Hy Jorden what i know is that Eucalyptus sp. have Repellent  effect but i think it is not harm for monarch butter flies but  its better do some preliminary expert first and observe the activity of Butter fly. 

I'm not asking if Eucalyptus hurts Monarch butterflies, I'm asking if insects that consume Eucalyptus leaves acquire poisonous compounds in their bodies and therefore become poisonous to predators.

The dried leaves and oil are used to make medicine. as well as used medicinally for many purposes, there isn’t enough scientific evidence so far to rate it as effective for any of them. Eucalyptus leaf is used for infections, fever, upset stomach, and to help loosencoughs. The leaf is also used for treating respiratory tract infections, whooping cough,asthma, pulmonary tuberculosis, osteoarthritis, joint pain (rheumatism), acne, wounds, poorly healing ulcers, burns, bacterial dysentery, ringworms, liver and gall bladder problems, loss of appetite, and cancer. In foods, dried eucalyptus leaf is used as a flavoring agent. even in manufacturing, eucalyptus oil is used as a fragrance in perfumes and cosmetics. It is also used as a mouthwash, antiseptic, liniment and ointment, and in toothpaste,cough drops, and lozenges. Eucalyptus leaf contains chemicals that might help control blood sugar. It also contains chemicals that might have activity against bacteria and fungi. Eucalyptus oil contains chemicals that might help pain and inflammation. It might also block chemicals that cause asthma.

so, from all of above positive effects of Eucalyptus we can assess that if this plant have health benefits as well food quality so leaves of plants don't have negative effect on your insect. 

Dear Jordan Landerman,

Consult the following link.After a comprehensive search i am in position that your have an adorable idea that needs to be checked.Previous evidences are very rear.I think to some extent this publication would satisfy your query.

http://www.ots.ac.cr/rbt/attachments/volumes/vol44-3A/31_Moreira_Eucalyptus.pdf

Looking for natural enemies of the "recently" introduced Eucalyptus globulus wood's (from 1895 as decorative trees and from the 1960s as crop) and namely of their pests (Phoracantha semipunctata and Gonypterus scutellatus) in Portugal, general natural enemies like wood peckers (larvae, pupae), ants (eggs), wolf spiders (adults) and specialized egg parasitoids (Avetianella longoi and Anaphes nitens) have been found. At least these natural enemies could cope with eventual "toxins" from the host plant, if ever those hadn't been detoxicated first by the plant consumers, subject which has not been specially examined though. The question is very interesting and deserves more analyses.

Hello Jordan, 

I have seen Extatosoma tiaratum being bred on Eucalyptus and fed to different lizards. The reaction of the predator did not show differences to the consumption of specimens feeding on bramble. That might be a hint. Additionally Phasmatodea have defensive sprays. I guess for that the consumption of Eucalyptus might be playing a more important role, than for an accumulation (check out Dossey et al. 2009; 2013; etc)