Mean (± SE) water potential (bar), and numbers of dry leaves, Mexican rice borer egg clusters, total eggs, entry holes, and exit holes per stalk of two sugarcane varieties maintained under well watered or drought stressed greenhouse conditions (Showler & Castro, 2010a)
When the availability of water is insufficient to maintain plant growth, photosynthesis, and transpiration, plants become water deficit stressed (Fan et al., 2006), a serious problem that reduces world crop production (Boyer, 1982; Vincent et al., 2005). While drought has profound direct detrimental effects against plants, including rendering otherwise arable regions less, or non-, arable, herbivorous arthropod populations and the injuries they cause can be affected by stress-related changes that occur in the plant. Moderate stress is known to heighten the nutritional value of some plants’ tissues and juices, in some instances to reduce concentrations of plant defense compounds, and even to select against predators and parasitoids that otherwise help reduce pest populations to economically tolerable levels, each of which can contribute toward greater pest infestations. Sometimes the injury inflicted on water deficit stressed plants is intensified even if numbers of the pest haven’t been affected, as in the instances of honeylocust spider mites,
Although severe water deficit stress that causes plant mortality usually renders plants useless to herbivores, chronic lower level or pulsed water deficit stress can enhance the nutritional value of plants to arthropods, resulting in selection preference, heightened populations, intensified injury to crops, and even outbreaks that affect production on area-wide scales. Twospotted spider mite,
Water deficit stress in plants can affect the amounts and composition of volatile compounds, and the concentrations of several kinds of nutrients beneficial to arthropod pests. Its associations with free amino acids and carbohydrates are chiefly described in this chapter because those two kinds of nutrients have been researched to an appreciable extent, permitting some conclusions to be drawn about arthropod host plant selection and levels of infestation.
2. Water deficit, host plant nutrient accumulation, and associations with phytophagous arthropods
Water deficit stress alters plant metabolism and biochemistry (Hsiao, 1973; Beck et al., 2007), and consequent changes to plant physiological processes have been reported as being factors affecting herbivorous arthropod host plant preferences, growth, and development (Mattson & Haack, 1987; Showler, 2012 ). Although soil dries in association with drought, evapotranspiration rates in affected plants are often maintained (Jordan & Ritchie, 1971) by elevated accumulations of free amino acids, especially proline, and other organic solutes (Janagouar et al., 1983). Osmotic stress in plants involves several interlinked molecular pathways that transmit signals and produce stress-responsive metabolites (Ingram & Bartels, 1996; Zhu, 2002), and gene transcripts associated with signaling can be up- or down-regulated minutes after stress induction (Seki et al., 2001; Showler et al., 2007). Water deficit stressed plants often have diminished osmotic potential (Labanauskas et al., 1981; Golan-Goldhirsch et al., 1989; Bussis & Heineke, 1998), heightened oxidative stress (Becana et al., 1998; Knight & Knight, 2001), and accumulations of osmolytes such as antioxidants, amino acids, carbohydrates, and inorganic ions, altering the attractiveness and nutritional value of the plant (Jones, 1991; Showler & Castro, 2010a). Reduced leaf water content relative to dry biomass in water deficit stressed plants, in combination with the increased quantities of nutritional metabolites (White, 1984; Dubey, 1999; Ramanulu et al., 1999; Garg et al., 2001), may contribute toward the increased nutritional value of plants per unit of surface area consumed by arthropods. It is likely that arthropods can perceive cues about host plant suitability from emission of plant volatile compounds, or semiochemicals.
Chemical cues from plants play a major, perhaps decisive, role in host plant selection and utilization by herbivorous arthropods (Schur & Holdaway, 1970; Fenemore, 1980; Waladde, 1983; Burton & Schuster, 1981; Ramaswamy, 1988; Salama et al., 1984; Udayagiri & Mason, 1995). Water deficit stress in plants alters plant metabolism which can affect quantities and combinations of volatile compounds (Apelbaum & Yang, 1981; Hansen & Hitz, 1982; Zhang & Kirkham, 1990). Apple trees,
Once the phytophagous arthropod has found or selected the host plant, contact chemoreceptors on many are important in the acceptance or rejection of a host plant based on the presence or absence of stimulant (
In addition to elevated levels of free essential amino acids, free proline, a nonessential amino acid that accumulates in most water deficit-afflicted plants, is a feeding stimulant for many phytophagous arthropods (Mattson & Haack, 1987; Städler, 1984). Dadd (1985) reported that a number of amino acids, particularly glycine, alanine, serine, methionine, histidine, proline, and γ-aminobutyric acid, were phagostimulants to a number of insect species. Amino acids that elicited the greatest response as feeding stimulants to southwestern corn borer larvae were determined to be arginine, histidine, lysine, methionine, phenylanaline, valine (essentials), alanine, glycine, and serine (nonessentials) (Hedin et al., 1990), but not proline.
