Here’s a list of some of the main pests that can affect onions:

Insects:

1. Onion maggot (Delia antiqua)
2. Thrips (Thrips tabaci)
3. Onion fly (Eumerus spp.)
4. Cutworms (Agrotis spp.)
5. Aphids (Aphis spp.)

Fungi:

1. White rot (Sclerotium cepivorum)
2. Downy mildew (Peronospora destructor)
3. Pink root (Phoma terrestris)
4. Fusarium basal rot (Fusarium oxysporum f. sp. cepae)

Viruses:

1. Onion yellow dwarf virus (OYDV)
2. Iris yellow spot virus (IYSV)
3. Onion mosaic virus (OMV)
4. Tobacco rattle virus (TRV)

Other pests:

1. Nematodes (Ditylenchus dipsaci)
2. Bulb mites (Rhizoglyphus spp.)
3. Rodents (such as voles and mice)

It’s worth noting that the severity and impact of these pests can vary depending on the specific region and climate, as well as the cultivation practices used.