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E. J. Armbrust
Alfalfa is the world's most valuable cultivated forage crop, and it is recognized as providing the best food value for all classes of livestock': In the U.S., there are nearly 30 million acres of alfalfa, which provide more than 50~o of the total hay crop. Alfalfa is exceeded in total acreage by corn, wheat, and soybeans, but its protein production potential per acre far exceeds that of each of these three crops.
The alfalfa weevil, Hypera postica (Gyllenhal), and the closely related species, the Egyptian alfalfa weevil, H. brunneipennis (Boheman), have been the most important pests of alfalfa in the U.S. The alfalfa weevil was first discovered near Salt Lake City, Utah in 1904, and for nearly 50 years it remained confined to 12 western states. However, in 1952 it was discovered in Maryland. From there, it spread rapidly throughout the remainder of the U.S. The Egyptian alfalfa weevil was first discovered in the U.S. in 1939, and it has remained confined mainly to California and Arizona.
Even though one might consider the alfalfa weevil complex as a single national alfalfa pest problem, various geographic regions have their own unique problems because the weevils' biologies and control requirements are extremely dependent on climatic factors. Because the weevil complex, and more specifically H. postica and its widely distributed parasite, Bathyplectes curculionis(Thomson), is of concern to growers on a national scale, it has been the key insect pest under investigation in this project. Other insects, both pest and natural enemy species, were also considered in the total arthropod complex on alfalfa. Whatever knowledge could be gained from specific studies on them has been incorporated into the program. Because of the diversity of alfalfa production and management and the biological differences in the weevil complex in relation to crop development in the specific region, many aspects of the research were conducted in each of several regions.
Because there is a vast amount of available information on the biology, ecology, and control of alfalfa insect pests, an immediate effort was made to search, categorize, analyze, and utilize the existing literature and unpublished data. By the end of the present project there will be published, indexed bibliographies on the spotted alfalfa aphid,' pea aphid Sitona species,' potato leafhopper, weevil complex, and weevil parasites. This represents over 9000 literature references of which two thirds have been indexed and entered into a computerized literature retrieval system. This accomplishment has been of valuable assistance to the biologists, as well as those involved in modeling efforts. In many instances, it was found that parasites had been established, cultural practices and timing of insecticide applications had been investigated, and considerable effort had been put forth in developing resistant varieties. These data have been integrated into a system that uses each factor in a complementary fashion.
There are over 2000 literature citations dealing with the weevil complex and its parasites, but it \vas apparent that certain aspects of alfalfa weevil and parasite biology needed special research attention. The modeling effort helped to distill out these unknowns and point to specific research needs. Especially needed was a better understanding of the mortality factors influencing fluctuations in the insect pest and natural enemy populations in order to develop a better insect management system for alfalfa, a system that depends increasingly on improved insights gained from the systems approach.
Alfalfa weevil larvae have green bodies and black heads and are 3/8 in. long when full grown. They feed on alfalfa plants for 3 or 4 weeks in the spring. During this time, they shed their skins three times. When full grown, they spin silken cocoons on the plants, within the curl of fallen dead leaves, or in litter on the ground. They change into adults in I or 2 weeks. After feeding for a short time in the spring, these adults fly to protected areas and enter a resting period. In many areas, most of the adults will return to the alfalfa fields in late summer and early fall. In some northern areas, the adults return to the alfalfa the following spring.
In southern areas, if temperatures permit, the weevils will lay eggs throughout the fall and winter as well as into the spring. Some of the eggs will begin to hatch about the time alfalfa is beginning its spring growth. In the more northerly regions, the larger number of eggs are laid in the spring. By the time the larvae emerge, the alfalfa is 6 to 10 in. tall and can tolerate more weevils than the southern crop.
Because population peaks vary from year to year, it is difficult to predict when spraying will be necessary. Existing methods of determining when to spray (such as percent tip feeding) are often confusing and do not consider crop height or weevil numbers.
