Oregano and lavender, in my experience, are exceeding drought tolerant and continue to produce nectar under hot and dry conditions. The same can be said about native asters. While there is some research showing that for shallow rooted annuals and perennials, less soil moisture can translate into less nectar and shorter bloom times, I have yet to see lavenders and oregano shorten their bloom period with excessive heat. Nevertheless, a weekly deep soak is good insurance during particular long stretches of hot dry weather (but I write this being on the cautious side - I suspect both plants can handle some pretty hot and dry conditions). 

Just a quick update. I heard from Jen Hayes, from our Garden Ecology lab. You can learn more about her work here:
https://blogs.oregonstate.edu/gardenecologylab/2021/10/14/2021-field-update-natives-nativars/

"We generally see a drop in visitation overall in the heat, but no obvious changes to any plant in particular. During this heat wave, and last year’s, we’ve tried to water all of our plants at least every other day, so that could potentially impact what we see at Oak creek. I’m not sure additional watering is necessary, especially for resistant/waterwise plants, we just don’t want to take any chances with the study plants.

I definitely agree about the lack of activity during the middle of the day, even in mid 80 degree weather it always seems like the insects are taking a “lunch break” with us between 11:30 and 1:00 PM".

Hi Sara,

Native bees certainly visit non-native plants. And length of bloom is also an important consideration (as is whether a plant can attract bees in the summer with limited watering).  In fact, a great study from Eastern Oregon pointed out that if you wanted to attract the greatest diversity of bees the biggest determination was having bee-attractive plants with different flower shapes. As the study points out: "The number and type of visiting bees were not influenced by the plant's native status. However, flower morphology (but not color) significantly affected types of bees visiting plants. Bilaterally symmetrical and medium tubular flowers, with nectar and pollen typically more difficult to reach, were associated with larger bees with longer tongues, while smaller, easily accessible flowers attracted smaller bees with shorter tongues. Our results suggest that certain plants are particularly useful for supporting abundant and diverse bee communities, and increasing diversity in the morphology of blooming plants is a key factor to consider when restoring riparian areas for bee pollinators".

Roof, S.M., DeBano, S., Rowland, M.M. and Burrows, S., 2018. Associations between blooming plants and their bee visitors in a riparian ecosystem in eastern Oregon. Northwest Science, 92(2), pp.119-135.