Caterpillar Growth Research Articles

Growth versus molting time of caterpillars as a function of temperature, nutrient concentration and the phenolic rutin.

A factorial experiment tested the effects of varying nutrient concentration (normal versus diluted), presence or absence of the phenolic allelochemical rutin and daytime temperature (20, 25 and 30° C) on growth, molting and food utilization efficiencies of tobacco hornworms (Manduca sexta). Two of the utilization efficiencies (approximate digestibility and efficiency of conversion of ingested food) were unaffected by temperature; the third one, efficiency of conversion of digested food, was affected by temperature but there was no consistent effect. Lower temperatures significantly increased the proportion of the stadium spent molting, with larvae at a daytime temperature of 20° C spending 9% more of the stadium in molting than larvae at 30° C. Growth time was not influenced by nutrient concentration. When temperature was low and rutin absent, molt time and the proportion of the stadium spent molting were affected by nutrient concentration. Addition of the phenolic rutin did not have an appreciable effect on growth time or digestive processes. However, it increased molting time by 7 to 14% and thus increased the duration of the stadium and reduced relative growth rate. These results indicate that the effect of food quality on growth rate is a function of the thermal conditions of insect herbivores. At cooler temperatures, a disproportionate increase in time spent molting, rather than altered food utilization efficiencies, contributed to lower growth rates. The consequences for larval growth of fluctuating temperatures due to diurnal cycles and the presence of predators forcing larvae into thermally suboptimal microhabitats are discussed.

Synchronized Group Foraging, Thermoregulation, and Growth of Eastern Tent Caterpillars in Relation to Microclimate

Foraging patterns of genetically similar colonies of Eastern tent caterpillars were continuously recorded in the field (in normal and artificially manipulated microenvironments) and in the laboratory. Frequency and duration of foraging bouts were similar despite average daytime body temperatures, which differed by as much as 15 C. Caterpillars which had access to radiant heat regulated their body temperature. Body temperature of caterpillars without radiant heating passively followed ambient conditions. Growth of tent caterpillars was positively correlated with body temperature and paralleled rates of food processing, as indicated by rates of feces production immediately after feeding. Behavioral thermoregulation enhances growth rates in tent caterpillars by increasing rates of food processing during inactive periods, presumably increasing the amount eaten in the next feeding bout.

The feeding behavior of a polyphagous caterpillar (Diacrisia virginica) in its natural habitat

The ground-dwelling woolly bear caterpillar Diacrisia virginica occupies a habitat characterized by low-lying forbs, grasses, and sedges where it feeds on more than 100 species of plants. The short effective range of its visual and olfactory senses restricts to 1 cm or less the distance from which it can recognize plants. Caterpillars meander randomly and blunder into food plants. Because of the dense and complex vegetational texture of the habitat, the random locomotion in the field, and the omnivorous diet of the caterpillars, only a small proportion of time need be spent wandering (2%). More than 90% of the time is devoted to inactivity which represents postprandial and trail resting. Meal size is regulated by state of deprivation and is modified by the position of the plant in the hierarchy of acceptability. With increasing deprivation larvae become less finicky and therefore seldom experience acute starvation. The effectiveness of plants in sustaining survival and optimal growth of caterpillars is only grossly correlated with acceptability. The field behavior of D. virginica seems to be well described by the thermostatic model of feeding which predicts time budgeting for animals as a function of physiological and ecological parameters.

Short-term damage-induced increases in tobacco alkaloids protect plants.

Leaf damage significantly increases the alkaloid content in undamaged leaves on damaged field-grown wild tobacco plants. Although field-grown pot-bound plants fail to exhibit the same damage-induced increase in alkaloid content, the ability to respond to leaf damage is restored 6 days after removing plants from their pots. Freshly hatched Manduca sexta larvae reared individually in the laboratory on the high-alkaloid foliage of damaged plants released from their pots gain less weight and eat less (57.2% and 45.7% of controls, respectively) than larvae fed low-alkaloid foliage from undamaged released plants. Moreover, larvae grow equally well on the foliage of damaged and undamaged pot-bound plants. The higher chlorophyll contents characteristic of damaged released plants did not negate the effects of the increased alkaloid contents on larval growth. Undamaged leaves from undamaged field-grown plants stem-fed nicotine solutions had elevated leaf nicotine and nornicotine contents. Larvae reared on these artificially induced leaves gain only 38.5% of the weight gained by larvae reared on low-alkaloid foliage. These results demonstrate that damage-induced increases in leaf alkaloids protect induced foliage from attack and are sufficient to explain the decreased growth of caterpillars on the foliage of damaged plants.

Relationships between leaf age and the food quality of cottonwood foliage for the gypsy moth, Lymantria dispar.

The cottonwood tree, Populus deltoides, continues to produce leaves late into the growing season, exposing midseason herbivores to leaves of a wide range of maturity. Gypsy moth larvae preferred and grew best on the oldest cottonwood leaves and suffered higher mortality and 85% less growth when fed young, expanding leaves. Concentration of phenolics in the youngest leaves was 3 times that in the oldest leaves and was negatively correlated with caterpillar growth rate. The active phenolics were not identified; tannin was present but its concentration changed more with season than leaf age.

