The Food Insects Newsletter
Food Conversion Efficiencies of Insect Herbivores
March 1993. Volume 6, Issue #1.
By Richard L. Lindroth
University of Wisconsin
In his classic children's book, The Very Hungry Caterpillar, Eric Carle describes the development of an increasingly voracious caterpillar, from egg hatch to metamorphosis into a beautiful butterfly. In addition to the character appeal of the larva and aesthetic quality of the illustrations, the book teaches some valuable lessons about the nutritional ecology of insect herbivores. The caterpillar hatched on Sunday: on Monday he ate through one apple, on Tuesday two pears . . . and on Saturday "he ate through one piece of chocolate cake, one ice cream cone, one pickle, one slice of Swiss cheese, one slice of salami, one lollipop, one piece of cherry pie, one sausage, one cupcake, and one slice of watermelon. That night he had a stomachache!"
What are the lessons we can learn? First, the older (and bigger) the insect is, the faster it eats. Indeed, consumption and growth rates increase exponentially with insect age. For example, leaf consumption by the forest tent caterpillar (Malacosoma disslria) is approximately 0.05, 0.2, 0.8, 2.9 and 18.0 square inches for instars 1-5, respectively. Second, the older an insect is, the more diversified its diet may become. Most herbivorous insects are specialists. feeding on only one or a few related species for their entire life span. But some insects are generalists; notable among these is the gypsy moth (Lymanlria dispar), which feeds on over 300 species of woody plants. For these generalist feeders, diets typically become increasingly diversified as maturity affords both greater mobility and increased capacity to detoxify the chemical defenses of plants. Third, for caterpillars. as for humans, some foods or combinations thereof may bring considerable discomfort.
These are basic principles of the discipline of nutritional ecology, which, in short, addresses what insects eat, why they eat what they do, and how efficient they are in doing it. The latter theme will be introduced in this paper. Several excellent reviews have been published on the topic and can be consulted for additional information (see References).
Insects, like all living organisms, require energy and nutrients to survive, grow and reproduce. The nutritional components (e.g., protein, carbohydrates, fats, vitamins, minerals) of ingested food may or may not be digested and absorbed. The proportion of ingested food that is actually digested is denoted by AD, the assimilation efficiency (also called "approximate digestibility"). Of the nutrients absorbed, portions are expended in the processes of respiration and work. The proportion of digested food that is actually transformed into net insect biomass is denoted by ECD, the efficiency of conversion of digested food. A parallel parameter, ECI, indicates the efficiency of conversion of ingested food (ECI = AD x ECD). In short, AD indicates how digestible a food is, whereas ECD and ECI indicate how efficient a herbivore is in converting that food into biomass. These efficiency values may be calculated for specific dietary nutrients as well as for the bulk diet. For instance, nitrogen use efficiencies are informative because levels of plant nitrogen (an index of protein) are often times limiting to insect performance.
Food conversion efficiencies may vary considerably within a species. One cause of such variation involves homeostatic adjustment of consumption rates and efficiency parameters such that an insect can approach its "ideal" growth rate even with foods of different quality in various environments. For example, insects that experience reduced ECDs due to increased respiratory costs may be able to compensate by increasing consumption rates or digestion efficiencies (ADs). Not all changes are homeostatic, however. For instance, many insects increase food consumption rates in response to low concentrations of critical nutrients such as protein. Increased consumption will accelerate passage of food through the gut and thereby reduce ADs. In our work with the gypsy moth we found that larvae reared on a protein deficient diet increased consumption rates by 3-4-fold, but overall ADs declined by nearly as much. Other nonhomeostatic changes in efficiency values may occur in response to plant allelochemicals. For example, compensatory feeding to increase intake of a limiting nutrient may simultaneously increase exposure to plant toxins, which in turn may reduce ECDs. In practice, however, it can be quite difficult to ascertain "cause" and "effect" responses with efficiency parameters. Does the insect eat more because digestibility is low, or is digestibility low because the insect is eating more? Efficiency parameters are so closely physiologically related that determination of "cause" and "effect" is not a trivial matter.
