The green mirid, Creontiades dilutus (Stal) (GM) is one of the important early season sucking pests in cotton in Australia. They prefer to feed on the growing parts of the cotton plant, particularly terminals and squares, causing shedding of squares and partial to complete wilting of terminals depending on the severity of feeding. GM feeding early in the season causes delay in maturity. Currently they are controlled mainly by the broad spectrum insecticides applied to control Helicoverpa spp., but this disrupts biological control of other major cotton pests. To increase our understanding of the pest, research on biology, behaviour, damage etiology and ecology of the GM was undertaken during the period 1994-97 in cotton growing areas of northern New South Wales.
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C. dilutus was studied extensively on cotton plants under glasshouse and field conditions and its life cycle and biology was documented. Effects of temperature on female fecundity and egg and nyrnphal development were also studied. Both fecundity and development rate were found to be maximum at around 30°C. Threshold temperatures for development of eggs, individual stages of nymphs and the total nymphal period were calculated.
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The oviposition pattern of GM on the cotton plants and the distribution of GM eggs in relation to the position of the main stern nodes were studied. GM preferred to deposit their eggs dorsally on the petiole end of 4 to 8 main stern node. Two factors, hardness and hairiness influence GM to discriminate preferred oviposition sites.
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Damage etiology of GM on cotton was studied in detail under glasshouse and field conditions. Both nymphs and adults caused similar damage to the cotton plant. The damage caused by GM was cumulative and very apparent as nymphs entered the 4th instar. GM confined their attack to young growing parts of cotton plants, especially squares and terminals, causing squares to shed and terminals to wilt. The shedding of squares depended on feeding site, frequency of feeding and the size or age of the square. The amount of damage increased with the increase of insect number. Their feeding extended to the vascular bundle region and during feeding the insect released pectinase that destroyed surrounding cells of the feeding point. GM feeding in the field did not cause any significant yield loss in irrigated cotton. However, their feeding at early stages of plant growth caused significant delay in maturity.
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The economic injury level for GM for the first time was calculated using maturity delay from field experiments by manipulating field population. An economic threshold (1 GM/m of row) was proposed which is higher than the currently recommended economic threshold and therefore will assist in the successful implementation of IPM programmes.
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Extensive studies were made on GM ecology during 1994-97 in two locations. GM was more abundant in early to maximum squaring stage in all seasons in both locations. Analysis of GM population suggested that populations were regulated by contest competition during early cotton season (October - December) and scramble competition during middle and late cotton season (January - March). Egg populations may have been regulated by delayed density dependent factors, possibly predators. Insecticides targeted for Helicoverpa, temperature extremities, heavy rainfall along with strong wind, predators, alternative hosts, intraspecific competition and emigration may also have influenced GM populations in the field. Other than these factors, overwintering hosts allowed the build up of populations toward the end of spring, contributing to GM invasion of cotton. Study on the overwintering hosts also suggested that in northern NSW cotton growing regions, GM populations continued throughout the winter, though population growth was reduced.