Self-Selection of Macronutrients in Two Populations of Grasshoppers
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Physiological Entomology (2008) 33, 264–273 DOI: 10.1111/j.1365-3032.2008.00619.x Discriminating tastes: self-selection of macronutrients in two populations of grasshoppers DENNIS J. FIELDING and LINDA S. DEFOLIART 1Agricultural Research Service, U. S. Department of Agriculture, Fairbanks, Alaska, U.S.A. Abstract . The capacity to self-select an optimal balance of macronutrients (protein and carbohydrate) is studied in two populations of Melanoplus sanguinipes F. (Orthoptera: Acrididae). One population derives from the subarctic (interior of Alaska) and the other from the temperate zone (Idaho, U.S.A.). Over the duration of the fourth and fifth stadia, Alaskan grasshoppers consistently self-select a diet cen- tred on a 0.90 ratio of protein : carbohydrate, whereas protein and carbohydrate intake by the Idaho grasshoppers is contingent on the particular food choices pre- sented to them. When restricted to imbalanced diets, the Alaskan grasshoppers develop more rapidly than the Idaho grasshoppers, regardless of diet composition. The Idaho grasshoppers also have a greater amount of lipid than the Alaskan grass- hoppers across all diets. Performance measures (body mass, survival, developmental times) are more sensitive to dietary imbalances in the Alaskan grasshoppers than in the Idaho grasshoppers. When fed diets with low, but balanced, proportions of pro- tein and carbohydrate, grasshoppers of both populations are able to increase con- sumption to compensate for the low concentration of nutrients. The results suggest that demographic responses of insects to changes in host plant quality, such as may result from climate change, may differ among populations within a species. Key words . Acrididae , Orthoptera , insect nutrition. Introduction when unable to meet its intake target, would do much to increase our understanding of demographic responses to Study of the nutritional ecology of insects has a long history altered food quality resulting from seasonal variations or a ( Waldbauer, 1968; Scriber & Slansky, 1981 ). More recently, changing climate. researchers have gained an appreciation of the importance of Many investigations of insect nutrition have been prompted the balance of macronutrients in an insect’s diet, especially by concerns regarding the effects of elevated levels of atmos- carbohydrates and protein ( Raubenheimer & Simpson, 1993, pheric CO2 on plant chemistry ( Bezemer & Jones, 1998; 2003; Lee et al. , 2004; Simpson et al. , 2004 ). Allowed to Coviella & Trumble, 1999; Barbehenn et al. , 2004; Zvereva self-select the composition of their diet, many insects con- & Kozlov, 2006 ). Atmospheric concentration of CO2 is sume carbohydrate and protein in proportions that are near expected to double within this century ( Prentice et al. , 2001 ). optimal in terms of fitness for the organism ( Waldbauer & Studies of C 3 plants exposed to elevated levels of CO 2 con- Friedman, 1991; Simpson et al. , 2004 ). This self-selected sistently show an increase in carbohydrates (up to 100%) and balance of macronutrients is referred to as their ‘intake tar- a decrease (up to 20%) in protein of leaf tissues ( Korner & get’ ( Raubenheimer & Simpson, 1993 ). Knowledge of an Miglietta, 1994; McGuire et al. , 1995; Poorter et al. 1997; insect’s intake target, and its survival, growth and fecundity Long et al. , 2004 ). These changes in leaf chemistry are often detrimental to insect herbivores ( Bezemer & Jones, 1998; Correspondence: Dennis J. Fielding, USDA/ARS Subarctic Coviella & Trumble, 1999; Asshoff & Hattenschwiler, 2005; Agricultural Research Unit, PO Box 757200, University of Alaska, Wu et al. , 2006 ), but insects can compensate for altered food Fairbanks, AK 99775, U.S.A. Tel.: +1 907 474 2439; fax: +1 907 quality in many ways: they may increase consumption in 474 1813; e-mail: [email protected] response to low levels of protein; they may increase the Journal compilation © 2008 The Royal Entomological Society 264 No claims to original US government works Discriminating tastes 265 efficiency of protein assimilation or alter allocation of pro- 75-W incandescent lamps. Hatchlings were fed romaine let- tein to different tissues; and they may use protein as an tuce and wheat bran through their first three stadia. Fourth- energy source if carbohydrates or lipids are limited ( Zanotto instar grasshoppers were weighed, sexed and placed into et al. , 1993; Yang & Joern, 1994 ). Higher temperatures may individual containers within 24 h of moulting. also increase digestive efficiency of insects to override any The containers were plastic food-storage containers effect of altered food quality ( Zvereva & Kozlov, 2006 ). (12.5 × 12.5 × 5 cm). Five 3-mm diameter holes