Insect herbivores, life history and wild animal welfare

Summary

  1. Life his­tory clas­sifi­ca­tion will hide some sig­nifi­cant differ­ences in the lives of wild an­i­mals. Not all species within a given clas­sifi­ca­tion pos­sess all of the traits as­so­ci­ated with that group even across all years or all lo­ca­tions. There­fore, when mak­ing moral de­ci­sions, one also has to con­sider how av­er­age qual­ity of life should be de­ter­mined in the face of large variance

  2. Among in­sect her­bivores, some lifes­pans are rel­a­tively long, some modes of death are very quick, and some small-bod­ied her­bivores may lead lives char­ac­ter­ized by am­ple food resources

  3. Although de­ter­min­ing the af­fec­tive states of wild an­i­mals from this data is im­pos­si­ble, it seems quite likely that the ma­jor­ity in­di­vi­d­u­als in some sub­groups, such as those sheltered from both the el­e­ments and pre­da­tion by feed­ing from within plant tis­sues, lead very high qual­ity lives

  4. Know­ing a group of or­ganisms pro­duce many offspring, have high mor­tal­ity rates, small body size and are short-lived is not suffi­cient to de­ter­mine that their lives are a net nega­tive (or pos­i­tive)

The ar­gu­ment from life his­tory sug­gests that since many species pro­duce many more offspring than sur­vive to adult­hood, are of small size and so sub­ject to many abiotic and biotic threats, and are short-lived rel­a­tive to hu­mans, that there is more suffer­ing than hap­piness in na­ture and there­fore we have a moral obli­ga­tion to end this suffer­ing (e.g., To­masik 2015). Here, we do not at­tempt to ex­am­ine such moral quan­daries (for a thought ex­per­i­ment on these is­sues see Bren­nan 2017). In­stead, we aim to im­prove the qual­ity of dis­cus­sions by ex­am­in­ing available data on one group of wild an­i­mals. Be­cause of their broad scope, some pre­vi­ous analy­ses of wild an­i­mal welfare have lumped many species to­gether at one pole of a con­tinuum of life his­tory strate­gies. Given these large group­ings, and some is­sues with the life his­tory clas­sifi­ca­tions them­selves, it is un­clear to what ex­tent this ap­proach ac­tu­ally in­forms us about an­i­mal suffer­ing in gen­eral. On the other hand, ex­am­i­na­tions of the lives of par­tic­u­lar species are also un­satis­fac­tory, since they re­fer to the spe­cific rather than the ma­jor­ity. By nar­row­ing our fo­cus to one group, we may be able to bring more data to bear on our in­tu­itions re­gard­ing wild an­i­mal welfare. In this post, we ex­plore the liter­a­ture re­gard­ing one group of or­ganisms that are clas­si­cally grouped in the “r-strate­gist” life his­tory cat­e­gory: ter­res­trial in­sect her­bivores.

Ter­res­trial in­sects are de­scribed by some as hav­ing net nega­tive welfare. They are one of the largest groups of ter­res­trial an­i­mals in terms of num­ber of species, bio­mass and es­ti­mated num­ber of in­di­vi­d­u­als. As May (1988) noted, “To a rough ap­prox­i­ma­tion, and set­ting aside ver­te­brate chau­vinism, it can be said that es­sen­tially all or­ganisms are in­sects.” Bar-On et al. (2018) es­ti­mate the bio­mass of ter­res­trial arthro­pods at 0.2 gi­ga­tons of car­bon, and in­sects are the largest clas­sifi­ca­tion in the arthro­pod phy­lum, ac­count­ing for over 80% of all de­scribed species in this group (Zhang 2013). For ex­am­ple, Höll­dobler & Wil­son (1990) sug­gest that 15-20% of the bio­mass of ter­res­trial an­i­mals is com­posed of ants. In­sects are ~1/​3 of all de­scribed species on the earth (Grimaldi & En­gel 2005), while es­ti­mates for the to­tal num­ber of in­sect species range be­tween five and seven mil­lion (e.g., Stork 2018).