Water deficit stress has also been associated with increased concentrations of carbohydrates (which have important roles in osmotic adjustment) in many plants (Schubert et al., 1995; Kameli & Lösel, 1996; Massacci et al., 1996; Mohammadkhani & Heidari, 2008). Corn plants with elevated soluble carbohydrate concentrations were preferred by the European corn borer for oviposition (Derridj & Fiala, 1983; Derridj et al., 1986), and styloconic sensilla of larvae and adults of three noctuid species were highly responsive to sugars, especially sucrose and fructose (Blaney & Simmonds, 1988). These two sugars are known to be important feeding stimulants for both life stages (Frings & Frings, 1956; Blom, 1978), and fructose, glucose, maltose, and sucrose have been identified as phagostimulants for other insects (Bernays, 1985). Electrophysiological recordings revealed that the maxillary sensilla styloconica of fifth instar African armyworm,
3. Water is a nutrient, too
Water deficit affects both the availability of water, which is a nutrient itself, to herbivores as well as the nutritional quality of dietary biochemical components that accumulate as osmoprotectants or for other purposes. When herbivorous arthropods are unable to have access to sufficient amounts of wager, their populations can decline. For example, aphid populations are reduced under conditions of continued and severe host plant water deficit ( Showler, 2012 ). Black bean aphid,
The greater nutritional quality of water deficit stressed plants can be offset by the condition that causes it: insufficient water. When provided with dried, ground material from water-deficit stressed tomato plants,
4. Some non-nutrient-related associations of water deficit with phytophagous arthropods
Host plant selection among insects also involves visual and physical factors such as leaf shape, color, and size (Ramaswamy, 1988; Renwick & Radke, 1988; Renwick & Chew, 1994; Showler & Castro, 2010b), and both constitutive and inducible plant chemical defenses can vary in response to water deficit stress (Lombardero et al., 2000), but visual and physical cues, and defensive compounds are not considered as being nutritional for the purposes of this chapter (although defensive compounds might loosely be considered as being types of nutrients, they mostly repel, interfere with feeding, or act as toxins). Concentrations of several classes of defensive secondary compounds tend to increase in plant tissues in response to moderate drought, including terpenoids (some of which are attractants (Mattson & Haack, 1987) and alkaloids (Gershenson, 1984; Hoffmann et al., 1984; Sharpe et al., 1985; Lorio, 1986; Mattson & Haack, 1987; Showler, 2012 ), but intensified drought stress can lead to reductions of these compounds (Mattson and Haack, 1987). Drought can also influence predator and parasitoid guilds that affect phytophagous arthropod populations ( Showler, 2012 ), but plant stress is not directly involved. Other mechanisms that might also contribute toward plant vulnerability to herbivorous arthropods under conditions of water deficit stress have been suggested (Mattson & Haack, 1987), including acoustical cues, detoxification of foods by drought stressed insects, and drought-induced genetic changes in arthropods, but they have not been well substantiated.
5. Multiple effects of water deficit: case study on sugarcane and the Mexican rice borer
The Mexican rice borer,
Eggs are mostly deposited in clusters within folds of dry sugarcane leaves, although eggs are also laid in folded green living tissue if available (Showler & Castro, 2010b). Van Leerdam et al. (1986) found 96% of the pest’s eggs on the basal 80 cm of sugarcane plants where most dry leaf tissue is located. The Mexican rice borer is not so much stress-oriented as it is nutritionally-oriented in that it prefers to lay eggs on dry foliage of plants stressed by limited water and of plants growing in enriched soil (Showler & Castro, 2010a; Showler & Reagan, 2012). Water deficit stress in sugarcane plants, however, unlike over-fertilized plants, offers increased quantities of dry, folded leaf tissue per plant, contributing to the crop’s vulnerability (Reay-Jones et al., 2005; Showler & Castro, 2010b). In a greenhouse no-choice cage experiment using sugarcane plants from which all dry leaf tissue was excised and removed from the cages, or placed at the bottom of the cages like a mulch, and intact (dry leaf tissue remained on the plants) sugarcane plants (controls), numbers of eggs and the degree of larval infestation was distinctly greater on the controls (Figs. 1 & 2; Showler & Castro, 2010b).
