The effect of Coccotrypes fallax (Coleoptera; Scolytidae) on the recruitment of Rhizophora stylosa (Family Rhizophoraceae) in North Queensland mangroves
Brook, Beth Mary (2001) The effect of Coccotrypes fallax (Coleoptera; Scolytidae) on the recruitment of Rhizophora stylosa (Family Rhizophoraceae) in North Queensland mangroves. PhD thesis, James Cook University.
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Abstract
The host specificity of Coccotrypes fallax (Coleoptera; scolytidae) infesting propagules of the mangrove tribe of Family Rhizophoraceae was investigated. Collections within the mangrove forest in three locations in North-east Australia of propagules of Bruguiera gymnorrhiza, B. exaristata, B. parviflora, Ceriops australis, C. tagal, Rhizophora apiculata and R. stylosa were made and examined for scolytid infestation. It was found that C. fallax infest Rhizophora spp to a much higher level than Bruguiera or Ceriops spp.
Seasonal and yearly variations in the level of infestation of R. stylosa propagules at each location were measured on transects within the locations. Numbers of propagules on the forest floor varied seasonally. Initially the number of propagules infested by C. fallax was small, peaking at densities of up to 2.8 infested propagules per square metre by late April. Regression of time on ground against level of infestation showed that propagules became infested after falling from the tree. Levels of infestation of propagules that remained on the site reached 100 % at some locations.
The effect of infestation on the recruitment of propagules and survival of seedlings was investigated. Damage caused by the feeding scolytid larvae was easily visible on dissection of the propagule and took the form of an irregular tunnel (feeding chamber) within the hypocotyl. The effect of infestation was assessed by recording number of surviving seedlings in October and correlating this with the infestation level within the site in the preceding March. A negative log linear relationship, between level of infestation and number of seedlings, was found. Plant out trials confirmed that infested propagules did recruit, but did not survive as seedlings.
The effect of temperature on the survival of the scolytids and their host propagules was investigated through immersion of infested propagules in hot water at 40°, 45° and 50° C for temperature periods of 5, 10, 15 and 20 minutes. Heat treated propagules were planted out in the field to assess effect of heat treatment on recruitment of propagules and subsequent survival of the seedlings. C. fallax are killed by exposure of host propagules to temperatures of 45° C for periods of ten minutes or more. Propagule recruitment and seedling survival were not significantly affected by the heat treatment.
The internal temperature of propagules exposed to direct sunlight was measured and temperatures of up to 55° C were recorded. Mortality rates of scolytids in propagules exposed to sunlight were significantly higher than in controls placed in adjoining shade. Collections of propagules from locations with high levels of insolation had no active infestation.
The host specificity of C. fallax, the level of infestation recorded in various locations and the effect of infestation on survival of Rhizophora seedlings suggest that C. fallax infestation on Rhizophora spp is a possible factor in succession within the Rhizophora zone. Host specific predation by C. fallax affects only Rhizophora. Ceriops and Bruguiera seedlings were not affected.
C. fallax infestation is not a factor in colonisation by Rhizophora of exposed substrate. Sunlight has the potential to quarantine propagules from the effects of any predation. This research demonstrates the importance of investigating both biotic and abiotic factors as well as their interactions in looking at factors affecting community structure.
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