Var­i­ous ap­prox­i­ma­tions of the to­tal num­ber of in­sect in­di­vi­d­u­als range be­tween 10^17-10^19 (Willi­ams 1960 , Höll­dobler & Wil­son (1990), Bar-On et al. (2018)). As small-bod­ied pri­mary con­sumers, her­bivores will be at the bot­tom of the trophic en­ergy trans­fer in any com­mu­nity. Since only ~10% of pri­mary pro­duc­tion passes be­tween trophic lev­els, this group and de­tri­tivores will make up ~90% of all ter­res­trial in­sects. As­sum­ing equal di­vi­sion be­tween these two groups and 1018 in­sect in­di­vi­d­u­als, ter­res­trial in­sect her­bivores will then com­prise ap­prox­i­mately 10^17 in­di­vi­d­u­als. In other words, this is one of the largest groups of ter­res­trial an­i­mals on the earth.

Life his­tory clas­sifi­ca­tion of ter­res­trial insects

Ter­res­trial in­sects are of­ten grouped near the “r” or “fast” end of life his­tory clas­sifi­ca­tions (see our prior post on life his­tory). The r- and K-se­lected clas­sifi­ca­tion scheme is based on the differ­ent evolu­tion­ary pres­sures in un­crowded, re­source rich en­vi­ron­ments and crowded re­source poor en­vi­ron­ments. (MacArthur and Wil­son 1967). K-se­lected species will fre­quently ex­pe­rience re­source scarcity and star­va­tion, while r-se­lected species will ex­pe­rience an abun­dant en­vi­ron­ment. Ex­treme r-strate­gies are small-bod­ied and short-lived, reach re­pro­duc­tive ma­tu­rity early, re­pro­duce once (semel­parous), have a large num­ber of offspring at that time, do not provide parental care, and have high ju­ve­nile mor­tal­ity (Pi­anka 1970). The fast-slow cat­e­go­riza­tion of life his­tory that has su­per­seded r- and K- clas­sifi­ca­tion has similar traits for “fast” species. How­ever, there will always be ex­cep­tions: egg­plant lace bugs provide ex­ten­sive parental care (Tal­lamy 1999), and ci­cadas can live for nearly 2 decades (Si­mon 1988). More­over, some species have life his­to­ries that are not cap­tured by these di­choto­mous clas­sifi­ca­tions. In par­tic­u­lar, those in harsh abiotic con­di­tions may have longer lifes­pans, slower growth rates, and lower re­pro­duc­tive out­put, be­cause of a larger in­vest­ment in sur­vival adap­ta­tions (e.g., arc­tic wooly bears Kukal & Daw­son 1989).

Analy­ses of ter­res­trial in­sect her­bivore life history

Here we re­port on available liter­a­ture data re­gard­ing fe­cun­dity, mor­tal­ity and lifes­pan in an effort to provide some quan­ti­ta­tive in­sight re­gard­ing the lives of ter­res­trial in­sect her­bivores. We con­clude that vari­abil­ity in life his­tory is the pri­mary char­ac­ter­is­tic of this group, but cer­tainly these species are, on av­er­age, shorter-lived, more fe­cund, and less likely to provide parental care than mam­mals.

Life span

Both r-se­lected and fast life his­tory clas­sifi­ca­tions de­scribe in­sects as short-lived, and most do live less than two years. Life spans of in­sects, how­ever, are quite vari­able. Carey (2001) notes that the be­tween-group vari­a­tion is enor­mous: for her­bivores, this range in­cludes aphids with a lifes­pan of weeks, to xylem-feed­ing bee­tles that take sev­eral years to reach ma­tu­rity, to ter­mite queens that can live for decades. This 5000-fold differ­ence in the life spans of in­sects can be con­trasted with the smaller 40-fold differ­ence in the life spans of mam­mals: from small ro­dents that live less than 2 years to hu­mans that live an av­er­age ~80 years.

