When you make a living growing produce, these are critical questions that demand answers. While it’s true that any of the scenarios mentioned will provide control, it’s the level of control that’s going to impact your bottom line. Insect populations and the resultant levels of crop damage can vary greatly from one approach to another.
Researchers will tell you that each of these strategies has to be evaluated on a case by case basis. Key variables include region, time of year, the pesticide(s) being partnered with Bt, and most importantly, the pest complex you’re up against. As a general rule, however, tank mixes often provide the best control but wind up being the most expensive option. So if you apply Bt products in rotation, which partner should come first?
That’s where resistance can play a big factor. Whatever the conventional partner, it’s generally accepted that some portion of the existing population is already resistant. If you partner Bt with a pyrethroid, for example, and apply the pyrethroid first, you’re going to select out the resistant population with the first application before coming back in with the Bt. By the third application, your level of resistance in the resultant population will be proportionally higher because the pyrethroid was used first.
Conversely, using Bt as the first partner will delay this effect – a result that can have significant effect on marketable yield. Bt will knock down the entire population and only upon the second application – with the conventional product – will the selection take place. Coming back in with Bt as the third application means that two of the first three applications have sidestepped the onset of resistance. By the end of the season, the ordering of these early applications can bring about a dramatic effect on the pest population and marketable yield.