Two field experiments were conducted at the Isabela and Juana Díaz Research Centers in 1982-83 to determine the effect of pigweed (Amaranthus dubius Mart.) density on yield of sweetpotato [Ipomoea batatas (L.) Lam]. Pigweed densities of 0, 4, 7, 13 and 27 plants/m2 were adjusted by hand at Isabela and those of 0, 11, 23, 45 and 91 plants/m2 were maintained at Juana Díaz. Pigweed population averaged 27 plants/m2 at Isabela and 91 plants/m2 at Juana Díaz under natural infestation. Sweetpotato yield was reduced significantly from 4 plants/m2 at Isabela and from 11 plants/m2 at Juana Díaz. The highest sweetpotato yield was obtained on weed-free plots with 11,823 kg/ha at Isabela and 27,724 kg/ha at Juana Díaz. The greatest yield reduction (91%) resulting from pigweed competition (at Juana Díaz) was at a density of 91 plants/m2. At Isabela, a density of 13 plants/m2 caused the greatest yield reduction (78%). The effect of pigweed on number of root-tubers followed a similar trend as the effect on yield. Dry weight of pigweed per plant was significantly greater at population densities of 4 and 7 plants/m2 at Isabela. At Juana Díaz, dry weight of pigweed was significantly higher at a population density of 11 plants/m2. The regression between sweetpotato yield and the different population densities of pigweed showed an inverse linear relationship at both localities.
GLYPHOSATE ON TOMATO AND SWEET PEPPER YIELDS
Two field experiments, both with Cubanelle and Yollow Wonder peppers (Capsicum annuum L.), were conducted at the Juana Díaz Research and Development Center in 1981-82. Weeds were induced to grow through periodic irrigations for 16 days before peppers were transplanted. Either glyphosate or paraquat at 1.12 kg ai/ha were then applied to control emerging weeds in two separate experimental areas 5 days before setting out transplants. Oxyfluorfen at 1.12 kg ai/ha applied 2 days before transplanting peppers caused the highest phytotoxicity (significant at P=0.05) as recorded during 14, 28 and 42 days after treatment in both experiments. Neither alachlor (1.68 kg ai/ha), oxadiazon, trifluralin nor pendimethalin applied 2 days before transplanting at 1.12 kg ai/ha was phytotoxic to peppers. There were no significant differences in Cubanelle or Yollow Wonder pepper yields among herbicide treatments and handweeded check. The highest pepper yields (47,771 and 44,964 kg/ha) were obtained in plots treated initially with glyphosate and later by either paraquat directed postemergence spray or oxadiazon pre-transplant at 1.12 kg ai/ha each, respectively. In the other experiment, hand-weeded check and preplant application of paraquat at 1.12 followed by oxyfluorfen at 1.12 kg ai/ha were the highest yielders with 47,771 and 44, 964 kg/ha, respectively.
Oxyfluorfen [2- chloro-1- (3-ethoxy-4-nitraphenoxy)-4-(trifluoromethyl) benzene] was evaluated throughout a series of experiments carried out from 1983 to 1986 at the Lajas Research and Development Center. In 1983 preemergence oxyfluorfen at 1.68, 3.36 and 6.72 kg ai/ha reduced significantly (P=0.05) the germination and plant height of pigeon pea cv. 2,B-Bushy. Preemergence oxyfluorfen at 1.32 kg ai/ha reduced germination of cv. Kaki by 20% in 1984-85, and by 5% in 1985-86. Oxyfluorfen at 0.33 and 0.66 kg ai/ha was found to be safe for germination of pigeon pea. There was severe injury from postemergence treatment to top of weeds and crop at 0.33 and 0.66 kg ai/ha. Phytotoxicity symptoms of pigeon pea disappeared after 9 weeks in all treated plots. The number of grass weeds was significantly reduced by Preemergence oxyfluorfen even with the 0.33 kg ai/ha rate and two direct applications of 0.25 kg ai/ha rate in both years. Broadleaf weeds were more effectively controlled by oxyfluorfen at 0.66 kg ai/ha applied over the top, followed by a direct application at 0.25 kg ai/ha. In 1984-85, the highest yield of pigeon pea (7,989 kg/ha) was obtained with oxyfluorfen at 0.33 kg ai/ha applied over-the-top plus the same herbicide at 0.25 kg ai/ha as post-directed spray. Pigeon pea yield with preemergence treatment of oxyfluorfen at 0.33 kg ai/ha plus two post-directed applications at 0.25 kg ai/ha ranked second, with 7,826 kg/ ha. There were nonsignificant differences (P = 0.05) in pigeon pea yields among oxyfluorfen treated plots, hand-weeded and non-weeded plots in 1985-86. Yields ranged from 6,470 kg/ha in hand-weeded plots to 4,428 kg/ha in non-weeded plots.
The feeding preferences of the herbivore Spoladea recurvalis Fabricius (Lepidoptera: Pyralidae) was determined through choice and no-choice tests in the laboratory with 12.7-mm leaf discs of the following weeds and crops: horse purslane (Trianthema portulacastrum L), common pigweed (Amaranthus dubius Mart.), botoncillo (Borreria ocymoides [Burm. F] DC), sugar beet (Beta vulgaris L.), sweet cherry pepper (Capsicum frutescens L.), tomato (Lycopersicon esculentum Mill.), cucumber (Cucumis sativus L.), and eggplant (Solanum melongena L.). Spoladea recurvalis preferred horse purslane as its primary food host plant. The pyralid larva also fed on leaf discs of common pigweed and sugar beet, the only vegetable crop serving as an alternate food plant. Larvae did not feed on the other plants studied (sweet cherry pepper, tomato, cucumber, eggplant and botoncillo), some of which were previously reported as alternate food host plants for the herbivore. This manuscript discusses the application of these findings for the biological control of horse purslane and the foraging behavior of S. recurvalis.
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