It is in­cor­rect to be­lieve that life span for any in­sect species is a sin­gle fixed age. The lifes­pan of a monarch but­terfly (Danaus plex­ip­pus) is ap­prox­i­mately 2-3 months when it is in the re­pro­duc­tive mode dur­ing the sum­mer months but 6-10 months when it is in the mi­gra­tory mode dur­ing the win­ter months (Grace 1997). Some com­men­ta­tors have con­fused the length of a sin­gle life stage with that of the en­tire lifes­pan of an in­di­vi­d­ual. For ex­am­ple, while some mayfly adults live only a few hours, mayfly nymphs take longer, up to two years, to ma­ture. For some species (e.g., Lepi­doptera: but­terflies and moths), these ju­ve­nile stages tend to be the largest pro­por­tion of the to­tal lifes­pan in tem­per­ate re­gions (e.g., see data in Danks 2006 for adult life spans). For oth­ers, (e.g., Coleoptera: bee­tles), the adult life stage tends to be the longest. For species with­out dis­tinct lar­val and pu­pal stages, the pre­re­pro­duc­tive stage also tends to be shorter than the adult stage (e.g., Hemiptera: true bugs). We note that these are all very coarse gen­er­al­iza­tions over very large groups. There are about 177,500 de­scribed species of Lepi­doptera, 400,000 of Coleoptera, and 79,000 of Hemiptera.

Across in­sect groups, parental care, monogamy, and eu­so­cial­ity (at least for queens) are all as­so­ci­ated with ex­tended life spans. As well, species that must seek out host plants that are scarce or widely dis­persed tend to be long-lived (e.g., Heli­co­nius but­terflies have widely-dis­persed host plants and only lay only a few eggs at a time. Th­ese species are long-lived for lep­i­dopter­ans in tem­per­ate lo­ca­tions, some­times ex­ceed­ing 4–6 months (Ehrlich 1987, Gilbert 1972).

As ec­totherms, to­tal lifes­pan and de­vel­op­ment time are also re­lated to tem­per­a­ture for in­sects. For ex­am­ple, at 10 °C Aphis gossypii take 75 days to reach re­pro­duc­tive ma­tu­rity, and live a max­i­mum of 103 days, while at 30 °C they take 5 days to reach re­pro­duc­tive ma­tu­rity and live a max­i­mum of 37 days (Ko­courek et al 1994). In ad­di­tion, in­creased longevity can be as­so­ci­ated with pe­ri­ods of re­source limi­ta­tion. Ex­treme life ex­ten­sions, such as di­a­pause for more than 10 years, usu­ally af­fect only a very small frac­tion of the pop­u­la­tion, and have only been recorded in about 64 species (Gill et al. 2017). How­ever, more mod­est ex­ten­sions, such as de­vel­op­ment over 2 years when 1 year is more nor­mal, are rel­a­tively com­mon (Danks 1992).

In ad­di­tion, species that are sub­ject to un­cer­tain or harsh en­vi­ron­ments fre­quently ex­hibit ex­tended longevity as­so­ci­ated with ex­tended or re­peated di­a­pause. Over­win­ter­ing di­a­pause nor­mally lasts for 9-10 months in the tem­per­ate zones (Gill et al. 2017). How­ever, Con­vey (1997) re­ports ex­tended lifes­pan in Antarc­tic arthro­pods as com­pared to their close phy­lo­ge­netic rel­a­tives in more tem­per­ate re­gions (e.g., 2-5 years vs ~1 year) as­so­ci­ated with re­peated di­a­pause over sev­eral win­ters. How­ever, lifes­pan can be short­ened or length­ened to deal with a brief sum­mer sea­son in po­lar re­gions (Danks 2004). Given this, we gen­er­ally ex­pect in­creased vari­abil­ity in lifes­pan with in­creas­ing lat­i­tude and also al­ti­tude (e.g., Laiolo & Obeso 2017).

Over­all, very short lifes­pans (less than 20 days) seem fairly rare (Danks 2006), and long lives (>3 years) are rarer still. Species from cool tem­per­ate re­gions tend to have longer life cy­cles with about one gen­er­a­tion per year (e.g., Danks and Foot­tit 1989), as do species liv­ing in ar­eas that have a dry sea­son. But we note that for many of these species, vari­able en­vi­ron­men­tal con­di­tions de­ter­mine how many gen­er­a­tions there are per year, and in ad­di­tion, the over­win­ter­ing gen­er­a­tion will have a longer lifes­pan than grow­ing sea­son gen­er­a­tions.

