Table 3. Examples of each magnitude of non-target impact on plants from weed classical biological control agents.

	Target species	Non-target species	Cause & predictability	Potential threat	References
Minimal	Ulex europaeus L.	Genista monspessulana (L.) L.A.S. Johnson	Deliberate release of two populations of Cydia succedana (Denis and Schiffermüller), one showed limited development on weeds including this one; Predictable	None, majority of examples	[24], [61]
Minor	Hypericum perforatum L.	Hypericum concinnum Benth	Deliberate release of Chrysolina quadrigemina in California causing damage and varying impact on H. concinnum, it is still common; Predictable	None, some examples	[80]
Moderate	Carduus nutans L.	Cirsium altissimum L. Spreng	Deliberate release of Trichosirocalus horridus causing damage on non-target native thistles; Predictable	Too early to tell; impact is uncertain (moderate impact may be too high); rare.	[81], [82]
Major	Carduus nutans L.	Cirsium canescens, Nutt.& Carduus	Deliberate release of Rhinocyllus conicus in the U.S. mainland following host range testing proving its safety for crops. Evidence of local population decline of Cirsium canescens; Predictable	High likelihood of some attacks on ∼28 species of native thistles; otherwise rare.	[19], [83]
Massive	Opuntia lindheimeri, Engelm., O. stricta, (Haw.) Haw., O. triacantha (Willdenow) Sweet	Opuntia spinosissima P.Mill.	Accidental release of Cactoblactis cactorum in the U.S. mainland following introduction against cacti in the Caribbean; severe feeding impact (threat of extinction without intervention?); Predictable	High likelihood of attacks on ∼87 native cacti species (too early to tell for most); rare.	[27], [45], [46], [70]