were drilled Many species of locusts and grasshoppers (Orthoptera: on each side for ventilation and the bottom was lined with Acrididae) are sensitive to food quality and populations may 3.2-mm mesh hardware cloth, which was pressed into the track food resources ( Belovsky & Slade, 1995; Hunter et al. , bottom of the container leaving room so that frass could 2001; Branson, 2003 ). The population dynamics of pest spe- accumulate on the bottom of the container but grasshoppers cies of acridids may result from subtle changes in the collec- would not come in contact with the frass. Dry meridic (a tive performance of individuals. Melanoplus sanguinipes F. combination of natural and chemically defined ingredients) is perhaps the most economically damaging grasshopper in diet was provided in ceramic boats (57 × 22 × 11 mm). Free North America and has a very broad geographic distribution water was available to the grasshoppers at all times. ( Pfadt, 1995 ). Many field studies show M. sanguinipes to be During the feeding trials, grasshoppers were housed in a a generalist feeder ( Mulkern et al. , 1964; Pfadt et al. , 1988; Conviron growth chamber (Conviron TC16; Controlled Fielding & Brusven, 1992 ). Considering the economic import- Environments Limited, Winnipeg, Canada). The temperature ance of this species, there are relatively few studies of its followed a roughly sine-shaped curve from a maximum of nutritional requirements and demographic responses to diet 36 °C to a minimum of 18 °C. Temperatures in the chamber quality using precisely defined diets. Joern & Behmer (1998) were at or above 30 °C for 13 h per day. Diurnally fluctuating demonstrate the fecundity of adults of this species in response temperatures were used for several reasons: to approximate to different concentrations of nitrogen and carbohydrate in the normal daily cycle experienced by grasshoppers in the defined diets, but we are unaware of any such studies exam- field; to provide a range of temperatures in case populations ining nymphal growth and development of M. sanguinipes in differed in their optimum temperatures for growth and feed- this context. ing; and to provide greater resolution in developmental times The broad geographic range of this species makes it a good (grasshoppers were inspected daily after the temperature had candidate for comparative studies to examine adaptations to dropped below 22 °C and it was assumed that any new adults different environments ( Fielding & DeFoliart, 2005 ). For the had moulted during the previous warm cycle). The photope- present study, one population of M. sanguinipes is derived riod comprised LD 16.5 : 7.5 h, with the dark hours centred from individuals collected from Idaho, U.S.A. (46.38°N, around the minimum temperature. 117.02°W, 450 m elevation) and the other from the interior of Experimental diets were introduced to third-instar grass- Alaska (64.00°N, 145.73°W, 400 m elevation). The growing hoppers to familiarize them with it, but lettuce and bran were season at the Alaska source is typically approximately available throughout the third stadium. Starting on the first 80 days less than that of the Idaho location. Also, precipita- day of the fourth stadium, grasshoppers were fed solely on tion patterns differ between the locations, with the interior of dry meridic diet (see below). The initial food given each Alaska receiving most of its annual precipitation in the sum- grasshopper was weighed, as were any additional offerings. mer, whereas, in Idaho, monthly mean precipitation is at a Grasshoppers were monitored daily so food and water could minimum during the summer months. Previous studies be replenished as needed. The date of the moult to adult was ( Fielding & DeFoliart, 2007 ) show that nymphal develop- recorded and the grasshoppers were then weighed and frozen. ment and growth occurs more rapidly in the Alaskan popula- Remaining food was collected and dried at 60 °C and weighed. tion under a variety of conditions. Furthermore, the Alaskan Adults were also dried at 60 °C and dry weights obtained. grasshoppers have a greater net retention of nitrogen than The experimental diets were all variations on the same those from Idaho. In the present study, the geometrical frame- base diet ( Fielding & DeFoliart, 2007 ), which contained work of Raubenheimer & Simpson (1993, 1999, 2004 ) was dried romaine lettuce, wheat germ, vitamins, yeast extract, employed to study the nutritional ecology of this species. cystine, glycine, cholesterol, flaxseed oil and cellulose. The The intake target for late-instar nymphs of these populations base diet was supplemented with pure protein (casein) and/or was indentified, and performance on imbalanced foods was pure carbohydrate (sucrose) and cellulose to achieve the vari- compared between these two populations adapted to very dif-