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SUMMARY(1) The dynamics of the seed bank of the tropical weed Mimosa pigra were investigated in two regions of northern Australia by monitoring changes in the seed populations beneath mature stands, by a seed burial experiment, and by measuring the seed output of populations.(2) Seed populations were generally very high compared to the range expected for secondary tropical vegetation. For example, in 1986 they varied from 8500 m-2 in one region to nearly 12 000 m-2 in another.(3) In the seed burial experiment, viability declined with time. The rate of loss declined with increasing depth of burial, and differed between the two regions, with exponential half-lives ranging from 99 weeks at 10 cm depth in a light clay to 9 weeks at 1 cm depth in a heavier black cracking clay. These seed loss rates were within the range reported in the literature.(4) The measured seed banks broadly agreed with the values predicted by a simple model based on the rates of seed loss found in the burial experiment.(5) Although the decline in viability of M. pigra seeds can be fairly rapid, it is argued that, because of the large soil seed population, rigorous control of seedlings would have to be maintained for some years after the eradication of mature plants.
Observations were made over a two-year period on the growth and natural enemies of Lantana tiliaefolia and L. glutinosa growing naturally at nine sites covering different climatic areas of Parana State, south Brazil. Lantana was found to be subjected to continuous attack by insects and/or fungi throughout the year, with different species causing damage at different times. Temperate-climate sites had a more diverse complex of insects and a greater intensity of attack than warmer-climate sites. Shrubs in sites which suffered severe insect attack, especially during the previous season, flowered and fruited significantly less than those which had suffered only light attack. Differential weighting of the principal phytophagous insects demonstrated that feeding by insects of some groups may be more damaging than that by others. The relevance of the results to the search for biological agents for control of lantana in Australia is discussed.
An investigation of the effects of 20 secondary plant chemicals on the survival, development, and feeding behavior of a polyphagous insect, Melanoplus bivittatus (Say), was made using chemically defined synthetic diets.When added to the basic diet, only one chemical, tigogenin, showed unqualified stimulatory effects on growths. Nornicotine dipicrate, solanine, tomatine, digitonin, and saponin caused all nymphs to die before they attained the adult stage, and santonine, indican, diosgenin, hecogenin, and lupinine produced leas severe toxic effects. Very few of the chemicals which permitted survival affected adult weight.Feeding preference experiments showed that some chemicals reduced feeding but none increased it. If it is assumed that the role of secondary plant chemicals in the feeding behavior of M. bivittatus is to indicate to the insect which diets to discriminate against, anomalies existed in that diets containing innocuous chemicals were sometimes discriminated against, whereas diets containing some chemicals producing lethal effects were accepted.Steroid chemicals exhibited the greatest effects on feeding behavior, survival, and growth, and it is postulated that the distribution of plant steroids may provide an insect – food plant association mechanism.
Survival of the non-parasitic stages of the cattle tick, Boophilus microplus (Canestrini), has been investigated over a period of 5 years in 25, 40, and 80 in. rainfall districts of north Queensland. Engorged ticks were exposed regularly in field plots and in Stevenson screens, and the times of egg hatchmg (prehatch period) and periods of survival of the larvae were observed. The prehatch period in the three districts followed the same pattern and was related to seasonal temperature changes, varying from less than 4 weeks in midsummer to over 13 weeks in midwinter. A relationship between minimum prehatch period and mean air temperature for ticks exposed in screens was established, but the data from ticks exposed in the field were too variable to establish a relationship between prehatch periods and soil temperatures. Hatching was recorded in every month of the year from all districts, but during the driest months in the 25 in. rainfall areas many ticks in exposed situations failed to reproduce. Larval survival and total longevity also followed a similar pattern in all districts. The longest survival periods were recorded for the progeny of ticks exposed late in the wet season from March to April, and the shortest survival periods for the progeny of ticks exposed during the dry season from August to September. Mean maximum total longevity for ticks exposed in field plots in the 25 in. rainfall district varied from 10 to 22 weeks, in the 40 in. rainfall district from 14 to 22 weeks, and in the 80 in. rainfall district from 15 to 26 weeks. The data on prehatch and survival periods are a suitable basis for formulating recommendations for tick control by pasture spelling and planned dipping.
The host range, distribution and aspects of the biology of Tingidae recorded from plants in the genus Lantana are reviewed. Tingids possess attributes predisposing them for use as biological control agents. Possible benefits from importation of a larger gene pool of Teleonemia scrupulosa are discussed. The host ranges of T. elata and Leptobyrsa decora were studied under simulated field conditions. Results indicated that both species are restricted ahnost entirely to L. caraara. Subsequently T. scrupulosa, collected from diverse habitats throughout its range, T. elata and L. decora have been liberated in Australia.
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