Fecundity

The num­ber of offspring pro­duced by ter­res­trial her­bivores varies with body size, parental care, and the en­vi­ron­ment. While in­fant care is not com­mon among in­sect species, it cer­tainly oc­curs, and is nega­tively cor­re­lated with re­pro­duc­tive out­put as ex­pected from a fast-slow life his­tory clas­sifi­ca­tion. Gilbert and Man­ica (2010a) ex­am­ined fe­cun­dity data for 220 non-car­ing, 23 offspring guard­ing, and 32 offspring pro­vi­sion­ing ter­res­trial in­sects (Note: this anal­y­sis was not re­stricted to her­bivores, but did ex­clude eu­so­cial species). Fe­cun­dity is defined as the num­ber of offspring pro­duced over the life­time of an in­di­vi­d­ual. Dung bee­tles species had the low­est life­time fe­cun­dity (~2 offspring), while mayflies had the largest (~4000 offspring). For ter­res­trial her­bivores in this dataset, the range ex­tends from 12-15 offspring for some leafmin­ers and wood-bor­ing bee­tles to ~3000 for cut­worms. The me­dian life­time fe­cun­dity (mea­sured as num­ber of eggs per in­di­vi­d­ual) was 138 but varied with the type of parental care (166 eggs for no care, 66 for guard­ing and 40 for pro­vi­sion­ing in­sects; all val­ues calcu­lated from Gilbert and Man­ica 2010b; but note the au­thors did not iden­tify the num­ber of eggs that were ac­tu­ally vi­able).

Con­trary to the fast-slow life his­tory clas­sifi­ca­tion, fe­cun­dity of­ten, al­though not always, pos­i­tively cor­re­lates with body size in in­sects (see Leather 1988 for dis­cus­sion). In species where par­ents provide no care or sim­ply guard eggs (e.g., the hi­bis­cus harlequin bug Tec­to­coris dioph­thal­mus ag­gres­sively defends newly laid eggs, Giffney & Kemp 2016), larger-bod­ied species pro­duced more and larger eggs. In species that pro­vi­sion offspring, such as bur­rower bugs that provide mint nut­lets to their nymphs (Se­hirus cinc­tus, see Kight 1997), those with larger bod­ies also pro­duced larger eggs but laid fewer (Gilbert and Man­ica 2010a). In this re­spect, the life his­to­ries of pro­vi­sion­ing in­sects re­sem­bled those of birds or mam­mals (tra­di­tion­ally viewed as slow or K-se­lected species) rather than those of re­lated species that in­vested less in parental care.

In ad­di­tion to the form of parental care and body size, en­vi­ron­ment also has an im­pact on in­sect fe­cun­dity. Her­bivores that feed on the plant ex­te­rior have higher fe­cun­di­ties (Cor­nell & Hawk­ins 1995), but this find­ing is par­tially con­founded by the fact the ex­ter­nal feed­ers tend to have larger body sizes than species that feed from within plant tis­sues (e.g., en­do­phyt­ics such as gall form­ers, stem bor­ers and leaf min­ers). More gen­er­ally, plant qual­ity can al­ter the fe­cun­dity of in­sect her­bivores (Aw­mack & Leather 2002).

Tem­per­a­ture also de­ter­mines fe­cun­dity, for ex­am­ple mean to­tal fe­cun­dity of Aphis gossypii ranges from 36 lar­vae per fe­male at 10 °C to 76 lar­vae at 30 °C. (Ko­courek et al. 1994). Broader en­vi­ron­men­tal con­di­tions also im­pact fe­cun­dity through both effects on time to re­pro­duc­tive ma­tu­rity and egg vi­a­bil­ity. For ex­am­ple, the de­vel­op­ing lar­vae of the arc­tic woolly bear moth, Gy­naephora groen­landica, feed on a north­ern willow species only in June when am­bi­ent tem­per­a­tures are rel­a­tively high and the host plant has the high­est nu­tri­ent con­tent. The lar­vae then leave the plant un­til the fol­low­ing sum­mer. Be­cause of the short grow­ing sea­son, the moth lar­vae take be­tween 7-14 years to reach ma­tu­rity (Kukal & Daw­son 1989, More­wood and Ring 1998), and fe­males have low life­time fe­cun­dity, pro­duc­ing ~ 5 vi­able eggs (Kukal & Ke­van 1987).

