.
If you find any eggs, remove them from the leaf and dispose of them. For hatched insects, you can create a squash bug trap by laying cardboard or newspaper around the plant. Overnight the bugs will seek shelter under the paper, and you can pick them out. If you control the foliage too, birds can help with the removal of unwanted insects.
Young pumpkin plants are vulnerable to slugs and snails, so covering with a cloche is ideal to ward off these garden pests.
Aphids can affect weak, young plants, but these can be washed off and removed. Keeping the plant well-watered and strong will help fend off aphids in the first place.
Mildew can also be an issue later in the season and this is when you have too much foliage reducing flow of air. Remove some leaves to allow birds and pollinators to access the area to eat pests and their eggs. However, due to the timing, the fruits will have likely developed and be ripening so this could impact the growth and taste of the pumpkins.