Abstract
Recently, resistance to the pyrethroid bifenthrin was detected and confirmed in field populations of western corn rootworm, Diabrotica virgifera virgifera LeConte from southwestern areas of Nebraska and Kansas. As a first step to understand potential mechanisms of resistance, the objectives of this study were i) to assess adult mortality at diagnostic concentration-LC99 to the pyrethroids bifenthrin and tefluthrin as well as DDT, ii) estimate adult and larval susceptibility to the same compounds as well as the organophosphate methyl-parathion, and iii) perform synergism experiments with piperonyl butoxide (PBO) (P450 inhibitor) and S,S,S-tributyl-phosphorotrithioate (DEF) (esterase inhibitor) in field populations. Most of the adult field populations exhibiting some level of bifenthrin resistance exhibited significantly lower mortality to both pyrethroids and DDT than susceptible control populations at the estimated LC99 of susceptible populations. Results of adult dose-mortality bioassays also revealed elevated LC50 values for bifenthrin resistant populations compared to the susceptible control population with resistance ratios ranging from 2.5 to 5.5-fold for bifenthrin, 28 to 54.8-fold for tefluthrin, and 16.3 to 33.0 for DDT. These bioassay results collectively suggest some level of cross-resistance between the pyrethroids and DDT. In addition, both PBO and DEF reduced the resistance ratios for resistant populations although there was a higher reduction in susceptibility of adults exposed to PBO versus DEF. Susceptibility in larvae varied among insecticides and did not correlate with adult susceptibility to tefluthrin and DDT, as most resistance ratios were < 5-fold when compared to the susceptible population. These results suggest that both detoxifying enzymes and target site insensitivity might be involved as resistance mechanisms.
Highlights
The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is considered the most important insect pest of field corn (Zea mays L.) in the U.S Corn Belt with estimates of over $1 billion in yield loss and control expenditures annually [1,2,3,4]
Similar relationships between larval progeny from these populations and the standard lab control colony in larval bioassays indicated that susceptibility differences were a heritable trait. These results provided initial evidence that resistance to bifenthrin has evolved in field populations that have been exposed for multiple years to pyrethroid insecticides
During 2014 and 2015, adult mortalities of all field populations to bifenthrin, tefluthrin, and DDT bioassays were statistically lower than the susceptible non-diapause and susceptible field populations collected from Saunders County, NE except for the Clay County population in the 2014 DDT bioassay (Fig 2)
Summary
The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is considered the most important insect pest of field corn (Zea mays L.) in the U.S Corn Belt with estimates of over $1 billion in yield loss and control expenditures annually [1,2,3,4]. The WCR has evolved resistance to many control methods including insecticides, crop rotation, and corn hybrids expressing Bacillis thuringiensis Berliner (Bt) toxins [7,8,9,10,11,12,13] making sustainable management difficult. In the western U.S Corn Belt, continuous corn is the predominant cropping pattern and often the most profitable under irrigation; crop rotation options are limited. This practice often leads to build-up of WCR densities and annual pest challenges [14] (Meinke et al 2009). In this system, a combination of tactics is needed to effectively manage rootworms. Insecticides are important complementary tactics to Bt corn to prevent silkclipping or reduce rootworm densities [15] (Meinke 2014)
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