Although insecticidal control has been the most widely used method, two other methods arc common. One is to manipulate the timing of the first harvest in the spring. After considering many factors such as numbers of weevil larvae, plant growth, and prevailing weather conditions, proper timing of the cutting date can achieve the same effect as the application of an insecticide. The other method involves biological control agents, such as parasites and predators. One of the most successful biocontrol agents is a small parasitic wasp, Bathyplectes curculionis. This wasp lays its eggs inside young weevil larvae. The wasp larvae develop inside the weevil larvae, and when they have satisfied their needs, they kill their hosts. Other parasites of the larval stage include B. anurus, B. stenostigma, Tetrastichus insertus; of the egg stage, Patasson luna; and of the adult stage, Microctonus colesi and M. aethiopoides.
A high level of resistance in alfalfa to Hypera spp. has not been found, but varieties with low levels of resistance have been developed. These previously developed varieties are of the dormant type and were tested exclusively against H. postica. Summers and Lehman demonstrated that resistance can be found in nondormant type alfalfas and that varieties developed for resistance to H. postica also resist H. brunneipennis.
The principal reason for a pest management program is that the three control methods are interrelated. For example, insecticides kill parasites and predators as well as alfalfa weevils. Harvesting alfalfa when many of the weevil larvae are parasitized will also reduce the parasite population. To help understand how the three methods of control work together, a computer-based mathematical model was developed. This model simulates field conditions. The pest management program resulted from laboratory analysis of the model followed by extensive field trials. The values in published charts were taken directly from these trials. The charts are designed to explain the model without requiring the use of a computer.
This program assumes proper soil pH, adequate fertility, and that the alfalfa is not under stress from drought, disease, or other factors. If these conditions are not met, it is possible that excessive feeding injury will occur.
In order for researchers to be able to compare results from alfalfa-weevil field studies, for each life stage, standardized research and sampling techniques that were based on a unit-area basis were agreed upon. It was further agreed that the sweep net should not be used for population studies until sweep-net catch could be related to absolute densities.
Where necessary, research results were obtained to refine current methods of sampling for each life stage. It was soon discovered that sampling intervals would best be based on physiological time as measured by degree days.
Adult Alfalfa-Weevil Sampling
Estimates of absolute density (weevils per square foot) for adult alfalfa weevils have been notoriously difficult to obtain. Even under the best conditions, about I man-hr is needed in the field, plus 3 days of extraction time, and the use of 20 Berlese funnels in the laboratory to estimate adult density for one field. A more typical case would require about three to four times this effort.
A study was undertaken in Illinois to obtain information about the relationship between sweep-net catch and absolute density. It was hypothesized that the ratio (M) of absolute density to catch per sweep would depend largely on alfalfa height, temperature, wind speed, relative humidity, and solar radiation intensity. Data were gathered between April 1973 and November 1975 from several fields in Illinois and Indiana, resulting in 29 data sets.
Multiple regression analysis was used to obtain an equation for predicting the ratio AT from environmental conditions. The best equation, based on analysis to date is
M = 0.2 + 107'3595 - 0.001343(T + 3 L)2 - 0.3744H + 0.0058 TH
where TH is temperature in °F, H is crop height in centimeters, and L is solar radiation in Langley units (cal/cm2/min). Use of this conversion equation is facilitated by using tables with which one can estimate Min about 15 sec.
This equation has not been field tested using independent data, and until that time it is premature to place much confidence in its predictions. It is expected that absolute density can be estimated within 20070 of the true mean 90°70 of the time, based on 200 to 500 sweeps (depending on catch per sweep), and that about 15 to 50 min will be required in each field.
Collection of Field Samples for Life-Table Construction
Using standard sampling techniques for each life stage of the weevil, population data for the weevil complex and for B. curculionis have been collected over a period of 3 to 4 years from one or more locations in Kentucky, California, Illinois, Indiana, Virginia, New York, Ohio, Utah, Michigan, and Nebraska. These data have been extremely valuable for determining age, specific survival rates and key mortality factors, quantifying the effects of populations dynamics and impact of parasites on population regulation, and field validation of models.
The life tables were based on I ft2 samples of alfalfa collected at random within each field. Eight age intervals were used to trace the course of each generation Graphic key factor analysis was used to study the factors responsible for changes in population density. By this method, the killing power, or k-value, of each mortality factor was estimated by taking the difference between the logarithm of population density before and after its action. Using these values, the data from Kentucky were plotted for 11 life tables. The results indicated that k" mortality of summer adults (to emigration), followed the same fluctuating course as did changes in generation mortality (K). Thus, total generation mortality was dominated by the magnitude of summer adult mortality (cause not specific), and this represented the key factor of importance in explaining generation survival of H. postica on alfalfa. At the same time, mortality of larvae due specifically to parasitism tended to compensate for the changes in adult mortality and reduce generation-to-generation variation in K and, thus, contributed a density dependent regulatory component.