Ju­ve­nile Mortality

Sur­vival tra­jec­to­ries for ju­ve­nile in­sect her­bivores are quite vari­able, but do show a trend of higher mor­tal­ity for younger stages. The most fre­quent cause of mor­tal­ity is preda­tors and par­a­sitoids. Cor­nell & Hawk­ins (1995) ex­am­ined 530 datasets for 124 species of her­bivorous in­sects that had 4 dis­tinct stages: egg, lar­vae, pu­pae and adult re­gard­ing the sur­vival of the ju­ve­nile stages to de­ter­mine the most fre­quent causes of death. Sprayed, caged, or lab­o­ra­tory pop­u­la­tions were ex­cluded from analy­ses, as were in­sects that had no dis­tinct pu­pal stage (i.e., these are species such as bee­tles and but­terflies, but not aphids). The most com­mon cause of death across both groups is preda­tors and par­a­sitoids fol­lowed by weather, plant defences and com­pe­ti­tion (Fig 1).

How­ever, the au­thors find wide vari­a­tion in sur­vival tra­jec­to­ries re­gard­less of whether the same species and pop­u­la­tion was sam­pled within the same year in differ­ent lo­ca­tions or be­tween years. Between species, this var­i­ance is even larger. For ex­am­ple, in most cases 50% of eggs of Da­cus oleae, the olive fruit fly, sur­vived to adult­hood, while less than 5% of cab­bage root fly (Eri­oschia bras­si­cae) eggs did. If an av­er­age across this wide var­i­ance is taken, there are similar sur­vival tra­jec­to­ries of ju­ve­niles across species groups, with slightly higher rates of mor­tal­ity oc­cur­ring in the early life stages and slightly lower rates in the later ones. Sources of mor­tal­ity shift as her­bivores grow. Phys­iolog­i­cal fac­tors, weather and plant fac­tors more fre­quently kill early stages, whereas par­a­sitoids and preda­tors more fre­quently kill later stages.

Ex­ter­nal feed­ers were more likely to be kil­led by par­a­sitoids and preda­tors, while plant fac­tors were more likely to kill species that feed from in­side the plant tis­sue (en­do­phytic species). This group of her­bivores had higher sur­vival in the older ju­ve­nile stages than species that fed from the out­side of the plant (Cor­nell & Hawk­ins 1995). On av­er­age about 70% of eggs be­came lar­vae, 20-30% of eggs be­came late stage lar­vae (de­pend­ing on whether the species was en­do­phytic or not), 5-20% be­came pu­pae and 2-10% of eggs reached adult stage. Th­ese find­ings broadly sup­port Price’s (1974) ear­lier claim of lower mor­tal­ity in lar­vae con­cealed in plant tis­sue, but not the claim that this group ex­hibited con­vex sur­vival curves, with most mor­tal­ity oc­cur­ring past the mid- to late lar­val stage.

Figure 1: Sum­mary of the fre­quency of mor­tal­ity fac­tors for all ju­ve­nile in­sect her­bivores in Cor­nell & Hawk­ins (1995).

Competition

The ini­tial idea be­hind r-se­lec­tion, that species on this end of the spec­trum are not limited by re­sources may be sup­ported by Cor­nell and Hawk­ins’s (1995) anal­y­sis of mor­tal­ity. Com­pe­ti­tion is a very small fac­tor in to­tal ju­ve­nile mor­tal­ity, and the vast ma­jor­ity of mor­tal­ity events are caused by preda­tors and par­a­sitoids. The weather, in­trin­sic fac­tors re­lated to re­pro­duc­tion (egg vi­a­bil­ity) and plant defences also ex­plained more mor­tal­ity than com­pe­ti­tion. This lower im­pact of com­pe­ti­tion is pos­si­bly be­cause there are very few species that reach very large pop­u­la­tion sizes (e.g, Ayres and Lom­bardero 2000, Faeth 1987). In ad­di­tion, even among these species, such pop­u­la­tion out­breaks are rather rare, usu­ally lo­cal and of short du­ra­tion (Ko­zlov & Zvera 2017).

