Influence of larval and adult nutrition on egg production of Oxyops vitiosa
G. S. Wheeler
Abstract. Leaves from different fertilizer
treatments of M. quinquenervia were fed to
neonate and adult O. vitiosa to determine the
influence of nutrient levels on agent survival, growth, development and fecundity. We found that larvae fed the low fertilizer leaves
had survival that was less than one half (30 ± 5.0%) that of larvae fed the high
fertilizer leaves (65 ± 10.4%). Additionally,
prepupal, pupal, and adult biomass were each significantly greater (each 1.1-fold) in
larvae fed leaves from the high fertilizer level. Thus
in mass rearing activities increased production will occur in fertilized treatments due to
increased larval survival. Furthermore, when
considering potential release sites of nascent populations of agents adult nutrition does
not impact fecundity. However greater establishment of larvae will be expected in higher
nutrient conditions.
Biological control efforts against the
Australian melaleuca tree Melaleuca quinquenervia
(Cav.) Blake (Myrtaceae) have resulted in the 1997 release in south Florida of Oxyops vitiosa (Center et al., 2000). Both the adults and larvae feed on leaves of the
melaleuca tree (Purcell and Balciunas 1994). In
Australia this insect is most abundant in situations where trees are growing rapidly,
often from suckers or landscaped plants (Purcell and Balciunas 1994). This suggests that larval performance (survival,
growth and development) and adult fecundity are influenced by relatively soft, high
nutrient foliage and therefore, establishment of this species in Florida for biological
control of M. quinquenervia will benefit from
knowledge of these nutritional limitations. In previous work conducted in Australia, the
pre-ovipositional period of the adults averaged between 42 and 46 d. Additionally, the females, when laying eggs,
produced an average of 1.8 eggs per d and at least some individuals continued laying for
more than 200 d (Purcell and Balciunas, 1994). Here,
I determine the effect of foliar nutrient levels on the larval survival, development time
and biomass gain, adult pre-ovipositional period, fecundity and longevity of O. vitiosa.
Plant quality. Melaleuca
quinquenervia plants were germinated and grown in 1 liter pots connected to a drip
irrigation system. Seedlings were
transplanted to one of two fertilizer levels 90, and 30 g/12 liter pot (Osmocote plus 15-9-12/ 12 liter pot;
Scotts-Sierra Horticultural Products Co.). Plants were grown in tanks receiving rainwater and
irrigation 3 times/week for approximately 3 months. Leaf
tips were removed and fed to larvae and adults as needed.
Insect. Eggs were collected from our laboratory colony
and the neonates were placed on leaves of one of the fertilizer treatments. Larvae were reared to the adult stage and data
were collected on larval survival, development time to the prepupal stage, and pupal
biomass gain. Prepupae were transferred to
Oasis floral blocks for pupation. At
emergence the adults were transferred to leaves of the same fertilizer quality and to the
alternate treatment leaves. In this way we
could distinguish between larval and adult nutrition on adult performance and fecundity. Moreover, we could determine the ability of adults
to compensatory for a poor larval diet. Adult
pairs were reared in cages with fresh M.
quinquenervia tips and eggs were counted and removed every 3 days. All insect rearing conditions occurred at 12:12
(L:D) h photoperiod at 28 °C. Data were
collected on the pre-ovipositional period, adult longevity, and the total number and rate
of eggs produced.
Larval
performance. Larval survival was significantly (F 1,4 = 9.19; P = 0.0387)
greater when fed the high fertilizer leaves (65 ± 10.4%) compared with those fed leaves
from the low fertilizer treatment (30 ± 5.0%; Fig. 1).
Larvae fed the high fertilizer treatment had greater prepupal (F 1,123 =
14.52; P = 0.0002), pupal (F 1,105 = 10.23; P = 0.0018), and adult (F 1,91
= 8.07; P = 0.0056) biomass (Fig. 1) compared with those fed the
low fertilizer treatment leaves. Development
time to the prepupal (14.7 ± 0.2d), pupal (22.2 ± 0.3d), and adult (30.5 ± 0.4d) stages
were not significantly influenced by the fertilizer treatments.
Adult
performance and fecundity. Surprisingly,
none of the adult performance and fecundity indices studied were influenced significantly
by the fertilizer diets. Female
pre-ovipositional periods averaged 47.4 (± 7.3 d) and were not significantly influenced
by either larval or adult diets. The
longevity of the adult females averaged (256.6 ± 18.2 d) and was not influenced by
fertilizer treatment. The total number of
eggs produced per female averaged 283.0 (± 48.3 eggs; range 1 to 1069) and was not
influenced by larval or adult diet. The
average number of eggs produced per female per day averaged 0.94 ± 0.17 eggs per day and
was not influenced by larval or adult diet.
These results indicate that the
greatest impact of M. quinquenervia foliage
growing under low fertilizer levels on O. vitiosa individuals
was higher larval mortality resulting in more than double the mortality rate compared with
those fed the high fertilizer leaves. Moreover,
larvae fed the lower fertilizer leaves developed into prepupae, pupae and adults with
reduced biomass. However, adult performance
and fecundity were not reduced when either, or both, the larvae and the adults were fed
low fertilizer leaves. Thus, equal numbers of
eggs were produced by adults regardless of diet, however the number of surviving larvae
will be much greater when fed high fertilizer leaves.
The importance of host nutrient levels for this herbivore species may be minimized
due to its tip-feeding behavior. Typically,
plants grown under nutrient-limiting conditions will shunt more nutrients to the actively
growing tip tissues at the expense to more mature foliage.
This phenomenon was demonstrated by our previous work as when M. quinquenervia was grown under low nutrient
conditions the nitrogen levels in the tip leaves are similar to those of plants grown in
intermediate or high nutrient conditions. Thus,
by feeding on the tip leaves O. vitiosa adults
minimize the negative impact of low nutrient hosts.
Possibly larvae are not able tolerate low nutrient conditions because of their
greater metabolic requirements for growth and development.
The implications of these findings for biological control of M. quinquenervia are several. First, the mass production of weevils at high fertilizer levels will be more conducive to larval survival. However, adult rearing can occur even on low fertilizer plants with little effect on longevity and fecundity. Second, when considering environmental factors for the selection of agent release sites, nascent populations of agents will establish and build up populations more rapidly in high fertilizer sites. If biological control agents are reared in nursery sites for local production and redistribution, fertilization of the plants will benefit larval survival and the ultimate production of the colony.
References