According to a study published in PLOS ONE, genetically modified Bt maize are more susceptible to aphids.

Within five out of six pairs that were evaluated, transgenic maize lines were significantly more susceptible to aphids than their near-isogenic equivalents, with the remaining pair being equally susceptible. The aphids feed from the phloem sieve element content and analyses of this sap in selected maize lines revealed marginally, but significantly higher amino acid levels in Bt maize, which might partially explain the observed increased aphid performance.

However, depending on the level of aphids in a region, this could be a beneficial, natural method of pest control against caterpillars:

Aphids produce honeydew, a sugar-rich substance that can be used as a food source by beneficial insects, such as the parasitic wasp Cotesia marginiventris. This parasitoid helps the plant when it is attacked by caterpillars. It kills these pests by laying its eggs in them. In cages with aphid-infested Bt maize, Cotesia wasps lived almost twice as long and parasitized 37.5% more caterpillars. Hence, an increase in the number of aphids might help to control caterpillars in areas where these are a major problem.

However, the researchers warn that in regions where aphids are considered to be a pest, growing Bt maize may not be advisable. Aphids mainly damage plants by transmitting viruses and using Bt maize might amplify this problem.