Quantitative Biology Studies
In the construction of insect models for the alfalfa weevil and B. curculionis, it soon became apparent that there were certain research areas needing special attention for both alfalfa weevil and B. curculionis. These were diapause rate, diapause duration and termination, adult survival, oviposition rate, and dispersal between habitats. A detailed quantitative understanding of these features as affected by different, or changed, environmental conditions was needed.
A literature search at the beginning of this project revealed few detailed studies concerning the biology of the parasites and the impact of insecticides on them. Even more limited were studies dealing with predators in alfalfa and their impact on the alfalfa weevil or its parasites. Hundreds of species of organisms are commonly found in alfalfa, and the role of many is as yet poorly understood. In these studies, to avoid disrupting whatever natural stability may exist in this ecosystem, the system was disturbed as little as possible. Thus, investigations of the natural control complex in relatively undisturbed situations became an essential part of this project. There has been wide variation, however, in the emphasis of effort on specific members of the natural enemy complex.
Mortality Factors Affecting B. curculionis
Applications of pesticides in the alfalfa ecosystem interrupt the normal relationship between alfalfa and its associated arthropod fauna. Cocoons of B. curculionis can be found in the litter for a period of I to 2 weeks for nondiapausing individuals and 10 to 11 months for diapausing ones. Bartell et al.' demonstrated that the construction of the cocoon, and the metabolic state of the individual within it, influences the penetration of insecticides.
Because the cocoons of diapausing B. curculionis remain in the litter from one spring to the next, they are vulnerable to insecticides over a much longer period of time, and to a variety of other factors such as weather, predators, parasites, pathogens, cultural practices, etc. Cherry and Armbrust found that invertebrate predation caused the heaviest mortality in these diapausing larvae. Specific predators were identified by exposing B. curculionis larvae to various surface-dwelling invertebrates found in alfalfa fields. In addition, field plantings of parasite larvae in modified screen mesh cages were used to determine the size of the predators involved and also to determine if litter density affected rates of predation or the species involved.
Results from feeding studies showed that spiders, Cicindelidae, Formicidae, and small species of Staphylinidae did not prey on B. curculionis larvae in cocoons. The two groups of predators of the parasite larvae planted in the field were mainly insects of moderate size, and their use of B. curculionis was not significantly affected by litter density. The greatest number of total predators (G. pennsylvanicus plus carabids) caught per day per pitfall trap and the greatest predation on field-planted B. curculionis larvae occurred concurrently during September and October. These data suggest that predation during fall (September and October) may be significant in reducing field populations of diapausing parasite larvae. Based on feeding studies and pitfall trap catches, C. pennsylvanicus and the carabids, Abacidus permundus (Say), Evarthrus sodalis le Conte, Harpalus pennsylvanicus de Geer, and Scarites subterraneus Fab., were the most significant specific predators on B. curculionis larvae. These and other invertebrate predators cause a far greater mortality to diapausing B. curculionis than the combined weather, hyperparasites, diseases, and the insecticide usage.
Economic Thresholds and Compensation of Alfalfa to Insect Damage
Early in this project some states, especially California, " began economic injury studies for the weevil complex, and certain other participating institutions cooperated with researchers at Purdue University to determine these levels under various biological and geographical conditions.' Also, in order to evaluate management practices and control measures, quantitative knowledge was required of the effects of insect defoliation on yield and quality. In New York, Liu and Fick compared yield and quality in systems cut twice and three times during the season where insect pests were controlled and not controlled. The greatest effect for a single harvest was in the second cutting of the three-cut system where feeding occurred in the early stages of regrowth. Indirect plant responses to insect feeding may partially compensate for direct losses or, on the other hand, cause further reductions in yield and feed quality. Fick and Liu found many indirect effects of insect defoliation that have to be considered in evaluating insect damage to alfalfa and included in plant and insect models.
