African Pine-Feeding Grasshoppers

These grasshoppers are excellent examples of indigenous insects that developed a preference for an exotic plantation-grown crop, in this case pines. Plagiotriptus pinivorus attained some prominence aſter causing persistently severe defoliation of exotic pines, especially Pinus patula, in Malawi in the 1960s, resulting in significant tree mortality. A smaller scale defoliation of P. patula was observed at Morogoro in Tanzania in the mid 1980s, which was attributable to another, very closely related species, Plagiotriptus hippiscus. Both of these grasshoppers are highly polyphagous, including herbaceous hosts, shrubs and both angiosperm and gymnosperm trees. The prime requirement for P. pinivorus seems to be access to evergreen or semi-evergreen vegetation in areas of moderate to heavy rainfall, i.e., mostly at altitudes between 1,525–2,135 m, but occasionally as low as 490 m.

Plagiotriptus pinivorus in Malawi exhibits three generations every 2 years, and the complete life cycle takes about 1 year. Nymphs and adults have been observed on pines throughout the year, except from December to late January. Copulation occurs anytime, but peaks from October to January and May to June. During copulation, the small male assumes a characteristic dorso-lateral position by clinging to one of the hind femurs of the female. Both males and females are promiscuous. About 7–20 days aſter the last mating, females seek bare soil and dig a shallow pit to lay a batch of up to six eggs. They then resume voracious feeding in the trees, before laying other batches of eggs at 17–35 day intervals.

Eggs incubate from 49–248 days, with an average of 115 days. The winter population hatches from April to May, maturing in November, while the summer population hatches from December to January and matures from May to July. Within the same batch, an average of 34 days and a maximum of 88 days may elapse between first and last hatch.

Nymphal peak emergence and rainfall are strongly correlated in February, allowing prediction of emergence two weeks in advance. Another smaller peak in emergence in August, however, cannot be explained by rainfall. The first instar nymph is ephemeral (about 12 h), and will molt immediately when reaching the soil surface, before feeding on ground vegetation for the next 2–3 weeks. Advanced instars complete their life cycle on trees, each instar lasting about one and a half to over two months. Young instars are wasteful feeders. There are generally six instars for males and seven for females. Despite the extra instar, females develop more rapidly and reach adulthood at about the same time as do males.

Adult males (Fig. 25) are about 1.5–2 cm long, moderately robust grasshoppers. Their abdomen, shield-like pronotum and greatly enlarged hind femora are strongly compressed. A minute set of non-functional wings, not found on nymphs, is hidden under the pronotum. The thread-like antennae are about one third the length of the head. The abdomen is strongly reflexed over the back in the male. The insect is largely leaf-green, but sports inconspicuous, small areas of blue, pink, red and white on various parts of the legs, wings, antennae and the pronotal ridge. Eyes are golden yellow. Females are about twice the size of males, more robust and generally less compressed. They are uniformly leafgreen, except for the golden yellow eyes and valves of the ovipositor. Their wings are also minute and hidden under the pronotal shield.

Numerous invertebrate and vertebrate predators, including skinks, birds and blue monkeys, as well as parasites were documented, but ultimately they were deemed insufficient by themselves to reduce populations of the grasshopper to nondamaging levels. As a result, sticky bands and chemical controls were relied on for monitoring and control purposes, respectively. In the 1960s in Malawi, gamma-BHC at 0.5% proved the most effective insecticide for ground and aerial applications at ultra-low volume formulations. Spraying of road banks was particularly recommended, as insects clustered there for oviposition in the bare ground. present; labial palpi long and porrect, 3-segmented; maxillary palpi short and 3-segmented; antennae short. Maculation is brown or gray, with various darker spots. Biologies and larvae remain unknown.

Figure 25 Plagiotriptus hippiscus female (above) and male (below) with size comparison (shaded)