Integrating Effects of Natural Enemies into Winter Wheat Greenbug Management

Presence/absence (sequential binomial) sampling methods have been developed that can provide quick and easy estimates of both greenbug and parasitoid densities in the wheat field. Specifically these methods were designated “Glance ‘n go” and “Glance 'n go plus parasitoids” respectively. These sampling methods were evaluated over the 2003-2004 growing season in 10 cooperator supplied wheat fields. In each field four 30 x 30 m plots were established. Each plot randomly received one of four common management treatments: 1. Glance ‘n go (sequential binomial sampling for greenbug densities), 2. Glance 'n go plus parasitoids (sequential binomial sampling for both greenbug and their parasitoid densities), 3. Prophylactic insecticide applications, 4. No insecticide applications. Whether data were analyzed for all ten fields, with a covariant for cheat Bromus secalinus L. competition, or by only analyzing data from those fields where an insecticide treatment was recommended and made on the Glance ‘n go managed plots, there were no significant differences in the grain yield or one hundred seed weights among treatments. This research indicates that Glance 'n go and Glance 'n go plus parasitoids management practices result in grain yields that are not statistically significant, than prophylactic insecticide applications. The management schemes are simple to apply and should save the wheat producer money since the producer will make fewer unnecessary insecticide applications.

Introduction

Wheat, Triticum aestivum L., is a multipurpose cereal crop that is grown in the Southern Great Plains for grain production, forage production or a combination of the two (Thompson 1990). Annually, some 6 to 7 million acres are planted in Oklahoma (Krenzer et al. 1999). In 1998, 6.4 million acres of winter wheat were planted in Oklahoma, of which about 4.3 million acres were harvested for grain with an average yield of 34 bushels per acre (Krenzer et al. 1999).
Wheat is attacked by many herbivores, including aphids that can inflict losses (Royer et al. 1998). Aphids are of particular interest because they have been observed to damage wheat from plant emergence to heading. Aphids reproduce rapidly, and are often not detected by farmers until their populations reach deleterious levels. These aphid pests include greenbug Schizaphis graminum (Rondani), Russian wheat aphid (Diuraphis noxia Mordviko), bird cherry-oat aphid (Rhopalosiphum padi L.), English grain aphid (Sitobion avenae Fabricius), and corn leaf aphid (Rhopalosiphum maidis Fitch) (Royer et al. 1998). Arguably the most important of these aphid pests is the greenbug.
Greenbugs infest a wide variety of crops and wild hosts throughout the central United States, feeding on over 70 graminaceous species many of which serve as secondary hosts when winter wheat and other grain crops are not present (Michels 1986). First reported in the United States as an agronomic pest of wheat in 1882 (Pfadt 1962), greenbugs can reach tremendous population levels in a short period of time (Starks and Burton 1977). Outbreaks occur in Oklahoma almost every year, and statewide infestations are reported about every 5-10 years (Starks and Burton, 1977). When population levels surpass economic injury levels, greenbug feeding reduces yield and crop quality (Elliott et al. 1994, Kindler et al. 2002). In Oklahoma, losses range from $0.5 to $135 million annually, though much of the losses are due to the expense of insecticide use (Starks and Burton 1977, Webster 1995).
Greenbugs are attacked by a number of predators and parasitoids (Royer et al. 1998). One of the most important examples of these natural enemies in the Southern Great Plains is Lysiphlebus testaceipes Cresson (Hymenoptera: Aphidiidae) (Kring and Gilstrap 1983). Lysiphlebus testaceipes is a common parasitoid of cereal aphids throughout temperate regions of North and South America and has great potential for destroying large numbers of greenbugs (Pergande 1902, Sekhar 1957, Wood and Chada 1969, Krombein et al. 1979, Salto et al. 1983, Jones 2001, Giles et al. 2003). Their effects on greenbug populations can be dramatic. Female L. testaceipes oviposits in all life stages of the greenbug. When parasitized as adults, greenbugs stop reproducing about three days after being parasitized by L. testaceipes (Spencer 1926, Eikenbary and Rogers 1974).
Because grain yield losses are directly related to greenbug population levels, a population assessment (sampling) is required to estimate the potential for economic losses, and whether insecticides are cost effective to apply (Royer et al. 1998, Kindler et al. 2002, Elliott et al 2003a,b). Historically, in Oklahoma, greenbug infestations have been estimated by relatively laborious methods that estimate greenbug populations per 0.3m of crop furrow, or on a per tiller basis (Royer et al. 1998). A simplified sequential binomial sampling system for classifying greenbug densities has been developed (Royer et al. 2002). Coined “Glance 'n go,” this method involves looking at randomly selected tillers and noting the presence or absence of greenbugs on each tiller. Because the proportion of tillers that are infested accurately corresponds with greenbug density (Giles et al. 2000), samplers can quickly classify high or low greenbug populations through sequential sampling. The important goal of Glance 'n go sampling is not to determine the exact greenbug density in the field, but rather to classify the likely density as being above or below the economic threshold (ET). The economic threshold is the density at which control measures should be taken to prevent an increasing pest population from reaching the economic injury level (EIL), at which significant losses to the producer occur (Stern et al. 1959). Due to the simplicity, timesaving, and ease of using Glance ‘n go, producers could be more likely to sample for greenbugs and make educated decisions about insecticide applications.
The Glance ‘n go method of greenbug sampling, while simple and easy to use, deals primarily with classifying greenbug population densities as being high or low and does not include sampling for important effects of natural enemies such as parasitoids. To remedy this deficiency, a complimentary sequential binomial sampling method for classifying parasitoid densities in winter wheat fields was been developed. Giles et al. (2003) demonstrated that when the proportion of tillers with parasitized aphids that are pupating (mummies) exceeds 10%, the actual percent parasitism could always be classified as being above 20%. Based on research by Jones (2001), when this level of parasitism is reached, greenbug population densities will quickly decline. Being able to simultaneously predict parasitoid efficacy for greenbug control during greenbug sampling would allow producers to reduce the misuse of pesticides, thus increasing producer profits, reduce the incidence of greenbug resistance to insecticides and reduce negative impacts of pesticides on the environment.