THE BIO-ECOLOGICAL DYNAMICS AND EFFICACY OF A FIELD INTEGRATED CONTROL STRATEGY OF INVASIVE PEST, Leptocybe invasa Fisher & La Salle, IN KENYA

Abstract

Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae) has been recorded in many tropical and sub-tropical regions of the world as a gall wasp attacking Eucalyptus species. Chemical control strategy is hampered by the fact that L. invasa completes much of its life cycle inside eucalyptus tissue and only expensive systemic insecticides may be palliative. It is not known whether integrated control of L. invasa would be feasible due to insufficient information on the insect’s biology and ecology. The urgent need for more information on the biology and ecology to pave way for its control generated the current study. The studies reported here investigated the colonization and the rearing procedure for L. invasa; its life cycle, oviposition requirements, foraging, potential of selected herbaceous plants as IPM components against the pest and variability of its attack on different Eucalyptus germplasm. Caged infested E. saligna seedlings were used as sources of L. invasa while caged healthy seedlings were used in L. invasa life cycle and susceptibility experiments with gall formation and mean gall numbers per seedling as response variables. Leptocybe invasa took nineteen weeks (128-131 days) from oviposition to adult stage at a room temperature of 25.5 oC with ten infested and caged Eucalyptus seedlings giving an output of 30± 6 insects per cage per day. L. invasa egg is white to light yellow in colour and round in shape. Egg diameter ranges from 0.09 mm to 0.14 mm (mean of 0.11± 0.01 mm). Eggs are laid singly, normally in a row beneath the epidermis of the host tissue (leaf midrib, petiole and succulent shoots). A white substance occurs on the oviposition spots 1.50 ± 0.29 days after oviposition. The larva of L. invasa is a minute, white legless grub. Larval stage takes 33 – 42 days with four larval instars and body length ranges from 0.13- 0.19 mm with a mean (± SD) of 0.16 ± 0.02 mm. The pupa of L. invasa has mummy-like appearance and is dark brown to black in colour. Mature pupa measures 1.1 – 1.2 mm in length. Pupal stage takes about 14 days. Adult L. invasa are black in colour and small in size. The mean (mean ± SD) body measurements of unsexed insects are as follows: Body length = 1.13 ± 0.07 mm; Head capsule width (eye to eye) = 0.25 ± 0.01 mm; Antennal length = 0.25 ± 0.00 mm; Number of antennal segments = 0.37 ± 0.02 mm; Wing length = 0.77± 0.09 mm; Abdominal length = 0.42 ± 0.07 mm; and Abdominal width 0.29 ± 0.01 mm. Adult stage takes 3-4 days under natural conditions but can be lengthened to 16 days if the adults are fed on 15 % sucrose. More eggs are laid by Leptocybe invasa (Hymenoptera: Eulophidae) in response to cues for oviposition in relation to olfaction stimuli than visual stimuli (p < 0.05) and patch residence time was greater than time spent in foraging from patch to patch (p < 0.05). Low nitrogen fertilization and moderate watering regime lowered the severity of attack by the gall wasp (2.6 ± 0.9 galls per seedlings) (p < 0.05) while high levels of nitrogen fertilization and high watering regimes increased the severity of attack by the pest (13.1 ± 0.9 galls per seedling) (p < 0.05). Leonotis nepetifolia, Schkuria pinnata Kuntz ex Thell and Tagetes erecta L. were repellent to L. invasa. E. saligana grown together with T. erecta had the least number of galls (4.2± 0.8), followed by those grown together with S. pinnata (6.0 ± 1.3), L. nepetifolia (7.4 ±1.8) and E. saligna grown alone (10.8  1.7) (p  0.05). Unlike S. pinnata, the planting of L. nepetifolia and T. erecta as companion plants significantly reduced height growth of the E. saligna (p  0.05). Eucalyptus saligna was the most susceptible species to L. invasa attack (15.43± 0.29 galls per seedling) while E. globulus and E. citriodora were resistant, having significantly fewer galls per seedling (86±0.07 and 0.94±0.07 galls respectively; p  v 0.05). Whereas E. camaldulensis seemed resistant in the presence of E. saligna, it was slightly susceptible to L. invasa attack when exposed to the insects alone. In the presence of E. saligna, gall count per seedling on E. camaldulensis was 3.21±0.33 while 7.11±0.24 galls per seedling was recorded when the species was alone (p  0.05). This study has provided a first report on L. invasa larval instars. The use of S. pinnata in particular and other herbaceous hosts studied as companion plants to E. saligna as part of IPM strategy against L. invasa has been recommended. Research geared towards elucidating the potential of biological control agents against the pest is recommended