SONAR is definitely effective on Eurasian watermilfoil. HOWEVER, our circumstances in the South are considerably different by June 1st, compared to the conditions found in central and northern regions. Our water temps are now in the upper 80's+, untreated plant biomasses are comparatively high by now, and most plants have long-since passed their peak growth-rate stage: all of which will impact the efficacy of SONAR on submerged plant species.
SONAR's mode-of-action is dependent upon post-treatment plant growth - the more rapid the growth, the better the results. As many submerged plants become seasonally mature, they become less responsive to SONAR treatments. That is why early-season treatments (rapid plant-growth periods) of SONAR are far more effective than mid or late season treatments (duckweed is an exception). The optimum SONAR treatment-period is usually dictated by the climatic conditions for the region in question (Feb-May in TX).
So, it is wise to keep the vastness of the US in mind (and the differing regional "seasons") when making treatment recommendations that depend upon a plant's physiological response, which is governed by regional climates and any abnormal fluctuations.
In simpler terms: a SONAR treatment for EWM at this time of the season (in Texas) will be very slow to do its thing � 60 to 120 days before �control� can be expected (lots of patience required). In the mean time, the infestation must be tolerated � and there�s always a risk that when the entire plant population does fall-out (around Aug/Sept), the DO level could go with it (without aeration to offset to O2 demand).
Alternately, AQUATHOL or REWARD could be used to rapidly take down sequential portions of the plant population. But, treat too much, too quickly and the DO will suffer the consequences.
SONAR is usually the less expensive treatment, PER-ACRE-TREATED, when the plant infestation occupies MORE than 25% (+/-) of the pond/lake. But, the pond/lake needs to be relatively static � ie. unlikely to receive flow-through events for 30+ days after a treatment. If it doesn�t fit this requirement (as in Lambo's situation), SONAR is a gamble (especially with ponds/lakes designed for storm water retention). In such cases, the other alternatives should also be considered.

Kelly Duffie
Cypress, TX