Biotic Stress and Yield Loss

the uncaged control used to generate yield loss relationships. Comparison of cagedand uncaged controls provides a measure of cage effects.The mobility of a pest may vary from crop to crop; thus pests may require confinementin one crop but not in another. For example, manual infestations of smallfall armyworm, Spodoptera frugiperda, larvae usually remain in the whorl of infestedcorn plants and do not typically require confinement. 12 Some plant-to-plant movementby larvae can occur, so typically a few border rows of uninfested corn are sufficientto prevent larval movement between plots. However, fall armyworms activelymove on the soil surface in a bermudagrass pasture. Jamjanya and Quisenberry 13used metal barriers to confine fall armyworm larvae in small plots of bermudagrassat varying densities to create a range of forage yield loss. In another example,Mailloux and Bostanian 14 confined nymphs of the tarnished plant bug, Lygus lineolaris,on strawberry plants using plastic barriers, whereas a similar study with adultswould require enclosed cages. However, if natural enemy populations are present,nonmobile arthropods may need caging to reduce pest mortality that would occur inunconfined plots.Typically, winged adults must be confined with cages to prevent their immediatemovement out of the designated plots. Cages must be sized to be practical whileenclosing a large enough area to provide meaningful results. Where crops are grownas separate plants such as in orchard crops, individual plants or (for large trees) individualbranches can be caged. However, for crops grown as a continuous populationof plants, including most agronomic and vegetable crops, a cage must enclose a representativeportion of the plant populations or crop canopy rather than individual orsmall groups of plants. Indeed, cage studies that enclose single plants, a stem, oranother portion of plants are of limited use in developing yield–loss relationships formany crops, because crop yield is determined on the basis of plant populations andnot individual plants. 15 A common cage size for agronomic crops encloses about2 m 2 of ground area with the height varying with crop height. In a typical cagestudy, 16, 17 yield loss by potato leafhopper, Empoasca fabae, was studied in alfalfa bycaging plots with cages measuring 1 m wide by 2 m long by 1 m high and coveredwith Saran screening. Highly mobile potato leafhopper adults were collected fromnearby fields using a D-Vac vacuum insect net, sorted and counted, and introducedinto cages in predetermined numbers. Leafhoppers were allowed to feed and lay eggson alfalfa plants for 14 days, after which cages were removed and populations ofnymphs were quantified and allowed to feed up to harvest. Other crops where cageshave been successfully used to study yield loss by arthropod pests include soybean, 18grain sorghum, 11, 19 barley, 20 pinto bean, 21 and oilseed rape.22, 23Wingless insects such as aphids or foliage-inhabiting lepidopteran larvae usuallydo not require confinement to prevent their movement from plots, although borderplants between plants often are necessary to minimize movement to adjacentplants. 24 Ground barriers have been successfully used to confine wingless insects thathave potential for movement such as ground-inhabiting lepidopteran larvae and beetles.25–28 Barriers typically are metal or plastic, measuring from 0.3 m to 1 m widewith a portion of the barrier buried in the ground. Open barriers have the advantageof not restricting light, rainfall, or air movement in enclosed plots. However, barriers,