Detailed descriptions of the population dynamics of the alfalfa weevil and its parasite have made it possible to develop management models. For example, defining the phonology of the weevil and its parasite has led to adjustments in management practices to maximize parasite survival and minimize weevil survival. Furthermore, the development of a simulation model for alfalfa growth has added another dimension to understanding the weevil/alfalfa interaction. Predictions of plant growth and phenology should allow for improved scheduling of management practices such as harvest.
In addition to the basic biology of the alfalfa plant, weevil, and parasite, an understanding of such phenomena as plant compensation to insect damage, disruption of ecological balance in the alfalfa crop system, and the economics of the weevil/alfalfa interaction will lead to a more realistic structuring of the alfalfa ecosystem. The results suggest new approaches to management of the alfalfa weevil. For example, this past season alfalfa researchers, in cooperation with Extension staff and alfalfa growers, continued an alfalfa weevil management program in alfalfa fields in Illinois, Kentucky, New York, Iowa, Utah, Virginia, and other cooperating states.
How to Use the Program
To use this program the following things have to be done:
1. Calculate degree-day accumulation by recording daily high and low temperatures from January I until the end of the alfalfa weevil season in late spring.
2. Count the number of larvae on a 30-stem sample.
3. Measure the height of 10 stems from the original 30.
4. Refer to Recommendation Charts, which provide directions for the entire weevil season. They tell the user either to resample, harvest early, or spray. They also indicate when the weevil season is over, and samples are no longer needed.
Measuring and Recording Temperature
A record of daily high and low temperatures should be kept from January I until the end of the alfalfa weevil season. This information can be obtained from the daily newspaper, local weather stations, radio or television, specialized county extension systems, etc. Once the daily high and low have been obtained, the next step is to convert this information into degree-days from published tables.
The degree-days used in this program are based on a developmental threshold of 48°F, since at temperatures lower than this little or no weevil development takes place. (Note that the degree-days used to calculate alfalfa weevil development are not the same as the degree-days quoted in weather reports.) In areas where alfalfa weevils lay eggs in the fall and winter, field sampling must begin when 200 degree-days have accumulated since January 1. In areas with no fall or winter egg laying, sampling need not begin until 400 degree-days have accumulated.
Counting Larvae and Measuring Plant Height
Each 30-stem sample from a field should be taken in a pattern that covers as much of the field as possible. This is important because the level of infestation varies across a field. For example, the problems are often worse on southern slopes because these areas tend to be more protected during the winter and warm up sooner in the spring. Field edges should be avoided because they give inaccurate samples. If possible, stay at least 50 ft from the edges.
At 30 evenly spaced intervals, carefully pick an entire stem (without dislodging any larvae) and place it in a 2- to 3-gal container. Stems at each location must be selected at random, and this can be done by picking the first stem the hand touches. Next, beat the 30 steams vigorously against the inside of the container for a few seconds. Transfer the larvae to a shallow pan for counting and record the number found. Randomly select 10 stems from the original 30 and record their average length to the nearest inch.
This process requires 20 to 25 min for a 15- to 20-acre field. In very large fields (40 acres or more), it may be better to take two or more 30-stem samples and average the results.
If the alfalfa was windrowed during harvest, the 30-stem samples should be picked from the windrow area whenever possible (after removal of the hay). If there are enough larvae on these stems to recommend spraying, it would be well to pick another 30 stems, avoiding windrow areas. If there are so few larvae on these stems that spraying is not recommended, spraying only the windrow areas will save on the cost of insecticide.
Samples should be taken more frequently early in the season than toward the end of the season. A field should be visited an average of every 7 days during the weevil season. levity an extremely early spring, a field could be sampled as many as 11 times or more.
A sample that is preceded by frost or beating rains can result in underestimation of population density. Numerous larvae may be found on the ground following these weather conditions. Although some larvae will probably fail to crawl up the plant, it is suggested that these kinds of fields be resampled the following day. Each time an alfalfa field is sampled, Recommendation Charts must be consulted to determine if spraying is needed. A field must be resampled 100 degree-days after spraying to make sure the spray was effective.
