Larval development. Monarchs complete almost all of their growth
during the larval stage. This stage lasts from 9 to 14 days under normal
summer temperatures, during which time they undergo five larval instars (Figure
Under typical spring and late summer temperatures in the northern part of
the breeding range, this time can double. From hatching to pupation,
monarchs increase their body mass about 2000 times.
Monarch eggs and larvae have a slim change of reaching adulthood; several
studies have documented mortality rates of over 90% during the egg and larva
stages (Borkin 1982, Zalucki and Kitching 1982, Oberhauser et al. 2001,
Prysby and Oberhauser 2004). This mortality stems from both biotic and
abiotic sources. Biotic factors that affect monarch survival include natural
enemies such as predation, diseases and parasites; and interactions with
their milkweed hosts. Abiotic factors include environmental conditions such
as adverse weather and pesticides. Many monarchs in natural populations are
killed by invertebrate predators that eat the monarchs themselves, or by
parasitoids whose larvae develop in and eventually kill the monarch larvae.
Diseases caused by bacteria, viruses, fungi and other organisms are also
significant sources of monarch mortality.
Prysby (2004) documented overall impacts of natural enemies on monarch
survival. By limiting predator access to monarch eggs and larvae with
exclosures placed around naturally growing milkweed plants, she showed that
both terrestrial and aerial predators represent significant sources of
mortality (Figure 8). In addition, she found that monarch eggs were less likely to
survive on plants on which ants had been observed, suggesting that ants are
important predators. This conclusion is supported by work in Texas by Calvert (1996,
2004), who found that monarchs inside exclosures were much more likely to
survive than those outside the structures. Calvert found that invasive fire
ants currently kill most of the monarch eggs and larvae present in many
areas in Texas, but thinks that pre-fire ant mortality may have been
similarly high, since these invasive ants displaced native ants that also
preyed on monarchs. In addition to predators, insect parasitoids are
important sources of monarch mortality in some locations. Prybsy (2004) and
the Monarch Larva Monitoring Project have both documented
mortality rates of from 10% to 90% in late instar monarchs due to tachinid
fly (family Tachinidae) parasitoids, but these rates are variable from
location to location and year to year.
Monarch eggs do not hatch in very dry conditions (Dunlap et al. 2000), and
dry weather can kill milkweed. Very hot weather also causes mortality;
several studies have shown that temperatures above approximately 35oC (95oF)
can be lethal to all stages (Zalucki 1982, Malcolm et al. 1987, York and
Oberhauser 2003). Likewise, extended periods in which temperatures are below
freezing can kill monarchs, although this has been best studied in
overwintering adults (Anderson and Brower 1993, 1996; Brower et al. 2004).
Threats due to very hot or very cold temperatures are magnified during the
breeding season, since monarchs are indirectly affected by conditions that
affect milkweed health and survival. Freezing temperatures and extremely dry
conditions are especially damaging to milkweed, and thus to monarchs.
Pupae. During the pupa stage the transformation to the adult stage is
completed in a process that takes about 9 to 15 days under normal summer
temperatures. The ecology of monarch (or any other lepidopteran) pupae is
unfortunately poorly-studied, at least partially due to the fact that it is
extremely difficult to find monarch pupae in the wild. Their green color
provides effective camouflage in a green world, and they appear to seek
sheltered spots to undergo this transformation. Important questions on how
larvae choose sites for pupation, how far they travel seeking these sites,
what habitat characteristics are important in promoting pupal survival, and
how much mortality from different sources occurs during this stage remain to
Adults (Figure 9). Non-migratory adults live from two to five weeks, while those
that migrate may live up to nine months. This difference is due to the fact
that overwintering monarchs are not reproductive, and can thus funnel more
energy into survival. In addition, the cool conditions in the overwintering
sites slow their metabolism.
Summer generation monarchs first mate when they are 3 to 8 days old
(Figure 10) (Oberhauser
and Hampton 1995), and females begin laying eggs immediately after their
first mating. Monarchs that overwinter do not lay eggs until spring
(although they may mate before this). Both sexes can mate several times
during their lives (e.g., Oberhauser 1989), and the ability of male monarchs
to force unwilling females to copulate makes them unique among the
Lepidoptera (Oberhauser 1989; Van Hook 1993; Frey et al. 1998). When females
mate with more than one male, it is generally the last male that fertilizes
their eggs (Solensky 2003, Oberhauser personal observation).
Since there is a delay between adult emergence and egg-laying, and also
because monarchs reproduce over a relatively long time period, maximizing
reproductive success also requires being able to survive predators,
environmental extremes and other sources of mortality. Adult survival during
the breeding season is another under-studied area of monarch biology,
despite its importance to monarch ecology. Full understanding of adult
ecology during the breeding stage of their lives will require measuring the
effects of nectar availability and quality, the distances that females will
fly to find milkweed host plants, the degree to which breeding monarchs
remain in one area or move, and the effects of abiotic conditions on adult
survival (Oberhauser 2004).
Human-induced mortality during the breeding season. As with many other
species, the most important source of human-caused mortality for monarchs is
habitat loss, especially the destruction of milkweed and nectar sources.
Milkweed is considered a noxious weed in some localities, and is often
destroyed. In addition, herbicides used to kill plants in agricultural
fields, near roadsides, and in gardens may harm milkweed and nectar sources,
and may also kill monarchs directly. This has probably become much more
important in agricultural fields with the widespread adoption of
herbicide-tolerant crops. In a study conducted in the summer of 2000, Oberhauser et al. (2001) found that most monarchs probably originated in
agricultural habitats. However, since that study, most soybeans grown in the
upper Midwestern US, the source of most overwintering monarchs (Wassenar and
Hobson 1998), are herbicide-tolerant. The increased use of herbicides
allowed by herbicide tolerant crops means that fields have many fewer
milkweeds than before (Oberhauser unpublished). Monarchs can also be exposed
to insecticides used to control insect pests in agricultural fields,
forests, and gardens. Many people worry that the use of insecticides to
combat mosquito-borne diseases like the West Nile Virus will kill monarchs
and other beneficial insects.
The risks to monarchs of corn genetically modified to contain Bt (Bacillus thuringiensis) toxin have received a great deal of attention (Losey et al.
1999; Jesse and Obrycki 2000; Oberhauser et al 2001; Sears et al. 2001;
Brower 2001). Bt corn produces a protein that is toxic to lepidopteran
larvae, and is effective against European corn borers, important
agricultural pests. However, the wind-dispersed pollen produced by Bt corn
also carries the toxin. The toxicity of the pollen produced by different
corn varieties varies significantly, and the varieties now on the market
have lower levels of toxin that some of the earlier varieties (Hellmich et
al. 2001; Sears et al. 2001). Most researchers who have assessed the risks
of this technology isolated corn pollen from other material shed by the
plant (particularly the pollen-bearing anthers) (Hellmich et al. 2001; Sears
et al. 2001), but Jesse and Obrycki (2004) found a consistent trend of lower
survival in Bt fields than non-Bt fields when larvae were exposed to Bt corn
pollen and anthers naturally deposited on milkweed plants within a corn
field. This finding suggests that the blanket conclusion that Bt corn poses
no risks to monarchs (Sears et al. 2001) should be revisited.
End of the section on "Monarch Breeding Ecology."
of this Review (Monarch Butterfly Ecology):
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