Intraspecific variation in food conversion efficiencies may also be related to insect development. ADs generally decrease, whereas ECDs increase, from early to late instars. In other words older larvae digest their food less completely, but that which they do digest is more efficiently utilized for growth. One study showed that values for AD and ECD change from 46% to 27% and 38% to 60%. respectively, for early and late instars of the desert locust (Schislocerca gregaria). Factors contributing to such changes are still largely unknown, but may include shifts in food selection, digestive physiology, metabolic rates, and body composition.
Food conversion efficiencies also vary greatly among species. and this variation is more closely related to feeding guilds than to taxonomic affinity. insects that feed on nitrogen-rich foliage generally have higher consumption rates and assimilation efficiencies than do insects that feed on nitrogen-poor foliage, and as a consequence grow and develop much faster. The classic example here is the difference between forb- and tree-feeders. Forb leaves typically have high levels of nitrogen and water. whereas tree leaves have lower levels of those substituents and higher levels of poorly digestible compounds such as cellulose, lignin and tannins. Accordingly, insects that feed on mature tree leaves exhibit growth rates half or less than those insects that feed on forbs. The relatively poor nutritional quality of tree foliage has had important consequences for insect life histories. In temperate regions forb feeders often have many more generations per year than do tree feeders. Among tree-feeders, numerous species have adapted to emerge and feed only on the especially nutritious early spring foliage, and thus have only one generation per year.
Other examples that demonstrate how the various efficiencies are strongly influenced by food quality include wood- and seed-feeding insects. Wood is tough and nutritionally poor. Thus wood-chewers have slow rates of consumption and digestion (much of which is accomplished by symbiotic microbes). The combination of these factors precludes all but slow growth rates in wood-feeders. In contrast, seeds are high in readily digestible carbohydrates and protein and low in fibrous material. Thus seed-feeders exhibit high ADs. Growth rates are nonetheless only low to moderate, due to low consumption rates and low ECDs. Low ECDs may result from a requirement of these insects to metabolize digested food in order to produce water.
Understanding of these basic principles of nutritional ecology can enhance our appreciation of insects as a food resource. Environmentalists and others concerned about nutrition and world food resources have long decried the reliance of some people on large animal protein (e.g., beef) as a dietary staple. The reasoning is that production of such high-quality protein is very inefficient; more food would be available if people ate the grain instead. This debate is complex and beyond the scope of this paper. Suffice it to say, however, that a major reason that large animals are inefficient in transforming plant biomass into animal biomass is that they have very high maintenance costs (i.e., low ECDs). Large amounts of energy and nutrients are used to maintain constant body temperatures. Insects, being "cold-blooded," are more efficient in transforming plant biomass into animal biomass.
Understanding of basic nutritional ecology may also improve selection of insect and plant species for large-scale insect production. For example, production will be more rapid with forb feeders than with tree-feeders and with leaf-feeders than with wood-feeders, other environmental factors equal. Want to know what plant/insect characteristics may be limiting production? Some simple input/output and growth measurements will tell whether production is limited by low consumption, poor digestibility, or inefficient conversion of assimilated food into body mass. Different corrective measures may be available for each situation.
This article benefited greatly from the content and inspiration of excellent reviews by Frank Slansky and Mark Scriber.
Scriber, J.M., and F. Slansky. 1981. The nutritional ecology of immature insects. Annual Review of Entomology 26:183-211.
Slansky, F., and J .M . Scriber, 1982 . Selected bibliography and summary of quantitative food utilization by immature insects. Bulletin of the Entomological Society of America 28:43-55.
Slansky, F., and J.M. Scriber. 1985. Food consumption and utilization. Pp. 87-163, in G.A. Kerkut and L.I. Gilbert (eds.), Comprehensive Insect Physiology, Biochemistry and Pharmacology. Vol. 4. Regulation: Digestion Nutrition Excretion. Pergamon Press, N.Y.
