Search

Winter canola Brassica napus L. (Brassicales: Brassicaceae) was introduced to U.S. Southern Great Plains (Kansas, Oklahoma, Texas) growers to manage some difficult-to-control grassy weeds in winter wheat Triticum aestivum L. (Poales: Poaceae). Two braconid parasitoids, Diaeretiella rapae (M'Intosh) and Lysiphlebus testaceipes (Cresson) (Hymenoptera: Braconidae) are active in this cropping landscape. Both wasps move between crops but D. rapae has a limited ability to develop in the main wheat aphid hosts, so L. testaceipes could influence D. rapae's ability to maintain itself when canola is absent in the landscape. We compared behavioral responses of naturally emerged D. rapae and wasps that were excised before emergence to odor volatiles of host plant, aphid host and aphid-infested plants using two plant/aphid combinations (wheat/Rhopalosiphum padi (L.) and canola/Brevocoryne brassicae L. (Hemiptera: Aphididae). We also compared parasitism rates of D. rapae that were naturally emerged and excised from R. padi or B. brassicae on subsequent parasitism rates of R. padi or B. brassicae hosts. Naturally emerged wasps responded more strongly to host plant and host plant + aphid odors compared to excised wasps regardless of the host origin. Neither wasp group responded to odors from aphids alone. Both wasp groups were most attracted to odors from aphid-infested host plants, regardless of the combination. D. rapae parasitism rates on canola-reared aphids were higher than on wheat-reared aphids. D. rapae parasitism rates were lower when switched from its original host to the alternate host. Results suggest that D. rapae faces challenges to maintain significant populations in the wheat/canola landscape of the Southern Great Plains, especially in years when canola is not locally present.

1. Aquatic macrophytes are abundant in ponds and canals that are constructed in semi-arid regions for water storage and conveyance, as well as in lakes that are culturally enriched. 2. Addition of Ca(OH) 2 to two hardwater ponds at 250 or 275 mg L -1 caused an immediate eradication of submersed aquatic plants. Although these ponds are well-buffered (alkalinity: 2.57-3.94 mequiv L -1 ; pH: 8.1-9.0), lime addition caused an immediate increase in pH of 0.2-3 units. 3. Application of 135 mg L -1 Ca(OH) 2 for 24 h or 210 mg L -1 Ca(OH) 2 for 65 h to two irrigation canals had no effect on macrophyte biomass at the lower concentration and duration, but resulted in the elimination of aquatic macrophytes 1 month after the higher concentration, longer duration treatment. 4. Unlike the macrophyte control achieved following application of 210-275 mg L -1 Ca(OH) 2 to ponds or canals, microcosm experiments in which lime formulation [slaked lime (Ca(OH) 2 ), calcite (CaCO 3 ), or a 1 : 1 mixture] and concentrations (up to 1500 mg L -1 ) were manipulated failed to elicit a consistent change in macrophyte biomass. Macrophytes in microcosms treated for the short-term (23-33 days) with ≥ 200 mg L -1 Ca(OH) 2 or a mixed Ca(OH) 2 /CaCO 3 formulation always lost pigmentation, but biomass was not consistently reduced. 5. Declines in macrophyte biomass following treatment of ponds and canals may have been triggered by a short-term rise in pH which, in these relatively warm (22-23 °C) alkaline (2.28-3.94 mequiv L -1 ) systems, would have resulted in low concentrations of free CO 2 and bicarbonate for photosynthesis.