On the other hand, meta-analy­ses con­sis­tently find ev­i­dence of nega­tive effects of one in­sect her­bivore species on an­other (Ka­plan & Denno 2007, Bird et al. 2019); how­ever, the sug­gested mechanisms for this im­pact are pre­dom­i­nantly in­di­rect effects (e.g., in­duc­ing stronger plant defences and in­creas­ing preda­tor pop­u­la­tions). Direct lethal im­pacts caused by food short­ages are con­sid­ered less com­mon. More­over, in a meta-anal­y­sis of 75 pub­lished stud­ies Vi­dal & Mur­phy (2018) found that preda­tors and par­a­sitoids have a larger im­pact on her­bivore abun­dance, sur­vival, growth and fe­cun­dity than plant qual­ity. There are also sig­nifi­cant benefi­cial effects of in­sect her­bivores species on each other (e.g., by over­whelming host plant defences, dis­tract­ing nat­u­ral en­e­mies, or by cre­at­ing shelters such as leaf mines and rolls; Cor­nelissen et al. 2016, Soler et al. 2012), so that there is no nec­es­sary di­rect re­la­tion­ship be­tween in­creas­ing num­bers of her­bivores and nega­tive im­pacts.

Pre­da­tion, pathogens and parasitoids

Nat­u­ral en­e­mies (par­a­sitoids, preda­tors and pathogens) emerged as the most fre­quent mor­tal­ity fac­tor (48% of all deaths) in Cor­nell and Hawk­ins (1995), sur­pass­ing ev­ery other source of mor­tal­ity over all life stages and, in some cases, ex­ceed­ing all oth­ers com­bined. Th­ese fac­tors were a larger source of mor­tal­ity in later ju­ve­nile stages. In a study of 78 in­sect her­bivore species with a range of 100-1000 in­di­vi­d­u­als for each, Hawk­ins et al. (1997) re­port close to 0% me­dian per­cent mor­tal­ity for eggs and early lar­val stages from nat­u­ral en­e­mies, al­though this statis­tic is par­tially due to the ex­tremely low mor­tal­ity among en­do­phytic species. In this study, nat­u­ral en­e­mies kil­led a me­dian of 1% of mid lar­val stage in­di­vi­d­u­als, 3% of late stage and 5% of pu­pal stage.

Hawk­ins et al. (1997) find that over­all ~4% of all in­di­vi­d­u­als are kil­led by par­a­sitoids, ~1% by preda­tors and less than 0.5% by pathogens. Th­ese au­thors re­port that many her­bivores, par­tic­u­larly species feed­ing within plant tis­sues, suffer lit­tle or no mor­tal­ity from pathogens. So while the au­thors note that dis­ease can be im­por­tant in some groups of in­sects (e.g., for­est Lepi­doptera My­ers 1993), on av­er­age it does not seem rep­re­sent an im­por­tant mor­tal­ity source in phy­tophagous in­sect pop­u­la­tions. Hawk­ins et al. (1997) also find that par­a­sitoid deaths are more com­mon in tem­per­ate re­gions than in trop­i­cal re­gions, where preda­tors have a some­what larger im­pact. In ad­di­tion, both Cor­nell and Hawk­ins (1995) and Hawk­ins et al. (1997) re­port that en­do­phytic her­bivores suffer lower mor­tal­ity by preda­tors. Leaf min­ers suffer the great­est par­a­sitoid-in­duced mor­tal­ity, while gall-form­ers, stem­bor­ers and root feed­ers suffer the least. Hawk­ins et al. (1997) note that for koino­biont par­a­sitoids (see be­low), at­tack will oc­cur in ear­lier lar­val stages, while death will oc­cur later. So her­bivore mor­tal­ity rates in­crease through time, where this mor­tal­ity in­cludes both both de­layed mor­tal­ity from koino­bionts as well as im­me­di­ate mor­tal­ity by later at­tacks from preda­tors and idio­biont par­a­sitoids.

