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The effects of an insect herbivore-resistant soybean genotype on relationships between four levels of a trophic system were examined in the laboratory using both greenhouse and field-grown plants. Pre-imaginal development of the predatory pentatomid, Podisus maculiventris (Say), was affected by soybean antibiosis in a manner similar to that of its lepidopteran prey, Pseudoplusia includens (Walker). Pre-imaginal development time was increased, and cumulative weight gain tended to be reduced on both greenhouse and field-grown resistant foliage; although mortality was increased on greenhouse-grown resistant foliage, it appeared unchanged on field-grown foliage. Reproductive capacity of P. maculiventris reared on P. includens larvae that were fed resistant soybean generally was unaffected, although peak progeny production was delayed and extended slightly. Pre-imaginal development and adult emergence of the egg parasitoid Telenomus podisi Ashmead from eggs of P. maculiventris reared on P. includens larvae that were fed resistant soybean were unaffected. However, the overall reproductive capabilities of this parasitoid were reduced. Our results demonstrate that plant antibiosis can influence the biology of organisms over four trophic levels, thus documenting a relationship heretofore undescribed. Determination of the final outcome of the interaction between plant resistance and biological control is difficult. The four trophic level model reported in this study further illustrates the complexity which hinhers a general understanding of these interactions.

Transgenic lines of soybean, Glycine max (L.) Merrill, expressing a synthetic cry1A gene (tic107) from Bacillus thuringiensis (Bt), were evaluated in screenhouse and conventional field trials for efficacy against lepidopteran pests. In screenhouse trials, Bt soybean and negative checks (isogenic segregants and parental lines) were evaluated against Anticarsia gemmatalis Hübner and Pseudoplusia includens (Walker) in the United States and against A. gemmatalis, Epinotia aporema (Walsingham), Rachiplusia nu (Guenée), and Spilosoma virginica (F.) in Argentina. Bt soybean exhibited virtually complete efficacy against each of these pests, whereas negative checks suffered significant damage. Bt soybean and negative checks also were evaluated in conventional trials against native populations of A. gemmatalis and P. includens in the southeastern United States. Each of these insects caused significant damage to negative checks in one or more locations, whereas Bt soybean exhibited virtually complete efficacy against these pests. In the laboratory, lyophilized leaf tissues from Bt soybean incorporated in artificial diet at a concentration representing a 25-fold dilution of fresh tissue caused complete mortality of A. gemmatalis and near complete mortality of P. includens neonates after 11 d, whereas mortality on negative checks did not exceed 10% for either insect. Average TIC107 expression approached or exceeded 50 microg/g fresh weight at V3 stage of growth and 200 microg/g by R6 stage of growth. These results demonstrate that expression of TIC107 in soybean can not only achieve highly efficacious control of several lepidopterans under field conditions but also provide a high dose for effective insect resistance management.

No abstract

Little is known about soybean [Glycine max (L.) Merr.] yield response to defoliation during the last half of the seed‐filling period (R6.3 to R7) and how it is affected by source/sink ratio. Because defoliating insect pests in the southeastern USA frequently attack soybean during this period, greater knowledge of yield responses to defoliation at this time would aid in making management decisions. Therefore, the objectives of this field study were to (i) determine yield losses to total defoliation near the temporal midpoint (R6.3) and three‐quarter point (R6.6) of the seed‐filling period; (ii) determine if alterations in source/sink ratio affect this response; and (iii) determine yield component mechanisms responsible for the yield reduction. ‘Centennial’ soybean was planted in 1991 and 1992 near St. Gabriel, LA. Treatments were no defoliation (control), 100% defoliation at R6.3, and 100% defoliation at R6.6 arranged as split plots within high, normal, and low source/sink ratios during seed filling (main plots). Defoliation at R6.3 resulted in 40% yield reduction, whereas defoliation at R6.6 caused a 20% yield loss. Lower yield resulted from reduced seed size. Source/sink ratio did not affect this response. Smaller seed size in both defoliation treatments resulted partly from reduced seed‐filling rate (28% less than control). Shorter effective filling period also contributed to smaller seed size, but this effect was influenced by treatments and years. In conclusion, 100% defoliation during R6.3 to R6.6 must be avoided to maintain optimum yield.