A Follow-up Interview with Dr. Lindroth
The Newsletter has never used this journalistic technique before, but it seems a good way of getting the most out of our invited experts while we have their attention. We'll designate the questioner as The FlN(The Food Insects Newsletter). It's too bad we're not in the fish business because it would make a great acronym.
The FIN: First, thank you Dr. Lindroth for accepting our invitation to set forth some basic principles of insect food conversion efficiency in the Newsletter and for taking additional time to respond to some questions. The food conversion efficiency of edible insects has important ecological and environmental implications. First question. Remembering that edible insects furnish not only protein, but fats, vitamins, and minerals, and, as a very high proportion of growth occurs in the last two larval or nymphal instars (about 95% in lepidopterous larvae as shown with your example, M. disstria), can we assume that the combined ECI for the last two instars is a valid (and the simplest) statistic for comparing food conversion efficiency (let's shorten it to FCE) between or within species in different situations? A second, related question. Do ecologists have any "rule-of-thumb" ECI level that is considered good, or is everything comparative and dependent on the quality of the food source?
Dr. Lindroth: If I had to select only one efficiency measure, ECI would be a good candidate, as it represents efficiencies of both digestion and how well digested food is converted to biomass. Bear in mind though, that insects can compensate for low ECIs to some degree simply by increasing their feeding rates. Thus two insects could have the same growth rate; one achieves it by eating less but being very efficient with what it eats, the other by eating more but being less efficient. Because so much of an insect's feeding and growth occurs in its last few instars, FCEs from that period are a very useful comparative measure. Another caution here is that dietary characteristics (nutrient deficiencies or toxins) may affect younger instars more than older instars, and if the impact is great enough, you'll never see those insects as older instars.
I'm reluctant to suggest what ECI values may be "good" or "bad"; they're really more useful in a comparative sense. What is "good" for one insect feeding on one substrate may or may not be "good" for another insect feeding on another substrate. What is most valuable is to compare different species (or races) feeding on the same food, or individuals of one species feeding on different foods .
The FIN: You pointed out that forb-feeders show higher FCEs than tree leaf-feeders because forbs are higher in nitrogen and water and lower in such hard-to-digest compounds as cellulose and lignin. I've seen combined ECI data (Scriber's) on only one forb-feeding edible insect, Spodoptera eridania (the southern armyworm). When tested on 10 varieties of alfalfa, combined ECIs ranged below 15% on six varieties, from 15.5-20.3% on three others, and showed an incredible 29.8% on Vernal alfalfa. Two questions. Do you know of any vertebrate meat animal that can come anywhere close to 29.8%? And secondly, how do you explain such great ECI differences at the plant varietal level?
Dr. Lindroth: Yes. As you'll see below, poultry can attain this level of efficiency. But their food source is grain, which is even richer than alfalfa.
Considerable variation in ECIs at the plant varietal level has not been well-studied, but may not be as unusual as one might expect. For example, in a study with gypsy moth larvae feeding on individual aspen trees from a common habitat, we found ECI values that ranged from 6% to 16%. In our case among-tree variation in levels of phenolic toxins greatly influenced ECI's and subsequent larval growth rates. I'm not at all surprised that differences of the magnitude you describe exist among plant varieties. Those differences probably result from differences in chemical or physical attributes of the varieties.
The FIN: In scanning ECI data, one can dream up some wild schemes. For example, Scriber also tested S. eridania on five kinds of clover and trefoil. The highest combined ECI was on Trifolium agrarium (yellow blossom sweet clover), 23 .6%. Now, commercial pond fish producers are looking for good sources of long-chain w3 polyunsaturated fatty acids, and lepidopterous larvae, in general, would be a rich source if they could be feasibly exploited. Yellow blossom sweet clover must do very well on poor soils, because it's along roadsides all over the country. And S. eridania has multiple generations per year. Maybe it would pay the fish growers to hire a young entomologist (or maybe put some research money into your lab) to look into the possibilities. Maybe the armyworms should be harvested at the end of the penultimate (second to last) instar. Scriber's data showed an incredible ECI of 56.9% for that instar on YBSC (it was even higher, 58.3%, on Vernal alfalfa.)