Predators

True preda­tors are less spe­cial­ized than par­a­sites and par­a­sitoids, and they usu­ally catch prey smaller than them­selves (Griffiths 1980). It has been sug­gested that the im­pact of death by pre­da­tion on the net value of a life may be par­tially de­ter­mined on the length of the pre­da­tion event with re­spect to length of life. Since in­sect life spans are ex­tremely vari­able, so are the length of ju­ve­nile stages. How­ever, as noted by Plant (2016), death by pre­da­tion is of­ten a rel­a­tively small pro­por­tion of even a very short life span. For ex­am­ple, video cap­ture of a gen­er­al­ist preda­tor Har­mo­nia axyridis (Asian multi-coloured lady­bug) feed­ing on aphids sug­gests con­sump­tion rates of 20-60 aphids in a one hour pe­riod, im­ply­ing that each prey in­di­vi­d­ual was con­sumed in a cou­ple of min­utes (Feng et al. 2019). In con­trast Michálek et al. (2017) used high speed pho­tog­ra­phy to cap­ture hunt­ing tac­tics in both a spe­cial­ist and gen­er­al­ist spi­der species. Spe­cial­ist preda­tors tend to con­sume rel­a­tively large prey. The au­thors recorded body part con­sump­tion rates, and if we as­sume that prey death oc­curred af­ter max­i­mum con­sump­tion time by the spe­cial­ist preda­tor, then the prey would ex­pe­rience about 300 min­utes of pre­da­tion. Us­ing these times, and not­ing that the ma­jor­ity of ju­ve­nile mor­tal­ity by pre­da­tion oc­curs in the late lar­val to pu­pal stage, which for the shorter lived species would be on the or­der of a cou­ple of weeks, the ex­pe­rience of death by a true preda­tor would range from 0.007-1.0% of to­tal lifes­pan (as­sum­ing death at 3 weeks). For species that tended to take longer to ma­ture (e.g., ~1 yr), and as­sum­ing that the late lar­val or pu­pal stage oc­curs oc­curs in the fall, the av­er­age es­ti­mate would be lower (~0.002-0.3% of to­tal lifes­pan, as­sum­ing death at 3 months).

Parasitoids

Across all stages, par­a­sitoids kill more her­bivores than ei­ther preda­tors or pathogen (Hawk­ins et al. 1997). Death by par­a­sitoid is most com­mon late lar­val and pu­pal stages (Hawk­ins et al. 1997). Koino­biont par­a­sitoids (cf Har­vey & Mal­ci­cka 2016), such as Cote­sia vestalis, al­low the host to con­tinue feed­ing and de­velop so that death takes quite a long time. In con­trast idio­bionts cause host de­vel­op­ment to cease once par­a­sitized, ei­ther by caus­ing death or paral­y­sis dur­ing ovipo­si­tion. There­fore, ju­ve­nile her­bivores in­oc­u­lated by par­a­sitoids can ex­pe­rience a much longer time frame to death (e.g., 12 days in one study). How­ever, this species group is enor­mous, and it seems difficult to make any gen­er­al­iza­tions re­gard­ing time to death by differ­ent classes of par­a­sitoids. Koino­biont par­a­sitoids of­ten rep­re­sent the most speciose por­tion of par­a­sitoid com­plexes (Mills 1994), so it is pos­si­ble that this is the most com­mon form of death for in­sect her­bivores.

The per­centage of time the in­sect her­bivore ex­pe­riences the effects of the par­a­sitoid will be highly vari­able, but re­lated to the age at which is par­a­sitized. If we as­sume in­oc­u­la­tion at the most com­mon time of early-mid lar­val stage, and a koino­biont par­a­sitoid that keeps the lar­vae al­ive for an­other 10 days of de­vel­op­ment, then ~40% of to­tal lifes­pan may serve as an es­ti­mate for species that ma­ture in about 60 days (death at just over 3 weeks), and ~20% for those that take about 1 year, where late lar­val or pu­pa­tion stage oc­curs at about 4 months (i.e., Septem­ber—Oc­to­ber in north­ern tem­per­ate re­gions, and there­fore death at ~6 weeks). How­ever, whether lar­vae be­ing eaten by par­a­sitoids feel pain dur­ing the ex­pe­rience is un­clear, given that both par­a­sitoids and par­a­sites can cause ex­ten­sive be­havi­our mod­ifi­ca­tion in their hosts (e.g., Chen et al 2017).

