Studies on population dynamics of the yellow headed borer, Dirphya Nigricornis Olivier (Coleoptera: Cerambycidae), a pest of coffee in Kenya
Studies were conducted in coffee plantations in Kenya and under laboratory conditions to determine whether durations and sessions of mating either impeded or enhanced both the ovipositional rate per day and fecundity of Dirphya nigricornis Olivier on susceptible varieties of Coffea arabica L. Stocks of wild and field bred beetles from three districts of Kenya were used in the studies. Caged pairs of beetles in sex ratio 1:1 installed on distal tips of coffee shoots were in addition used to relate the oviposition behaviour to egg niche location and to elucidate through timing and observations from stop watches the oviposition rhythms if any in the beetle. Variable populations were also initiated and used to establish the mortality factors for the egg, larval, pupal and adult stages of the pest as wTell as evaluating the efficacy of the braconid, Iphiaulax varipalpis Cary as a biocontrol agent of the pest in Kenya. A series of complementary studies were also conducted to evaluate the actual damage by the larvae to coffee stems and the losses in yield that resulted from their boring. These parameters were critical in understanding the population dynamics of the pest. Caging of the beetles singly showed that the mating of JD. nigricornis influenced its oviposition through the durations (minutes) of mating and the number of times the species mated. Long durations of 280 minutes and six repeated sessions of mating enhanced significantly (P < 0.05, 0.01, 0.001) the production of upto 7 eggs per day per female. This led to the maximum fecundities of 21.47 and 29.50 eggs per female of first and second brood beetles, respectively. Short and single matings below 36.41 minutes or 1.30 sessions either impeded oviposition or had minimal influence on it. The behaviour of the beetles when timed and observed before and during oviposition using 20 x 15 cm wire-mesh cages on coffee tips showed that the ovi- positional period lasted 11.3 + 1.8 minutes. It comprised of a search period of 3.0 + 0.5 minutes, a gnawTing period of 1.0 + 0.2 minutes, an egg deposition period of 5.0 + 0.1 minutes, and a sealing period of 2.0 + 0.3 minutes. Within that time, the females were able to integrate various plant factors such as preferability of the 12 distal internodes, their green colour and girths of 0.9 + 0.1 cm and the possibility of peeling the bark with least force of approximately 8-11 mg/mm2. The females when observed and timed both within and between days of oviposition demonstrated that they started ovipositing about the same time during each reproductive day after a constant pause of 22.83 + 1.19 to 31.17 + 7.97 hours between days. During each day of oviposition, there occurred two rhythms with eggs laid at seven intervals of < 110.0, 186.0 and 260.5 minutes respectively. When the occurrence of the egg stage was sampled and categorised per season during the years 1982, 1983 and 1984 in the primary, secondary, tertiary, quarternary and sucker shoots that comprised the coffee canopies at the two sites, this revealed that D^. nigricornis constructed single, double, treble, quadruple or other multiples of egg niches. They were predominantly located within 142.0 and 215.0 cm of the mid zones of canopies usually 0.5 to 30.0 cm off shoots but concentrated significantly (r = -0.44 to r = -0.99) in tip internodes irrespective of shoot category. The main causes of the fluctuations in the populations of the beetles as assessed under field conditions were namely: brooding, incidence and prevalence of parasites and predators of the larval, pupal and adult stages as well as the prevalence of abiotic factors. The latter factors also acted on the egg stage. Brooding produced two different generations which occurred after 351.46 - 354.30 and 586.86 - 588.33 days from every single egg population. The parasites and predators which were recognized and recorded were: Iphiaulax varipalpis Cary, Microplitis sp, Camptotypus (Hemipimpla) sp, Ectopsocus sp, Mirid sp. Dacnodes caffra Dohrn, Pheidole sp. Acantholepis sp, Tapinoma sp, Crematogaster sp, Tetramorium sp and Technomyrmex sp. The incidence of parasitism was low in the larval stage and absent in the pupal and adult stages. Contrarily, there was a trend toward larval, pupal and adult predation. Field collected samples and specimens of the braconid, _I_. varipalpis when reared in polyethylene sheet tubes under controlled laboratory conditions of 23.0 C + 1.0 at 70% r.h. and used to evaluate its ability to control the beetle in perspex cages showed that while field parasitism was as low as 10.72%, this rose to 56.66% in the laboratory because the parasite detected the hosts very easily in the latter which was minimized in the thick canopies of the plants. A relationship of the damage to the plant canopy and yield by the boring larvae when evaluated from readings taken of incidence of bores and the amount of destruction by larvae of known'ages revealed that the damage occured significantly (P < 0.001) when a length of 14.45 cm had been bored by the beetle by killing approximately 2.5 cm of it, but only after 1.45 nodes had completely withered up. This was often the case when at least 16.39 trass bores were visible on’ the bark. However, economic losses in terms of reduced weights of cherries only occurred when 15.9 cm and beyond of the main stem had been bored.