Dr. Lindroth: You're right, the possibilities are great. As you know better than I, a minor shift in one's thinking about insects as food can open up many new avenues of research and application.
The FIN: Unfortunately, many more of the major edible insect groups seem to feed on trees and grasses, or even wood, than feed on forbs. Tests on two species of edible grasshoppers, Locusrana migratoria and a species of Melanoplus, fed on several kinds of grasses showed combined ECIs in the range of 10-15% and 8-11%, respectively. Two questions. How do ECIs in the range of 10-15% compare with other grass-caters such as cattle? (I believe there is a rule-of-thumb in cattle husbandry that 15 lbs of hay puts on a pound of gain). As grasshoppers are generalists, if they were reared on forbs, should we expect higher ECIs?
Dr. Lindroth: As I alluded to in the article, FECs are generally higher for insects than for vertebrates. One must be careful in making such comparisons, however. One problem is that insect values are reported on the basis of dry weights, whereas livestock values are reported as "gain" which typically includes 70% water. After adjusting for water weight, ballpark figures for efficiency of gain are seen below. Clearly, the insects are superior to mammals when fed the same rood. FCEs of vertebrates can approach or even surpass those of insects when they are fed especially nutritious and digestible food such as grain.
Chicken (grain) 30%
Pigs (grain) 11 %
Beef (grain) 5%
Beef (grass) 3%
About rearing grasshoppers on forbs: I would expect higher ECIs than when reared on grass.
The FIN: Larvae of the giant silk moths (Family Saturniidae) are a major food insect group, especially in Africa. Most of these are tree-feeders, and as you indicated in general for tree-feeders, most have only one generation per year. I don't know of any ECI data on African species, but data by Scriber and Feeney on nine North American species on 21 host species showed combined ECIs ranging from 7.1 to 15.8 (ECIs above 10 on nine of the 21 larva/host combinations). Doesn't it seem that, even with ECIs at the relatively low range of 10-15%, if the forest was properly managed for caterpillar (and termite) preservation (as has been recommended in several instances by researchers in Africa), it would be about as productive for animal agriculture as grassland? Is there a short answer for this complex question, or is the question not as complex as it seems?
Dr. Lindroth: On the surface the reasoning seems sound. But a number of complicating factors come to mind; the answer really is complex. For example, because grasslands have coevolved with large grazing mammals grasses can recover remarkably well from extensive grazing. Remove the same percentage of green foliage from a forest habitat and you'll not have the forest for long. And then there are the practical matters of harvest, etc. It is probably much easier to harvest 1000 lbs of large animal biomass from a grassland than an equivalent amount of insect biomass from a forest! This is not to say that management of forests for insect production should not be considered, just that the comparison with grassland systems is fraught with problems.
The FIN: Several important food insect groups develop in wood, including decaying or rotten logs. As would be expected, most have long life cycles, one or more years, for example in the beetle families, Buprestidae and Cerambycidae. Palm weevils of the genus Rhynchophorus (Family Curculionidac). however, complete development in only two or three months in palm logs. Is this an exception to the "feeding guilds" principle that you mentioned (feeding guilds more important than taxonomic affinity in determining food conversion efficiency), or what would explain such relatively fast development on such poor food?
Dr. Lindroth: This is an interesting example. I don't know the answer, but I can hazard a guess. Most trees are dicotyledons and the woody tissue of these species is loaded with lignins, tannins, etc. Palm trees are monocotyledons: they are more closely related to Kentucky bluegrass than to oaks or maples. I know next to nothing about the chemical composition of palm logs, but would suggest that they have higher levels of particular nutrients (e.g., nitrogen, sugars) and/or lower levels of lignins and tannins than occur in the wood of dicots.
The FIN: Thanks again, Rick, and a final question. Are forbs and herbs the same thing?
Dr. Lindroth: Not quite. Herbs arc non-woody plants, including both monocots and dicots. In temperate regions they "die back" to ground level at the end of the growing season. Forbs are herbs that are not grasses (dicots).
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