Limited information exists on the insecticide susceptibility of redbanded stink bug, Piezodorus guildinii (Westwood) (Heteroptera: Pentatomidae), despite its impact on soybean, Glycine max (L.) Merr., production in Brazil and the United States. Therefore, this study set out to 1) determine baseline levels of susceptibility to currently recommended pesticides using topical and vial bioassays, 2) determine the levels of esterase activity in populations in the United States and Brazil, and 3) compare control among products in field trials. In topical bioassays conducted in the United States using technical grade materials, the LC50 values of lambda-cyhalothrin, acephate, and methamidophos were 4-25, 141-295, and 40-151 ng per insect, respectively. The LC50 values of imidacloprid and thiamethoxam were 11 and 27 ng per insect, respectively. In vial bioassays conducted in the United States using technical grade materials, the LC50 values of cypermethrin, acephate, and methamidophos were 0.4-0.9, 3.8, and 1.6 microg per vial, respectively. In topical bioassays conducted in Brazil by using commercially formulated products, the LC50 values of acephate, methamidophos, endosulfan, and imidacloprid were 0.90-1.9, 0.4-0.6, 1.5-6.6, and 0.2-0.3 microg per insect, respectively. In vial bioassays conducted in Brazil using commercially formulated products, the LC50 values of endosulfan, methamidophos, and lambda-cyhalothrin were 4-32 and 2-24 microg/cm2 for thiamethoxam and imidacloprid. Esterase activity in Louisiana (United States) populations ranged from 251 to 658 nmol alpha-naphthol formed/min/mg protein. Esterase activity levels in Londrina (Brazil) populations averaged 163 nmol/min/mg. In field tests, P. guildinii in Louisiana were controlled by organophosphates thiamethoxam and imidacloprid and in Brazil, with combinations of neonicotinoids and pyrethroids.

Relationships between defoliation, light interception, and grain yield for soybean [Glycine max (L.) Merr.] during the last half of the seed filling period are not fully understood. Because defoliating insect pests in the southeastern USA frequently attack soybean during this period, knowledge of such relationships would aid development of criteria for economic insecticide application. Our first objective was to determine the efficacy of using light interception and leaf area index as criteria for economic insecticide application to manage defoliating insect pests. The second objective was to determine the effects of partial and total defoliations during seed filling on grain yield and grain yield formation. ‘Centennial’ soybean (Maturity Group VI) was planted in 1993 and 1994 near St. Gabriel, LA. Experimental design was a randomized complete block in a split‐plot arrangement. Main plots were the temporal midpoint and 3/4‐point seed‐filling stages. Split plots were four defoliation treatments: 0% leaf removal (control), 37 to 41% leaf removal, 56 to 61% leaf removal, and 100% leaf removal. At the temporal midpoint, grain yield declined linearly as light interception fell from 93 to 50%. Thus, for a crop achieving near‐maximal light interception, insecticide application would be recommended whenever light interception at the midpoint seed‐filling stage fell below 93%. Yield and light interception were not closely related at the 3/4‐point seed‐filling stage. Maintenance of maximal grain yield (yield at 0% defoliation) required retention of at least 39% of the leaf area at this time. In conclusion, light interception potentially could be used as an insecticide application criterion at midpoint seed‐filling stage, but not the 3/4‐point. Partial defoliation at the midpoint will affect grain yield if it reduces light interception below 93%, whereas at the 3/4‐point total defoliation is required to affect grain yield.

Delayed maturity in soybean, Glycine max (L.) Merr., occurred in response to infestation by southern green stink bug, Nezara viridula (L.), in 4 yr of field studies. Maturity delays followed stink bug infestation that occurred only during the pod set and filling stages (R3-R5.5), and infestations at R3-4 and R5 resulted in delayed maturity more consistently than did infestation at R5.5. Infestation levels of six stink bugs per 0.3 m of row for 7-14 d generally were required to delay soybean maturity. The greatest impact on seed yield and quality parameters followed stink bug infestations that occurred during R3-R5.5, which corresponded closely with the periods of infestation that resulted in delayed maturity. If both delayed maturity and yield reduction are considered, the pod elongation through late pod filling stages were most critical for protecting soybeans from southern green stink bugs.