Field Infestation of Phaseolus Vulgaris by Acanthoscelides Obtectus (Coleoptera: Bruchidae), Parasitoid Abundance, and Consequences for Storage Pest Control
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by RERO DOC Digital Library PEST MANAGEMENT AND SAMPLING Field Infestation of Phaseolus vulgaris by Acanthoscelides obtectus (Coleoptera: Bruchidae), Parasitoid Abundance, and Consequences for Storage Pest Control 1 2 I. SCHMALE, F. L. WA¨ CKERS, C. CARDONA, AND S. DORN Institute of Plant Sciences, Applied Entomology, Swiss Federal Institute of Technology (ETH), Clausiusstrasse 25/NW, 8092 Zu¨ rich, Switzerland Environ. Entomol. 31(5): 859Ð863 (2002) ABSTRACT Over a period of 3 yr we collected 19 samples (1 kg each) of recently harvested beans (Phaseolus vulgaris L.) from eight small-scale farms in Restrepo, Valle de Cauca, Colombia. Initial infestation by Acanthoscelides obtectus (Say) was low, but frequent. At harvest, 90% of the bean samples were infested by the weevil. The average level of infestation was 16 weevils per 1,000 beans, with a maximum of 55 weevils. Infested beans usually carried multiple larvae witha maximum of 13 larvae per bean. Emergence data indicate that oviposition by A. obtectus in the Þeld is conÞned to a very short period before harvest. This relatively narrow time window can be exploited for proper timing of control measures. Only one species of parasitoid, Horismenus ashmeadii (Dalla Torre) (Hymenoptera: Eulophidae), was recorded, emerging from 21% of the samples. Samples with parasitoids had an average of Þve parasitoids per 1,000 beans, witha maximum of 12 parasitoids. Thisrepresented a parasitization level of 18%. During the 16 wk of storage, two weevil generations emerged, which caused visible damage in 0.5 and 34% of the beans (average of 14%). Although H. ashmeadii was successful in attacking the Þrst generation of A. obtectus in the Þeld, it failed to attack or develop under storage conditions. This indivates H. ashmeadii cannot serve as a postharvest control agent. KEY WORDS Acanthoscelides obtectus,beans, Bruchidae, parasitoids, oviposition behavior, storage pest MANY SPECIES FROM the family Bruchidae are world- though market prices are then usually at their lowest. wide pests of beans bothin theÞeld and in storage. CIAT (1986) estimated that this results in a 50% in- After hatching from the eggs, Þrst instars penetrate the come loss compared withprices thatcould be ob- bean seeds. The beetles develop within the seeds to tained if beans were safely stored for a few month. emerge as adults leaving typical damage in the form of Furthermore, it is expected that spreading the bean the empty feeding chambers. In addition, to this direct supply over a longer period would stabilize bean damage, indirect damage arises from contamination prices in general (van Schoonhoven 1976). by excrement, pheromone, and dead insects, which Acanthoscelides obtectus is one of two major bruchid can cause allergic reactions in humans (CIAT 1986). species causing damage to stored beans in Latin Amer- Weevil damage can also lead to a lower germination ica. It prefers cooler climates at higher elevations, and rate of damaged bean seeds (Zacher 1930). thus, can be found in mountainous and subtropical In 1986, CIAT (1986) estimated economic losses regions where it is the only insect pest of stored beans due to bruchid damage in Colombian warehouses to (Cardona 1989). A. obtectus is known to infest beans be 7.4% after 45 d of storage. Other sources reported in the Þeld by ovipositing eggs loosely in growing pods damage levels of 35% losses in Mexico and Central (Cardona 1989). Despite its economic importance, America, and 13% in Brazil after different storage little or no information is available on the timing and times (cited in CIAT 1986). However, these data are level of this Þeld infestations. Available observations difÞcult to compare, because the types of beans and indicate that females deposit their eggs in the drying storage varied, and losses are known to be directly pods (Thiery and Jarry 1985). In addition, to this lack correlated withduration of storage. of knowledge on herbivore-plant interactions, little is Colombian farmers respond to the bruchid problem known about natural enemies attacking A. obtectus in by selling their commodity at the time of harvest, even the Þeld. In this study we wanted to quantify natural levels 1 E-mail: [email protected]. and the timing of Þeld infestations by A. obtectus, and 2 Centro Internacional de Agricultura Tropical (CIAT), Cali, Co- to investigate the subsequent dynamics of the herbi- lombia. vore populations under typical Andean storage con- 0046-225X/02/0859Ð0863$02.00/0 ᭧ 2002 Entomological Society of America 860 ENVIRONMENTAL ENTOMOLOGY Vol. 31, no. 5 ditions. We also surveyed parasitoids attacking A. ob- laboratory and kept in glass jars in a chamber at 20Ð tectus in the Þeld by taking samples of various sites 25ЊC