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>& Mary’s suggestion to add 'non-target’ to the 'bt spruce budworm’ search does indeed reveal hundreds of finds, many of which did look specifically at the effect of Bt on non-target organisms. I skimmed a few of these but it would take quite a bit of work for someone outside the field to get a sense of the size and importance of the ‘bycatch’ problem with Bt spraying. Peter’s concern about the effects of dry conditions on breeding success of insects is timely, and this must be important. Adults do drink water, but larval stages for herbivores get most of their water from the food plant and metabolism of the food, so should be OK initially unless the leaves shrivel up; some insects can also absorb water through the cuticle contact, and even from moist air when the relative humidity (RH) exceeds a certain level. The main concern then would be water loss under dry conditions, which is dominated by evaporative losses from the spiracles of the tracheal system, which depends in turn on the inevitable interaction of ambient temperature and local RH; also, some insects like flies lose water by excreting liquid waste. Evaporative losses are almost directly proportional to something that’s a useful practical indicator called the saturation deficit (the difference between the saturation vapour pressure of a body of air at that temperature, and the actual vapour pressure encountered, low at low RH). So if the RH is really low at the height level larvae occupy, they will lose a lot of water. Actual water content of different insect species varies a lot (~50% to >90%). The level where some common insects studied succumbed and died occurred when this had fallen to around 60% water content. Under lesser extremes, metabolism and development are greatly retarded by water loss under low RH, from the egg stage on up. For example, the moth Ephestia needs 33 days to complete the larval stage at 75% RH, but 50 days at 33% RH; its water content is reduced from 74% to 57% by this shift*. All this interacts with temperature e.g. from heating by the sun, and the ability to resist lethal temperatures, usually up in the mid-40°C range. So from a variety of interactions, it would be expected that conditions of extreme dryness and high temperatures would lead to retardation and even failure of development in normally abundant insect species. Steve (Hfx) *the usual starting point for this type of detailed info is V.B. Wigglesworth’s compendious 'The Principles of Insect Physiology' (7th (final) edition, 1972, Chapman & Hall), e.g. Chapter 14, ‘Water and Temperature’. On Aug 27, 2016, at 10:17 PM, pce@accesswave.ca wrote: > Without wishing in any way to distract us from discussion of the consequences of spraying Btk, I wonder if there is more than one significant mechanism affecting butterfly numbers this summer. > > It was a very dry summer in many parts of the province. Is there any connection between amount of rainfall and the success of larval and pupal life-stages? It does seem that after a decent rain we get a flush of Cabbage butterflies, almost as if they have been waiting as pupae for some moisture to help them complete their development and emerge as adults. > > Does anyone have any knowledge about the effect on lep pupae of rainfall? Does it affect emergence rates somehow? > > Peter Payzant > > > > On 2016-08-27 9:53 PM, Bev Wigney wrote: >> Last night, I did find a couple of references to Bt killing of non-target insects, and Leps in particular. >
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