As you might guess, the Witch Hazel Aphid causes galls to grow only on that plant.
This Gall is Found on Only One Species of Plant
![Witch Hazel Aphid galls.](https://npr.brightspotcdn.com/dims4/default/5a8533f/2147483647/strip/true/crop/800x534+0+0/resize/880x587!/quality/90/?url=http%3A%2F%2Fnpr-brightspot.s3.amazonaws.com%2Flegacy%2Fsites%2Fwltr%2Ffiles%2F201606%2FWitchHazelAphid_Galls.jpg)
John Cooper, via Wikimedia Commons
As you might guess, the Witch Hazel Aphid causes galls to grow only on that plant.