Thermal Ecology, Behavior, and Growth of Gypsy Moth and Eastern Tent Caterpillars

Eastern tent caterpillars (Malacosoma americanum) are behavioral thermoregulators while gypsy moth caterpillars (Lymantria dispar) are thermal conformers. Differences in thermal ecology can be related to differences in the environments these species encounter. Temperature sensitivity of growth rates differs in a predictable manner; the growth rates of gypsy moth caterpillars are temperature independent at ecologically relevant temperatures of 25°—30°C, and the growth of the tent caterpillars is very temperature dependent. Growth rates of tent caterpillars in the field are greater than that expected for the average ambient temperature of 15°, and closely approximate growth in the laboratory at 25°, near the average Tb exhibited in the field. The higher growth rates are a result of the tent caterpillars' elevated body temperatures. These data are used to develop hypotheses for the evolution of caterpillar thermal responses.

Ant Constancy at Passiflora Extrafloral Nectaries: Effects on Caterpillar Survival

I examined the role of ants in controlling the magnitude and distribution of caterpillar survival at Corcovado National Park, Costa Rica. Ants of 29 species were observed visiting the extrafloral nectary glands of Passiflora vitifolia and P. quadrangularis. Plants attended by ants had ants present 60% of the time. Approximately equal numbers of plants attended by ants and plants without ants were treated with Tanglefoot to exclude walking predators from a single branch on each plant. Heliconius ismenius caterpillars were placed on the treated plants and an equivalent set of unmanipulated controls, and their growth and survivorship were measured. Forty—eight percent of caterpillars died over a 2—d period on unmanipulated plants with many ants, while only 20, 22, and 23% died on unmanipulated plants with few ants, Tanglefoot—treated branches with many ants outside the Tanglefoot barrier, and Tanglefoot—treated branches with few ants outside the Tanglefoot barrier, respectively. This suggests that ants were the only significant walking predator of Heliconius ismenius caterpillars in this habitat, and that °75% of the deaths observed on control plants were on plants with ants. After 4 d all caterpillars were replaced by a second set of caterpillars. Measurements of these latter individuals revealed that mortality was 62, 46, and 24% on control plants on which caterpillars previously survived for 0, 2, and 4 d, respectively. No trend was observed on branches with ants excluded. This indicates that the effects of ant predation are locally stable, i.e., that ant constancy causes plants to be heterogeneous in their suitability as host plants. Caterpillars thus are much more likely to survive on some plants than on others. Analysis of caterpillar growth rates did not reveal any effects on survival; i.e., slow—growing caterpillars survived as well as fast—growing ones.

Does tent caterpillar attack reduce the food quality of red alder foliage?

We assayed the quality of red alder trees for western tent caterpillar growth and survival to test the hypothesis that caterpillar feeding stimulates plant defenses in both attacked and adjacent trees. Three years of high tent caterpillar density were necessary before deterioration in foliage quality occurred, and even then only foliage from trees which were almost completely defoliated in the current year reduced the growth of caterpillars. Both tent size and mean egg mass size increased after the second year of high density which indicates that good conditions still existed for tent caterpillars after 2 to 3 years of heavy feeding.Egg masses which were moved to areas where trees had not recently supported a high caterpillar population produced significantly smaller tents than endemic controls in 1982. Therefore the small tent and egg mass size of the high density population in 1982 was inherent to the insects rather than modified by food source. In 1983 the tents from introduced egg masses were as large as naturally occurring tents.If lightly attacked trees within areas of high caterpillar density are better defended against insect attack, this does not show up in their ability to support caterpillar growth and survival. We found no evidence to support the hypothesis that trees communicate insect attack and stimulate chemical defenses in adjacent trees. Reduced foliage quality seems to be a result of extensive insect damage rather than a defense against insect damage.

Growth of Herbivorous Caterpillars in Relation to Feeding Specialization and to the Growth Form of Their Food Plants

Larvae of 9 species of swallowtail butterflies (Papilionidae), 10 species of bombycoid moths (Saturniidae and Bombycidae), and of the southern armyworm (Noctuidae: Spodoptera eridania) were reared under standardized conditions on mature leaves of many of their typical food plants. Growth rates, feeding rates and efficiencies of food and nitrogen utilization of larvae in their penultimate and final instars were measured by standard techniques. The study revealed a clear relationship between larval growth and growth form of the food plants quantified as leaf water content. Larvae grew faster and more efficiently on herbaceous plants than on the foliage of shrubs and trees. These differences were greater than could be accounted for by variation in the degree of feeding specialization among the insect species tested. The particular leaf characteristics responsible for the relationship between plant growth form and larval growth are not known, but they probably include leaf water content, nitrogen content, toughness, and fiber content. Since trees are likely to be more apparent to enemies than are most herbaceous plants, our results are consistent with recent suggestions that the mature foliage of apparent plants is generally poorer food for herbivores than is the foliage of unapparent plants. However, the extent to which the low food value of mature tree leaves is actually a consequence of the selective action of herbivores remains an open question.