White grubs

The white grubs (i.e. larval stage) of several beetle species in the family Scarabaeidae commonly infest the roots of pineapple plants. Scarab species reported feeding on pineapple roots include, in Australia: the southern one-year canegrub (also known as rugulose canegrub, nambour canegrub), Antitrogus mussoni (Blackburn), Christmas beetle, Anoplognathus porosus (Dalman), rhopaea canegrub, Rhopaea magnicornis Blackburn, squamulata canegrub, Lepidiota squamulata Waterhouse (= Lepidiota darwini Blackburn, Lepidiota leai Blackburn, Lepidiota rugosipennis Lea), noxia canegrub, Lepidiota noxia Britton, and Lepidiota gibbifrons Britton (Waite, 1993); in South Africa: Adoretus ictericus Burmeister, Adoretus tessulatus Burmeister, Trochalus politus Moser and Macrophylla ciliata Herbst; and in Hawaii: Chinese rose beetle, Adoretus sinicus Burmeister, and Anomala beetle, Anomala orientalis Waterhouse (Carter, 1967). The species Heteronychus arator (Fabricius) is found in Africa and Australia, where it is referred to by the common names black maize beetle (Petty, 1976a) and African black beetle (Waite, 1993), respectively. The species above vary in the levels of damage they cause to pineapple. Additional scarab species that attack pineapple may also exist in these areas and other locations where pineapple is grown. Scarabs are not limited to pineapple in their feeding habits and may attack a wide range of plants. The various species of beetles can be separated in both the larval and adult stages by morphological characters on the body (Carter, 1967; Petty, 1977a).

In most cases, adult scarabs do not significantly damage the pineapple plant, if at all. Many species do not feed on pineapple plants as adults. However, exceptions do exist, such as the adult stage of H. arator which occurs in Australia, New Zealand and South Africa (Waite, 1993; Petty, 1977a) and bores into the lower stems of the pineapple plant. Adults of A. sinicus in Hawaii may riddle or completely destroy pineapple leaves, while the larvae rarely attack the roots (Carter, 1967). Fortunately, adult A. sinicus infestations are typically spotty in an area. On the other hand, adults of A. orientalis typically remain in the soil and lay their eggs in the vicinity of where they developed (Carter, 1967).

Adult scarab females are free-flying and choose the locations where they will lay their eggs in moist soil. Egg deposition preferences for soil conditions and type vary among scarab species (Waite, 1993). Eggs are oval in shape and, after hatching, the first-instar larvae feed on organic matter in the soil. Older scarab larvae develop within the soil among the roots of their host plants (e.g. pineapple). They feed upon organic matter within the soil as well. Although white grubs are not immobile, they do not disperse far from where the eggs were laid. White grubs are easily identified by their white or ivory-coloured, 'C'-shaped bodies, which are soft and plump. The posterior quarter to third of the larval abdomen is commonly a dark blue-grey colour, due to the contents of the digestive system. Grubs have three pairs of legs near their anterior end and a tan to dark brown head capsule (Waite, 1993). They may injure pineapple plants by: (i) feeding on the roots, which interferes with nutrient and water uptake and transport (Carter, 1967; Petty, 1978a; Waite, 1993); (ii) weakening or destroying the roots that anchor the plants in the soil (Waite, 1993); and (iii) wounding plant tissues, which enables secondary plant pathogens to enter the plant (Carter, 1967). If infestations are severe, a crop may be lost, especially in the ratoon crop (Waite, 1993). The length of the scarab developmental cycle varies among species and climatic conditions, but generally they grow slowly compared with most insect pests and may require 1-2 years to complete development to the adult stage (Waite, 1993).

Recognition of white-grub infestations is difficult until significant injury to pineapple plants becomes obvious, commonly in the ratoon crop (Waite, 1993). Plants may become stunted, wilted and chlorotic (Petty, 1978a). Severely affected plants are easy to pull out of the ground (Waite, 1993). Additionally, pathogens, such as Pythium fungus and root-knot nematode, may infect the plant (Carter, 1967). Areas designated for pineapple plantings should be inspected for the presence of white grubs prior to planting the seed crop. Larvae in the soil may be uncovered using a spade or found during cultivation of the soil (Waite, 1993). Adult beetles may be monitored using light traps (Petty, 1977a). Thorough cultivation of the soil will reduce white-grub populations. A preplant soil treatment with long-term residual activity is appropriate for areas where white grubs are historically a recurring problem (Waite, 1993). Given the long production cycle of pineapple (i.e. seed crop and ratoon crops), the long-term effectiveness of chemical soil treatments is limited. Discoveries of white-grub infestations after planting are problematic because of the difficulty in controlling them. Delivery of chemicals to the insects is a challenge. Natural enemies of these pests do exist (insect predators, para-sitoids and pathogens, as well as birds, toads, wild pigs and rodents), but the levels of control are not typically adequate (Carter, 1967; Petty, 1976b). Petty (1976b) reviews other control methods, but none appear overly successful or practical.

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