Early instars feed on living leaf tissue, under fresh leaf sheaths, and some tunnel into the leaf midrib; later instars bore into the main stalk (Wilson, 2011). Injury from stalk tunneling results in deadheart, decreased sugar production, and stunting or lodging of stalks sometimes so severe that harvest becomes unfeasible (Johnson, 1985; Legaspi et al., 1997; Hummel et al., 2008). Tunnels within host plant stalks are packed with frass, blocking entry of predators and parasitoids (Hummel et al., 2008). Pupation occurs within the stalk after mature larvae make emergence holes protected with a thin window of outer plant tissue (Hummel et al., 2008). In the Lower Rio Grande Valley, a life cycle takes 30–45 days, and there are 4–6 overlapping generations per year (Johnson, 1985; Legaspi et al., 1997). Tunneling damage and the insect’s prevalence has made it the key sugarcane pest of south Texas, displacing the sugarcane borer,
Approximately 20% of sugarcane internodes are injured by Mexican rice borers in south Texas, and larval entry holes also provide portals for red rot, resulting in additional loss of sugar (Van Zwaluwenberg, 1926; Osborn & Phillips, 1946; Johnson, 1985). On some varieties of sugarcane, up to 50% bored internodes have been reported ( Johnson, 1981 ); Mexican rice borer injury results in losses of US$575 per hectare of sugarcane (Meagher et al., 1994) and US$10–20 million annually (Legaspi et al., 1997, 1999). Projected economic consequences of Mexican rice borer infestation of Louisiana includes US$220 million in sugarcane and US$45 million in rice (Reay-Jones et al., 2008). In corn, stalk boring and secondary infection by stalk rot pathogens can cause shattering, lodging, and complete collapse of stalks (Showler et al., 2011) such that by season’s end >50% of stalks of susceptible varieties are destroyed (Showler, unpublished data).
A connection between irrigation practices and severity of Mexican rice borer infestation was first suggested by Meagher et al. (1993), and later studies indicated that drought stressed sugarcane is preferred for oviposition because there is more dry leaf tissue and the nutritional value, at least in terms of a number of important free amino acids, is enhanced (Tables 1 & 2) (Muquing & Ru-Kai, 1998; Reay-Jones et al., 2005, 2007; Showler & Castro, 2010a). Although severe water deficit stress of sugarcane reduces sugar production, some cultivars under moderate stress accumulate sugars (Hemaprabha et al., 2004), and Mexican rice borer preference among species of host plants (Showler et al., 2011) has been associated with concentrations of fructose (Showler, unpublished data). Differences in oviposition preference were not observed on excised dry leaf tissue regardless of whether the sugarcane plant from which it originated was water deficit stressed or well watered; hence, the expression of sugarcane vulnerability or resistance appears to require the pest’s ability to detect nutrients in living leaf tissue (Showler & Castro, 2010b). Although a sugarcane cultivar with some degree of resistance to the Mexican rice borer was still better protected than a susceptible variety under drought conditions, water deficit increased injury to the crop by ≈2.5-fold in each (Reay-Jones et al., 2005). Reay-Jones et al. (2003) also reported that high soil salinity, a stress factor that also heightens free amino acid accumulations in plants (Labanauskas et al., 1981; Cusido et al., 1987), increases Mexican rice borer infestations in sugarcane. Further, relatively high concentrations of organic matter incorporated into soil of the Lower Rio Grande Valley (and conventionally fertilized with nitrogen) resulted in 18% more stalk production per sugarcane stool but this effect was offset by substantial increases in Mexican rice borer infestation, causing stalk weight, length, and percentage brix reductions relative to sugarcane fertilized with conventional nitrogen fertilizer or chicken litter (Showler, unpublished data). The composted soil was associated with greater accumulations of free amino acids and fructose (Showler, unpublished data). These associations reveal that the pest is not responding simply to water deficit, but instead to nutritional enhancement of the plant whether moderated by stress or by other factors.
In addition to water deficit stress associations with Mexican rice borer preferences for physical (
Water deficit might initially appear to affect herbivorous arthropod populations because of a single factor, but the associations of the Mexican rice borer with water indicate a more complex relationship that can involve physical, biochemical, and ecological factors. Levels of Mexican rice borer infestation are likely influenced by low water availability in at least three ways, only one of which is directly related to the nutritional status of the crop. Drought changes many environmental conditions relative to arthropods, such as soil condition, leaf size and color, lignification of plant cell walls, secondary protective compounds, and natural enemy activity, but accumulations of nutrients, particularly free amino acids and carbohydrates, unlike the other drought-related conditions, directly result from water deficit stress to the plant. This plant stress response to water deficit influences levels of pest infestations by causing the plant emit volatile semiochemicals and by enhancing the nutritional quality of the plant. Water deficit can also make it difficult for some plant sucking insects (