Although the Recommendation Charts are designed to allow decision making in a routine fashion, unusual weather conditions may require a few modifications. With several alfalfa fields, unseasonably warm weather early in the season (such as during February) could make it difficult to finish sampling all fields within the prescribed degree-day ranges. In this case, starting or finishing 5 to 10 degree-days on either side of the range will not be detrimental. However, subsequent sampling should be adjusted to coincide with the next range on the chart. This situation will be rare. Usually, the rate of weevil development and plant growth varies enough from field to field so that, after the first sample of the season, all fields will rarely be sampled on the same day.
This program is offered as an alternative for deciding when to spray alfalfa weevil. Older methods are really no better than rules of thumb and are often confusing. As a result, treatment thresholds will vary greatly with the observer.
Although the Alfalfa Weevil Pest Management Program has received extensive testing, it is being continually improved and will receive further revision and refinement. The program offers the farmer, pest management consultant, dealer, and others methods for determining the timing of insecticide applications for control of the alfalfa weevil.
1. Davis, D. W., Nichols, M. P., and Armbrust, E. J., The literature of arthropods associated with alfalfa. 1. A bibliography of the spotted alfalfa aphid, Theriophasis maculate (13uckton) (Homopterahidae), 111. Nat. Hist. Surv. Biol. Notes, 87, 1974.
2. Harper, A. M., Miska, I. P., Manglitz, G. R., Armbrust, E. I., and Irwin, B. I., The literature of arthropods associated with alfalfa. 111. A bibliography of the pea aphid, Acrythosiphon pisum (Harris) (Homopterahidae), Unit. Spec. Publ., 50, 1978.
3. Morrison, W. P., Pass, B. C., Nichols, M. P., and Armburst, E. I., The literature of arthropods associated with alfalfa. II. A bibliography of the Sitona species (Coleoptera:Curculionidae), 111. Nat. Hist. Surv. Biol. Notes, 88, 1974.
4. Sorensen, E. L., Wilson, M. C., and Manglitz, G. R., Breeding for insect resistance, in Alfalfa Science and Technology, Hanson, C. H., Ed., American Society of Agronomy, Madison, His., 1972, 371.
5. Summers, C. G. and Lehman, W. F., Evaluation of non-dormant alfalfa cultivars for resistance to the Egyptian alfalfa weevil, J. Econ. Entomol., 69, 29, 1976.
6. Wedberg, I. L., Ruesink, W. G., Armbrust, E. I., and Bartell, D. P., Alfalfa weevil pest management program, Univ. Ill.. Coll. Agric. Coop. Ext. Serve Circ., 1136, 1977.
7. Bartell, D. P., Sanborn, I. R., and Wood, K. A., Insecticide penetration of cocoons containing diapausing and nondiapausing Bathyplectes curculionis, an endoparasite of the alfalfa weevil, Environ. Entomol., 5, 659, 1976.
S. Cherry, R. H. and Armbrust, E. J., Field survival of diapausing Bathyplectes curculionis a parasite of the alfalfa weevil, Environ. Entomol.,4, 93L 1975.
9. Cherry, R. H. and Armbrust, E. J., Predators of Bathyplectes curculionis, a parasite of the alfalfa weevil, Entomophaga(3), 323, 1977.
10. Cherry, R. H., Armbrust, E. I., and Ruesink, W. G., Lethal temperatures of diapausing Bathyplectes curculionis (Hymenoptera:lchneumonidae), a parasite of the alfalfa weevil (Coleoptera:curculionidae), Great Lakes Entomol.,9, 189, 1976.
11. Koehler, C. S. and Rosenthal, S. S., Economic injury levels of the Egyptian alfalfa or the alfalfa weevil, J. Econ. Entomol., 68, 71, 1975.
12. Hintz, T. R., Wilson, M. C., and Armbrust, E. J., Impact of alfalfa weevil larvae feeding on the quality and yield of first cutting alfalfa, J. Econ. Entomol., 69, 749, 1976.
13. Liu, B. W. Y. and Fick, G. W., Yield and quality losses due to alfalfa weevil, Agron. 1., 67, 828, 1975.
14. Fick, G. W. and Liu, B. W. Y., Alfalfa weevil effects on root reserves, developmental rate, and canopy structure of alfalfa, Agron. J.,68, 595, 1976.
Copyright © 1981 CRC Press. CRC Handbook of Pest Management in Agriculture Volume III.
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