Weather

Fol­low­ing un­known or mis­cel­la­neous fac­tors, the weather was the sec­ond largest known cause of mor­tal­ity (11%) in Cor­nell and Hawk­ins (1995), and this fac­tor had a larger im­pact on early ju­ve­nile states. Weather-gen­er­ated mor­tal­ity was due mainly to rain­fall and over­win­ter­ing deaths. The higher fre­quency of weather-gen­er­ated mor­tal­ity in early lar­val stages is prob­a­bly due to higher risk of dis­lodge­ment by rain­fall. The au­thors sug­gest that the im­por­tance of weather in later stages prob­a­bly de­pends on whether these stages over­win­ter. Of course, species that live within the plant tis­sues (e.g., gall-form­ers, stem bor­ers and and leaf-tiers) were less af­fected by weather than species that live on plant sur­faces. Gre­gar­i­ous species are also less likely to be af­fected by weather con­di­tions. For ex­am­ple, tent cater­pillars raise their tem­per­a­tures by bask­ing in groups and also con­struct elab­o­rate group shelters (Stamp and Bow­ers 1990). We also point out that even ex­tremely harsh con­di­tion are not nec­es­sar­ily a prob­lem for species adapted to those lo­ca­tions. Arc­tic woolly bear win­ter mor­tal­ity is quite low (~13%) be­cause the lar­vae seek out well pro­tected lo­ca­tions and have spe­cial phys­iolog­i­cal adap­ta­tions for cold har­di­ness (Kukal et al. 1987, 1989).

There­fore, mor­tal­ity of ju­ve­nile her­bivores can be quite high, but then again, the ju­ve­nile stage may be the longest life stage for some of these species (e.g., Lepi­dopter­ans). Mor­tal­ity of ju­ve­nile her­bivorous in­sects is dom­i­nated by par­a­sitoids and preda­tors, in that or­der, in tem­per­ate re­gions. En­do­phytic species are less likely to die from these causes (with the ex­cep­tion of leaf min­ers). Ju­ve­nile in­sect her­bivores seem to be sel­dom short of food (al­though food qual­ity may vary), and are only oc­ca­sion­ally sig­nifi­cantly im­pacted by weather.

Conclusion

This ex­am­i­na­tion of data on in­sect her­bivores sug­gests that life his­tory clas­sifi­ca­tion will hide some sig­nifi­cant differ­ences in the lives of wild an­i­mals. In par­tic­u­lar, claims about large fe­cun­dity, lack of parental care and short lives are not cor­rect for all species that are rep­re­sented as be­long­ing to a sin­gle group within a life his­tory clas­sifi­ca­tion, nor are they true for all years or all lo­ca­tions of the same species. There­fore, when mak­ing moral de­ci­sions about the qual­ity of wild an­i­mal lives, it seems likely that one also has to con­sider one’s po­si­tion re­gard­ing how av­er­age qual­ity of life should be de­ter­mined in the face of such large var­i­ance.

Set­ting aside prob­lems with gen­er­al­iza­tion, we note that the mere ob­ser­va­tion a large group of or­ganisms pro­duce many offspring, have high mor­tal­ity rates, small body size and are short-lived is still not suffi­cient to de­ter­mine that their lives are net-nega­tive. Even if death is very painful, it is un­clear that ex­pe­riences pre­vi­ous to death are not suffi­ciently plea­surable to com­pen­sate. While this pos­si­bil­ity has of­ten been dis­missed on the grounds that these an­i­mals have such a short lifes­pan, some of the data we ex­plore sug­gest that some ju­ve­nile lifes­pans are rel­a­tively long, some modes of death are very quick, and that small-bod­ied her­bivores may of­ten lead lives char­ac­ter­ized by am­ple food re­sources. In fact, it seems quite pos­si­ble that the ma­jor­ity in­di­vi­d­u­als in some sub­groups, such as those sheltered from both the el­e­ments and pre­da­tion by feed­ing from within plant tis­sues, lead very high qual­ity lives. We are care­ful to note, how­ever, that at­tempt­ing to de­ter­mine the af­fec­tive states of wild an­i­mals from this data is cur­rently im­pos­si­ble.

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Credits

This es­say is a pro­ject of Re­think Pri­ori­ties. It was writ­ten by Kim Cud­ding­ton. Thanks to Ja­son Schukraft, Daniela Wald­horn, David Moss, Mar­cus Davis and Peter Hur­ford for com­ments. If you like our work, please con­sider sub­scribing to our newslet­ter. You can see all our work to date here.