and 70% RH. All newly emerged insects were over 3 yr. removed from the jars and counted at weekly inter- vals. Samples were discarded after 5 wk, which covers the development time of the weevil under given con- Materials and Methods ditions. This ensured recovery of the majority of All observations were made using the commonly emerging adults and prevented counting F1-genera- cultivated susceptible bean variety Phaseolus vulgaris tion individuals. L. variety Diacol-Calima (Leguminosae). Percentage bean damage was calculated. Beans Initial Bean Damage. To investigate the level of were counted as ÔdamagedÕ when at least one emer- natural Þeld infestation by A. obtectus, a total of 19 gence hole made by A. obtectus was visible at the time bean samples of Ϸ1 kg eachwere taken from eight the samples were collected. Because larvae inside the small-scale farms in Restrepo, Valle de Cauca, Colom- seeds are not visible they have no direct impact on bia (1,360 m above MSL). Four farms had their Þelds value, and thus were not counted as ÔdamagedÕ. situated in the center of the bean-growing area, where Also, the intensity of infestation was calculated us- beans are permanently present in the Þeld, the other ing the following formula: intensity of infestation ϭ four farms had their Þelds situated outside the center total number of emergence holes/number of damaged of the bean-growing area, where crop rotation re- seeds. sulted in bean-free periods. Sampling took place at 15 dates over a 3-yr period (1997Ð1999). Samples were Results taken on-farm within the Þrst week after harvest. Time of harvest was recorded for each sample. Initial Bean Damage: Weevil Emergence. An aver- At the time of collecting the sample, every bean age of 26, 13, and 14 A. obtectus adults emerged from seed was examined for insect damage, in particular 1,000 bean seeds collected at harvest in the years 1997, weevil or parasitoid emergence holes. Subsequently, 1998, and 1999, respectively. Therefore, the mean Þeld the samples were kept in glass jars in a laboratory infestation over the whole period was 16 weevils per chamber at 20Ð25ЊC and 70% RH. All newly emerged 1,000 seeds, witha maximum of 55 weevils. Only two insects were removed from the jars and counted at of the 19 samples were uninfested (both in 1998). In weekly intervals. Samples were discarded after 5 wk, the 3 yr, emergence started the third, the fourth, and which covers the development time of the weevil the Þfth week after harvest, respectively (Fig. 1). under the given conditions. This ensured that the Acanthoscelides obtectus adults emerged over a period majority of emerging adults was recorded, but pre- of 3 wk, following a roughly normal distribution. vented the counting of F1 generation individuals. Parasitoid Emergence. Hymenopteran parasitoids Storage Monitoring. To investigate the population were observed in 11 samples collected from four farms dynamics of A. obtectus, beans were harvested in 1998 in the center of the village. They uniformly belonged and 1999 from seven representative Þelds in Restrepo. to a species in the genus Horismenus (Hymenoptera: Three Þelds were situated in the center of the bean- Eulophidae), which was determined to be either growing area, where beans are permanently present in Horismenus ashmeadii (Dalla Torre) or an unde- the Þeld, and four Þelds were situated outside the scribed species close to H. ashmeadii (witha longer center of the bean-growing area, where crop rotation female gaster) (det.: Christer Hansson, Institute of resulted in bean-free periods. Bags containing 40 kg of Zoology, Lund, Sweden). In this article, the species is bean seeds eachwere Þlled and stored on-farm. The referred to as H. ashmeadii. Emergence of H. ashmea- climatic conditions over 25 yr in this region are 19.6ЊC; dii occurred over the same period as emergence of A. 82.25% RH, and 86.75 mm precipitation (GIS, Centro obtectus, but there was greater variation between Internacional de Agricultura Tropical, Cali, Colom- farms withrespect to thepeak of emergence (Fig. 2). bia). Precipitation during the experimental period was The fraction of samples yielding parasitoids was 100, elevated due to the climatic phenomenon ÔEl Nin˜ oÕ 55, and 25% in 1997, 1998, and 1999, respectively. In and rainy and dry seasons were less distinct than parasitized samples, an average of 6, 3, and 18 H. normal. After harvest, bean pods were spread out for ashmeadii adults emerged from 1,000 bean seeds in sun drying and subsequently threshed. A 1-kg sample these same years. Over the whole period, a maximum of beans was removed to evaluate Þeld infestation of 12 parasitoids per 1,000 seeds emerged representing levels, and 40 kg of seeds eachwere poured into seven a parasitization rate of 18%. polypropylene bags and stored on-farm for 16 wk. Storage Monitoring. During 16 wk of storage, two Over this period, 0.5 kg of beans were removed at bruchid generations emerged (Fig.