Submitted to: Florida Entomologist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/15/2013
Publication Date: 3/1/2013
Citation: Fitzgerald, D., Tipping, P.W. 2013. Effect of insect density and host plant quality on wing-form in Megamelus scutellaris (Hemiptera: Delphacidae) . Florida Entomologist. 96:124-130. Interpretive Summary: Megamelus scutellaris Berg (Hemiptera: Delphacidae) is a South American insect that feeds on waterhyacinth, Eichhornia crassipes Mart. (Solms) by sucking the sap. One characteristic of this species is the ability of adults to grow long wings and disperse when they get too crowded or run out of food. However, high densities of this insect may be necessary to damage this weed so the higher the tolerance of this insect for crowding, the more likely it will be able to build up large numbers. This research examined the effects of different insect densities and plant quality on the likelihood the adults would grow long wings and disperse. Feeding was limited to a single generation with densities anywhere from 0 to more than 350 per plant. None of the experimental treatments resulted in a single adult with long wings indicating that, at the plant qualities used in these experiments, this biological control agent can tolerate crowding. This result increases the chances that this insect will be able to build up large populations which may negatively impact waterhyacinth.
Technical Abstract: Megamelus scutellaris Berg (Hemiptera: Delphacidae) is a South American species that feeds on waterhyacinth, Eichhornia crassipes Mart. (Solms). This species exhibits significant wing dimorphism whereby fully winged adults (macropters) are capable of flight while those with reduced wings (brachtypters) are not. The wing form is determined by a developmental switch that is triggered by environmental factors including crowding, host plant quality, temperature, and photoperiod. This study examined the influences of insect density and host plant quality on M. scutellaris wing dimorphism, development, and biomass as well as their effects on E. crassipes. Two experiments exposed a single generation of M. scutellaris to lower and higher densities of conspecifics on low and high quality plants. The first experiment involved transferring second instars to test plants at loads of 50, 5, or 1 g of fresh weight plant biomass per nymph, which resulted in mean densities of 2.0, 15, and 69.1 nymphs, respectively, per low and high quality plant. A second experiment exposed test plants to 2, 4, or 20 M. scutellaris adults for 7 d and allowed their progeny to develop into adults which ultimately produced densities of 0, 56.5, and 352.4 F1 adults per low and high quality plant, respectively. No macropterous adults were produced in any treatment combination in either experiment. Several plant variables were affected by insect densities and plant quality including the mean relative growth rate, the change in leaf number, and the percentage of dead leaves on a plant. Megamelus scutellaris appeared to have a relatively high density threshold for macroptery within the range of host quality used in these studies. This may promote more brachyptery which, in turn, may increase the chances of this insect reaching damaging densities in field populations of E. crassipes.